Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 6. Obtaining glaze materials 6.1. Materials suppliers 6.2. Materials from natural sources 6.3. Other sources of materials 6.4. Storing, packaging and labeling

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

6. Obtaining glaze materials

6.1. Materials suppliers

In countries with large ceramics industries, there are suppliers that specialize in collecting and distributing raw materials. These may be mining companies that can supply specific items like clay and feldspar. If these can be obtained directly, it saves the costs of middlemen. However, these companies often deal only in large quantities. For the small producer, it is often best to get supplies from reliable distributors.

6.1.1. LOCAL SUPPLIERS OF CHEMICALS

General chemical suppliers or pharmacies often have many of the necessary ingredients for glazes (which are often used in other industries). They are useful for obtaining small amounts of chemicals, but often their prices are high.

6.1.2. SUPPLIERS OF OTHER INDUSTRIES

Glaze materials are often available from other types of suppliers. For example, agricultural suppliers can provide calcined limestone. Paint industries use materials such as iron oxide and opacifiers.

6.1.3. IMPORTED MATERIALS

Imported materials should only be considered if there are no local sources, as they are expensive and often customs and import regulations make it difficult or impossible for the small producer to obtain them. On the other hand, it is often worth paying the additional price, if it makes possible production of special glazes or decoration effects that are in demand in the market.

In Thailand, for example, where there is a large export market for decorative ceramics, many producers import clay, glazes and overglazes from Japan. Their profit comes from cheap labor and high value added.

6.2. Materials from natural sources

Small producers can mine their own materials if these are available in the area. Historically, pottery centers located themselves where the necessary clay and glaze materials were available. Where stoneware clay and high temperatures are used, it is possible to make glazes from low-temperature clay alone. Generally, stoneware glazes are made from the basic ingredients of feldspar, quartz, limestone and clay, which are quite common. Wood ash is another common base for high temperature glazes. The process of mining, selecting and grinding is quite time-consuming, and with the advent of modern transportation it is often cheaper to purchase materials from suppliers.

In Nepal, we developed low-temperature glazes based on borax, which must be imported. The bulk of the glaze is composed of local materials such as rice husk ash (for silica), limestone and local clay, which are all easy to get and cheap.

6.2.1. CRYSTAL ROCKS

Igneous rocks

When the young earth slowly started to cool, different minerals formed crystals in the mass of molten rocks (magma). A variety of crystalline rocks were formed differing in composition according to their locality. For example, the igneous rock called basalt was created at a great depth and contains little feldspar compared to granite, which formed near the surface.

If rock cools very slowly, crystals have time to grow large, whereas rapid cooling produces small crystals. This process is still going on today where movement in the crust of the earth causes deep layers of molten materials to rise to the surface. An erupting volcano lets out hot magma, which cools quickly. The resulting volcanic rocks have microscopic-size crystals, since the rapid cooling allows little time for crystals to grow.

The most common crystal rocks used in glazes are feldspar and quartz. If a piece of granite is picked up and broken in two, the fresh faces of the stone will show a shiny surface and the crystals of the different minerals can be identified. The black crystals are mica or tourmaline. The yellow, white or red colored crystals with a pearly shine are different types of feldspar. The clear colorless crystals are quartz. The weathered surface of the granite will most probably show a rough surface with many holes, where the soluble feldspar crystals have been washed away by rain, whereas the less soluble crystals of mica and quartz remain. Coarse granite (known as pegmatite) often breaks up in weathering, leaving large pieces of quartz and feldspar lying on the ground. These can be collected, ground and used in glazes.

Volcanic rocks

These are rocks formed by the action of volcanoes, often in the form of molten lava that flows out of the volcano. The crystals in the rock are extremely small because the lava cooled very fast. Lava is essentially a glaze and can be used as the basis of high temperature glazes.

6.2.2. SEDIMENTARY ROCKS

Sedimentary rocks are made of materials produced by the crumbling of old rocks. All rocks eventually break up in the course of time when exposed to weather, and the broken-up rock particles are carried away by water. These particles of clay and sand are transported to lower lying areas or to the sea where they settle one layer upon the other. In the span of millions of years, the growing weight of sediments causes the deeper layers to compact and gradually turn into rocks, called sedimentary rocks. Much later, the movement of landmasses sometimes turns the whole area upside down, so that the old sea floor, with its sedimentary rocks, becomes a new range of mountains.

Figure 6.2.1.B. A coutout of a section of the crust of the earth shows a continental plate moving under another. The friction of the plates generates heat, which melts rocks and feeds a volcano. Rain falls and old rocks are weathered and washed to the sea creating new layers of sediments. Later the sediments are compressed into rocks.

The upper part of new mountains consists of sedimentary rocks resting on deeply set igneous rocks. Sedimentary rocks like sandstone, shale and slate can often be recognized by their layered structure. Limestone is a sedimentary rock created by the skeletons of billions of small animals that lived in the ancient seas. Gypsum is formed by chemical sedimentation in areas where seawater evaporates on a large scale. This produces a high concentration of gypsum which forms crystals like the formation of salt crystals in a glass of salty water.

For the glazemaker, sedimentary shale can be a source of glaze. At high temperatures, shale melts and with a few additions will produce glazes that are usually brown. Although shale often does not slake in water, it can be ground in a pan mill and used in glaze.

6.2.3. METAMORPHIC ROCKS

Igneous and sedimentary rocks are sometimes changed into new forms by high temperature and pressure. Marble is an example of a metamorphic rock formed from the sedimentary rock limestone.

6.2.4. HOW TO GET INFORMATION

Local authorities

First of all, information about the geology and the minerals of the region should be gathered from local authorities, like industrial development organizations, agricultural institutions, National Geological Institutes or mining corporations. They may have little information and the authorities may even say that no materials are available in the region. However, that is often not true and should not keep anybody from looking on his own.

Practical people

It is worth talking to people who make water wells, and builders of dams and roads. They sometimes have useful information about the minerals of the region. Farmers in the area will know about the upper layers of soil on their fields and about local rocks. Sometimes glaze minerals are used for other purposes, like whitewashing houses or medicine.

The best source of information is often other potters.

6.2.5. LOOKING FOR MINERALS

Good places to look for minerals are in riverbeds, where many different types of rocks will wash down from the mountains above. Although most of these may not be useful, it is often possible to find quartz and feldspar. Any rock with an unusual color is worth testing. Rocks that are unusually heavy may contain metallic oxides. For the potter, however, there are few rocks that are directly useful, other than quartz, feldspar and limestone, and some of the volcanic rocks.

Other minerals that are useful in glazes are sodium and potassium compounds, which sometimes form on the edge of lakes, particularly in desert areas. These usually look like a white powder and are soluble in water.

6.2.6. TESTING

To begin with, the most useful test is to take a small sample of the material, place it in a clay bowl and fire it in a regular glaze firing. This will indicate if it melts or not. If it melts, it certainly can be used in a glaze. Materials that do not melt should not be automatically rejected, as many useful glaze materials (such as calcium carbonate and quartz) only melt when combined with other materials. The simplest way to find out if they are of use is to make a line blend of one of your standard glazes, combined with the unknown material.

Rock minerals can be identified by their crystal shape, color, specific gravity and hardness. If you are seriously looking for rock minerals there are good books presenting most common minerals with color photos.

Hardness

Mohs' scale of hardness is based on the hardness of 10 different minerals:

Window glass and a penknife are about 5.5 and a metal file about 6.5.

Two materials have the same hardness if they cannot scratch each other. Quartz can scratch feldspar but not topaz. In the field a piece of glass and a penknife are used to find out if the hardness of a rock is higher or lower than 5.5.

Chemical analysis
If a testing laboratory is available, samples can be sent there for chemical analysis. This is usually expensive but may be helpful if the material looks useful after firing.

6.3. Other sources of materials

Recycled materials are often useful in glazes. These may be by-products from other industries, such as rice husk ash or bone meal, or waste materials. Some other sources of useful materials are discussed below.

6.3.1. METALLIC OXIDES

Metallic oxides are used as coloring agents in glazes. Commonly available are:

Iron oxide, which can be obtained by scraping rust from old steel. It is often possible to get this from paint and hardware suppliers, who use "red oxide" for coloring paint and cement.

Manganese dioxide, which is the main ingredient in torch batteries (the black substance which can be removed from old batteries).

Copper oxide, which can be collected from makers of copper pots. The oxide is the black powder that forms on the surface of copper when it is heated. Another way is to fire copper wire in the kiln and to use the resulting black copper oxide.

6.3.2. ASHES

Wood ashes are used as the basis for high temperature glazes, since they contain sodium, potassium, silica and other ingredients. Early glazes were often simple mixtures of wood ash and clay. Most wood ash is suitable for this purpose, but each type of wood will produce different characteristics and will have a different melting point. So it is important to have a consistent supply. Ash must be sieved to remove unburned material and is usually washed in water and dried before use. If it is not washed it contains more fluxes but they are soluble and make the glaze slip caustic.

At cone 8 to 11, a good starting point is 2 parts ash, 2 parts feldspar and 1 part clay. Ash glazes have the following general limits:

Rice husk ash contains more than 90% silica, so it can be used instead of quartz in many cases. For accuracy, it should be burned white - if there is much black carbon in it, it will make calculations incorrect. In the Appendix the chemical composition of different ashes is given.

6.4. Storing, packaging and labeling

If you use local materials, they will change from time to time. For this reason, it is best to store as much material as possible and to check each new batch by trying it in a standard glaze. For example, feldspar tends to be variable and, as the mine is used, the chemical composition will change. Suppliers of feldspar usually keep several large storage areas of material from different parts of the mine. In order to keep it uniform, they mix the different feldspars together when supplying.

Some materials are damaged by water. Borax, boric acid, soda ash and plaster of parts should all be kept in a dry place. In particular, soda ash absorbs water (up to 7% after one year, 11% after two years) and will thereafter no longer be effective as a slip deflocculant; and plaster will not set correctly after damp storage.

When you get local materials, each batch should be kept separately and labeled with date and source. It is often a good idea to purchase more material when your old supply is about 50% finished and to test it to see if it is the same or not. If it is not greatly different, the new material can be mixed with the old and your glaze will not change unexpectedly.

A good labeling system is very important, as most glaze chemicals look rather alike. Never depend on your memory - keep a permanent label on the bag or jar of material. Additionally, if you order bags of material from a supplier, ask him to label the outside of the bag, and also to put a label inside the bag as labels are often lost in shipping.

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 7. Frits and fritmaking (introduction...) 7.1. Why make frits? 7.2. Frit production 7.3. Frit kilns

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

7. Frits and fritmaking

Most low-temperature glazes require fluxes that are either poisonous (lead) or water-soluble (sodium and potassium). Traditionally, these materials were used raw, but this is not satisfactory for modern potters. Raw lead is poisonous and sodium/potassium are water-soluble. Borax is sometimes used raw in glazes, but these glazes cannot be stored for a long time, as the borax will go into solution or form crystals.

The principle of fritmaking is very simple: molecules of poisonous or soluble fluxes should be chemically combined with glass-making materials to eliminate these undesirable characteristics.

A frit is a combination of a flux or several fluxes (lead, borax, boric acid, potassium carbonate) that is combined with other insoluble materials (quartz, feldspar, lime etc.), melted in a kiln to form an insoluble glass, and ground to be used as the base for making glazes. (Many low temperature glazes are simply 90% frit and 10% china clay).

Fritmaking is not usually practical for the small producer, as it takes time and requires a special kiln and a ball mill for grinding. On the other hand, if reliable frits are not commercially available, the potter may have to produce his own. Frit glazes are more expensive than raw glazes, but their convenience usually makes up for the additional cost.

There are many different commercially available frits, all designed for different temperatures, surface qualities, coefficients of expansion, and color responses. The potter trying to decide which frit to use must depend on the supplier, as formulas are usually kept secret. Suppliers will give advice on which frit is best for the potter's purpose. There are two main types of frit:

Lead frits

These are all designed to provide lead in a nontoxic form. Lead oxide is combined with other materials to give desired properties of surface, opacity, and color response. The standard lead frit is called lead bisilicate and is simply a combination of lead oxide and silica, which combines the lead in an insoluble form. This can be used as the base for a large variety of lead glazes.

Other frits used commonly in the tableware industry are called lead-borosilicate frits, which combine the desirable properties of both lead and boron and are generally safer to use.

WARNING! Lead frits can still be poisonous, and glazes made from them can be poisonous if they are not combined with sufficient silica to combine with all the lead molecules.

Leadless frits
These are based on boron compounds, again combined with other materials. Because glazes compounded with lead are difficult to control for lead release, leadless frits are recommended for small producers.

7.1. Why make frits?

Frit making is only suggested if reliable commercial sources are not available. Often frit manufacturers are not interested in supplying small amounts. Dishonest frit manufacturers sometimes sell bad batches of frit to small producers. Even though frit making is complicated, the small producer who makes his own frits at least has the process under his own control.

Raw borax glazes can be used, but they must be used immediately after mixing or problems will result from the soluble borax. This may be satisfactory for art pottery but, if consistent results are needed, it is better to use fritted glazes.

Similarly, raw lead glazes are widely used. This is a danger for the workers, who will eventually develop lead poisoning unless they take extreme care in handling the glaze. Modern industries never use raw lead glazes, and industrialized countries all have severe restrictions on the use of lead in glazes. In developing countries, workers in industry suffer from lead poisoning, and it is the responsibility of the industrialist alone to take care of the workers' health. As lead poisoning takes several years to develop, many factory owners do not understand the seriousness of the problem and continue to harm their workers. IT CAN TAKE UP TO 20 YEARS TO DEVELOP SYMPTOMS OF LEAD POISONING!

7.2. Frit production

Frit composition

All the soluble materials are included in the frit batch along with silica, in order to form a glass when fired in the frit kiln. Other ma" serials may be included for modifying the frit or helping to melt it.

The main frit raw materials are:

Silica sand, SiO₂
Rice husk ash, almost 95% SiO₂
Borax, or sodium borate, Na₂B₄O₇ 10H₂O
Boric acid, H₃BO₃
Limestone, CaCO₃
Feldspar, soda and/or potash, K₂O & Na₂O
Al₂O₃ 6SiO₂
Clay, Al₂O₃ 2SiO₂
Zinc oxide, ZnO
Zircon,ZrSiO₄ (opacifier)
Red lead oxide, Pb₃O₄
Other materials like talc, barium carbonate and bone ash may be added.

In order to have a frit with low viscosity that easily runs out of the kiln, the clay or alumina of the glaze is not added to the frit. However, in order to make the ingredients insoluble, 2-3% kaolin should be included in the frit.

Work flow

The work flow for frit production is shown in Fig. 7.2.0.A. It is better economy to prepare large frit batches when firing a continuous-type frit kiln.

Figure 7.2.0.A. Work flow of frit production.

Prepare materials

All materials for frit need to be clean, dry and ground to pass through a 60-100-mesh sieve. The finer the material, the easier it will be to melt it. If rice husk ash is used as the source of silica, it should be well-burned to a white color, so that unnecessary carbon is not introduced. If there is a large amount of black carbon, this will decrease the amount of silica available. The content of carbon in rice husk ash may vary more than 30% from batch to batch. If materials are wet, they should be dried completely so that the weight of water is not included in the recipe. In frit calculations, the loss on ignition (see page 146) needs to be included to account for loss of material during firing.

Blend materials

Weigh the materials accurately and blend them together dry. WEAR A DUST MASK! Small amounts can be mixed by hand in a bucket, and larger amounts can be mixed with a shovel on a clean cement floor. After mixing the frit materials they are screened through a 16-mesh sieve (mosquito net) to ensure thorough blending or the materials are run through a hammer mill.

Melt the frit in a kiln

There are many different systems for melting frit, which are described below in section 7.2. In each system, the principle is to thoroughly melt the frit until all ingredient! are combined. Most frit is melted at 1150°C to 1250°C.

Check the frit

A sample of molten frit should be taken and examined to see if the melt is complete The frit should be uniform, without particle! of unmelted material.

With continuous frit kilns, the rate of feeding raw frit and the speed of the melted frit must be adjusted so that all the material melts completely and has time to mix' properly.

Quench the frit in cold water

The molten frit is poured into cold water, which "shatters" it into small pieces that can easily be ground. With continuous melting and discharging it is necessary to let fresh cold water run continuously.

Grind the frit

If the frit is quenched correctly, it will be easy to put it directly into a ball mill and grind it until it can be passed through a 100-mesh sieve. The granulated frit may be first dried and then stored in bags until it is needed for glaze making. Then it is ball-milled together with clay and other glaze materials. Alternatively, the still wet frit is ball-milled first.

Sieve the wet frit

When the frit is removed from the ball mill, it should be sieved through 100 mesh to remove any large particles that were not ground.

Dry the frit

The wet frit is settled, excess water is poured off, and the remaining frit can be spread out to dry, either in the sun or in a dryer.

Test the frit

Each batch of frit should be tested for correctness. The simplest way is to fire it in a kiln on a specially made flow tester, along with a sample of correct frit (page 131). If the frit flows evenly to the control sample, it will probably be correct but should be double-checked by trying it in a standard glaze.

Additionally, the frit should be tested for solubility in water. A sample amount is boiled in water for several hours, then allowed to sit for 2 weeks. If crystals do not form during this time, the frit can be considered stable. If crystals form, it means that there is not enough silica/alumina in the frit and the composition will need to be changed (frit calculations page 144). The causes of crystal formation could also be with the frit firing, e.g. overcharging, too short a firing time and improper mixing.

The finished tested frit may be sold to other ceramics producers either as a milled powder or in granular form.

7.3. Frit kilns

There are many different kinds of frit kilns, which are selected according to the amount of frit that needs to be regularly produced.

Normally, each type of frit -transparent, opaque, lead - requires a separate kiln to prevent contamination. When one kiln is used for several frits, it must be cleaned out before each different batch by melting frit in it to remove most of the old batch. This contaminated frit is then kept separately, to be used as "clean-out" frit before changing to different compositions.

7.3.1. CRUCIBLE FRITTING

Small amounts of frit for testing are easily made in a fireclay crucible. The crucible with frit is fired together in a glaze firing, which will melt the frit into a solid block of glass. After firing, the crucible is broken away from the frit and the frit can be crushed and ground. It is a good idea to first paint the inside of the crucible with china clay slip, as this will make it easier to separate the frit. NOTE: Frits containing boric acid often cannot be melted successfully this way, as the boric acid melts at a very low temperature and flows to the bottom before the rest of the ingredients melt. Frits with rice husk ash may also be difficult to melt in this way, because the upper layer of the frit melts first sealing off the frit mixture so that the carbon remaining in the ash cannot burn out. Carbon is highly refractory and it will prevent the frit from melting.

This is only suitable for test production and is not a safe method, since the pot often cracks, resulting in frit running out, destroying other ware, kiln furniture and the kiln lining.

CAUTION: Borax frits boil during melting with a great increase in volume. The crucible should be filled only half with frit, and a tile placed over the top to prevent boiling over.

7.3.2. CRUCIBLE KILN

For fritting small amounts of frit a simple frit kiln is shown in Fig. 7.3.2.A. It can be fitted with several crucibles arranged in a row for melting different frits at the same time. The crucibles can be loaded with raw frit from the top. The fuel economy of this type of kiln is less than for the other kilns.

Figure 7.3.2.A. Coal-fired frit kiln with three crucibles.

7.3.3. OPEN HEARTH KILNS

Open hearth kilns consist of a tank made of firebricks, which is set in a crossdraft kiln. The kiln may be fired by coal, firewood, oil or gas. The hot flue gases heat the arch over the frit. The arch in turn heats the frit. In batch-type frit kilns, the frit melt is checked by drawing out some melted frit with an iron rod for inspection.

After the frit is completely melted, a hole at the bottom of the tank is opened and the frit flows out into cold water. Then another batch of frit may be charged from an opening in the arch.

Figure 7.3.3.A. Open-hearth frit kiln for coal firing.

The melting of several tonnes of frit may take 6-12 hours consuming 1-1.5 tonne coal per 1 tonne melted frit.

7.3.4. CONTINUOUS FLOW

The continuous-flow frit kiln uses a kiln with a sloping floor, made of fireclay refractories. The raw frit is introduced at the upper end and, as it melts, it flows down while mixing to an exit chute by the burner and then into cold water. The kiln shown in Fig. 7.3.4.B was developed in Nepal. It uses a steam/kerosene burner, but any forced draft oil or gas burner can be used.

Figure 7.3.4.B. DSide elevation of a continuous flow kiln.

The rate of flow is controlled by introducing limited amounts of raw frit. Too much frit at one time may result in incomplete melting. If the frit runs very fast through the kiln, the low melting materials will not melt properly together with the silica. This may be a cause of water-soluble frit.

The frit can be slowed down in the kiln by making less of a slope and by putting some obstacles in the way (like kiln shelf supports).

7.3.5. ROTARY

Rotary frit kilns are large refractory-lined cylinders, which have a burner (gas or oil) that passes through them. The raw frit is introduced, and the kiln rotates full turns (or back and forth) as the frit melts. This has the double purpose of ensuring good mixing and of transferring the heat of the firebrick lining to the frit as this constantly moves over it. When the frit is completely melted, the kiln is turned so that the frit flows out through an opening into cold water.

Figure 7.3.5.A. Front and side elevation of a rotary frit kiln. It consists of a firebrick-lined steel drum resting on rolers. It is gas-or oil-fired.

7.3.6. FUEL ECONOMY

If much frit is to be produced, fuel economy is an important factor. In general, the more frit that can be made at one time, the lower will be the fuel cost. In a continuous frit kiln, it takes several hours to heat the kiln sufficiently to melt the frit at maximum speed -this preheating period consumes a lot of fuel. It is best to fire several hundred kg of frit at the same time to reduce firing costs.

Frit industries generally use rotary kilns, as they are the most economical for long, continuous use. However, the continuous kiln developed in Nepal by the Ceramics Promotion Project compares favorably with standard fuel/frit ratios obtained with rotary furnaces.

Examples of fuel to melted frit ratios are:

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 8. Preparation of glazes (introduction...) 8.1. Raw materials requirements 8.2. Grinding glaze materials 8.3. Weighing, mixing, using batch cards 8.4. Sieving 8.5. Suspending and binding agents 8.6. Density specific gravity 8.7. Old glazes, problems 8.8. Test your glazes! 8.9. Commercial production of glazes

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

8. Preparation of glazes

Glazes should be prepared in a systematic manner in order to prevent mistakes. Most problems with glazes come from simple things, like incorrect weighing, mistakes in identifying raw materials or not sieving the glaze correctly.

Glaze mistakes are expensive, as they can result in the loss of an entire kilnload. For this reason, it is important to have the right person in charge of making glazes -cleanliness, orderliness, careful record-keeping, and reliability are required.

Most small producers do not need a large variety of glazes -in fact, many use only one or two standard glazes and achieve variety by changing the colors, doubleglazing or using engobe decoration.

Designing a glaze is somewhat like choosing a paint in the paint store. First of all, you must decide if you want a glossy or matt surface, transparent or opaque. Then you can add different colors.

Base glaze

The base glaze is simply the combination of materials that melts at the desired temperature. It is either transparent or opaque, matt, semimatt, glossy etc. without any particular colon

Glaze additions

These are usually coloring oxides that are added to the glaze. In Nepal a glaze supplying system serving small producers was established. A base glaze was supplied in 5kg bags and 8 different colors were supplied in small bags that produced standard colors when added to 5 kg base glaze. The small bags contain coloring oxides mixed with a small amount of base glaze so the colors disperse more easily in the base glaze.

8.1. Raw materials requirements

Raw materials need to be as reliable as possible and always ground to the same mesh. If obtained from a glaze supplier, the materials are usually ground to at least 100 mesh. Because materials that are finely ground melt more easily, some ingredients may be as fine as 400 mesh. This is particularly true of quartz -200-mesh quartz will produce a different result than 400-mesh quartz.

When you get new raw materials, they always should be tested before using them in production. The best way is to try them in a standard glaze that you know well and to compare the results with the known glaze.

8.2. Grinding glaze materials

There are several steps in grinding glaze materials. Since many of them (feldspar, quartz, limestone) come as rocks, they first need to be reduced to pebble size. Feldspar and quartz rocks are first calcined to make them soft enough to crush. Calcining means firing to just above 600°C. This can be done in the cold spots of a biscuit firing or for large productions in a special kiln. Crushing of small amounts can be done with a hammer (use eye protection), and large amounts are usually done in a jaw crusher.

8.2.2. BALL MILLING

Ball mill operation

Ball mills are used for fine grinding of ceramic materials. The material has to be reduced to sand size (2 mm or less) before grinding in a ball mill.

Some typical uses of ball mills are:

- grinding clay that does not easily slake
- preparation of casting slips
- grinding of body additions like feldspar, quartz and glass powder
- grinding of frit granules grinding of glazes
- grinding of engobes and terra sigillata
- preparing color pigments for glaze, engobes or bodies.

There are two main types of mills:

- Large mills with an axle system are called ball mills.
- Small mills are called pot mills or jar mills.

These are usually small (up to 5 lifer) porcelain jars or plastic jars, which rotate on two rubber-covered rollers.

Conical ball mill

For large production conical ball mills are used. Various sizes of pebbles are used and the material is fed from one end and discharged at the other. Variation of the centrifugal force caused by a conical 30° slope at the discharge side classifies both pebbles and material so only fine material is discharged.

Vibro energy mill

This is a new type of grinding machine consisting of cylindric grinding chamber suspended on springs and vibrated at high frequency with the help of an excentric mounted on an electric motor. The chamber is completely packed with very hard small cylinders between which the material is filled. The vibrations make the small cylinders grind against each other and the material to be ground. The vibrating mill is better at ultrafine grinding and is more energy-efficient than ball mills.

Lining

The grinding action takes place between the pebbles, and not between the pebbles and lining. Therefore a ball mill with a steel drum can work without a lining (except for white body, where rust particles will cause discoloration). Pebbles constantly falling on a steel drum make a lot of noise. A lining will reduce the noise and at the same time prolong the life of the steel. Traditionally, linings are made of porcelain or stoneware bricks set in a cement mortar, using high alumina cement and coarse silica sand. Common cement can be used if necessary but may cause pinhole problems in glazes. The bricks should be dense and vitreous. A porcelain body for lining bricks and pebbles (fire to 1250°C or higher) is:

One type of brick is made concave to fit the curve of the drum and another type is made for the end walls of the drum.

Linings can be made from granite, quartzite or similar hard rocks (not limestone or marble). They are cut to shape and set in a high alumina cement mortar. They last far longer than porcelain bricks. Stoneware bricks can be used for the end walls, which are worn out more slowly.

Instead of a hard lining, thick rubber sheet glued to the inside makes a very long-lasting and quiet lining.

Pebbles

Pebbles or balls can be made from vitreous clay bodies. However, it is often cheaper to collect stones of granite, quartz or quartzite along riverbeds. Flint, a variety of quartz, is excellent for pebbles. The hardness is tested with a penknife to make sure it is above 5.5 (see Mohs' scale). Pebbles of limestone are not satisfactory, as they contaminate the glaze. The shape should not be flat or elongated but spherical. (Cylinders of equal diameter and length are sometimes used to obtain particles with less variation in particle size.) Size should be between 2.5 and 5 cm in diameter.

Pebbles wear out, so occasionally take out all the pebbles for inspection. Those that are broken or flat should be discarded. In large mills, pebbles are removed when they are less than 2-3 cm. In small mills pebbles smaller than 1.5-2 cm are discarded.

Figure 8.2.2.E. A cross section of a ball mill running at speeds from 30-90% of critical speed. At 30% the grinding takes place mainly between pebbles and lining, at 70% a good cascading rolling produces efficient grinding, and at 90% very little grinding takes place.

As the pebbles grind down, they contribute a small amount of material to the glaze. Usually this is not enough to make a difference in the glaze. However, if you have glaze problems that cannot be traced to any other cause, the ball mill pebbles should be checked.

Ball mill speed

Grinding of material takes place between the pebbles of the ball mill as they roll down the slope of the cylinder. If the speed is too high, the grinding action stops because centrifugal force stops the pebbles from falling.

This happens when the cylinder is running at its critical speed. Critical speed is calculated from the inside diameter of the cylinder:

(r = internal radius in meters)

The actual speed of the ball mill should be 60 -80 % of critical speed. Small ball mills can be closer to 80 % and large ones closer to 60 % Appropriate speed can be read from Fig. 8.2.2.F.

The most efficient grinding is achieved when the pebbles roll as shown in the center ball mill of Fig. 8.2.2.E. The pebbles cascade in a steady stream, and grinding takes place between the pebbles. The speed of the ball mill at 80% is too high. The pebbles have started to fall freely and this causes excessive wear as the pebbles hit one another and the lining.

Unfortunately it is not possible to look inside during milling, but if the pebbles make a low, rumbling sound the speed is correct. If they make a loud banging noise, the speed is too high or there is too much water, charge or pebbles in the mill. Porcelain jar mills crack if they run at too high a speed.

Figure 8.2.2.F. Graph of ball mill speeds.

Charge

With a speed of 60 -80% of critical speed the charge should be:
(by volume)

When the mill is filled to maximum capacity, the speed should be closer to 60% of critical speed. The water content should be enough to produce a thin slip. After filling, about 30 % of the volume should remain empty. If you measure all the materials separately, total volume may seem to be 85% of ball mill capacity. However, since the water and material fill the spaces between the balls, this will still result in 30% empty space.

Example

A ball mill with new lining measures inside:

width 0.64 m, diameter 0.445 m

60%-80%
of critical speed = 31.7 rpm -42.3 rpm

Charge:
(by volume)

A typical glaze has a density (specific gravity) of approximately 2.7. That means that the glaze charge should be 24-30 kg.

Ball milling time

The time for ball milling varies with the hardness of materials. Soft materials such as frits may require only 2-3 hours, whereas hard materials like quartz can take 24 hours or more.

When you ball-mill standard materials, it is important to mill each batch for the same amount of time. For this reason, it is a wise investment to purchase a timer switch for the mill. This will avoid human errors. Too much ball milling can cause glaze crawling.

Operating procedure

Before each operation:

1) Check that the ball mill is clean inside.
2) Check that pebbles fill half of the ball mill -refill if necessary.
3) Fill in materials (20-25% of mill volume).
4) Fill water until pebbles and material are just covered.
5) Be very careful about correct ball milling time. If possible, use an automatic timer.

After operation:

6) After emptying the ball mill, clean it thoroughly with water by filling it and running it with the pebbles. If the same material is to be ground, cleaning is not needed.

Every month:

7) Empty the pebbles out and remove all pebbles that are too flat or less than 2 cm in diameter.
8) Inspect the inside lining for signs of wear, and repair as necessary.

8.3. Weighing, mixing, using batch cards

Weighing glaze ingredients

First, you must have an accurate scale. This can be a small balance, such as is used by jewelers, or a triple beam balance, which is faster to use. Spring scales are not accurate enough, nor are postal scales. For large quantities, the most accurate low-cost balance is the common beam balance which uses standard weights.

Batch cards

For best results, a batch card system should be used. These are simply cards that have the glaze recipe written on them. As each ingredient is weighed, it is checked off on the list. When all materials are weighed, the batch card is given a number (usually the date). The same number is written on the glaze container. This makes it easier to find out the problem when the glaze does not work correctly.

Figure 8.3.0.B. Example of a glaze batch card used for quality control.

Water

The ingredients are then added to a container which already has the approximate amount of water in it. CAUTION: The water must always be clean. After mixing, the water is adjusted. It is always best to start with less water than required. If the glaze is too fluid, it is difficult to remove excess water.

Containers

Containers for glaze should be plastic or wooden. Metal containers cause contamination and rapid settling of glazes.

8.4. Sieving

Glazes are normally sieved through a 100mesh screen. The glaze should be poured through without forcing it. Never use your hand to force glaze through a sieve, as this will quickly break down the wire mesh. A brush should be used instead.

8.5. Suspending and binding agents

Because glazes are mixtures and not solutions, they tend to settle at the bottom of the container. Normally, the clay content of the glaze will be sufficient to keep them in suspension during application. However, some glazes tend to settle as a cement-like layer on the bottom and are difficult to stir. These glazes require the addition of a suspending agent.

Suspending agent

The most common suspending agent is bentonite, in 1-2% additions. This will normally not be enough to affect the glaze when fired. Dry bentonite cannot be added to wet glaze, as it will just form lumps and be impossible to mix in thoroughly. Instead it should either be mixed separately with water into a thin slip and then added to the glaze or it should be added to the dry glaze and mixed in well before adding water.

Binder

Another common problem is that some glazes tend to be powdery, and come off when loading the kiln. For this problem a binder is added.
Bentonite also works as a binder and is the simplest to use. Another common binder is CMC gum (carboxymethyl cellulose), which is available in either liquid or powder form. The liquid can be used directly, about 1%. The powder needs to be dissolved in water (1:10) overnight and then is added to the glaze as liquid.

Organic binders such as gum arable, wheat flour, sugar or starch (0.1-0.5% of dry glaze) are sometimes used. These have the disadvantage of fermenting. They should be used immediately after mixing, or if stored a few drops of chlorine bleach or formaldehyde can be added as a preservative.

Addition of 1% raw borax produces a hard surface that does not powder when painted on.
Flocculation

Addition of a flocculation agent will make the glaze more creamy. The pottery will absorb the water more easily so glaze is picked up faster.

This works better in combination with clay or bentonite. Common flocculants are: Epsom salts (magnesium sulfate), calcium chloride, calcium nitrate and borax. They are prepared by adding 100 g flocculant to 200 ml hot water and the solution is added to the glaze one tablespoonful at a time (up to 1% of dry glaze weight). Plaster of parts (already set) can also be used.

Flocculation is also used for nonporous ware often in combination with a binder. The creamy glaze forms a thick loose layer that stays on the nonporous surface.

Deflocculation

When the glaze is deflocculated it becomes more fluid with the same amount of water. This is sometimes used for glazing nonporous ware that cannot absorb water. Sodium silicate and soda ash are the most common deflocculants and they are prepared in the same way as flocculants.

CAUTION: Binders, deflocculants or flocculants should only be added after the glaze is ball-milled.

8.6. Density specific gravity

Most potters judge the consistency of their glaze by experience and feel, or by test application to a few pieces of biscuit to see if the thickness is correct. The standard test is to check thickness with a fingernail, which is a very accurate test for an experienced glazer. Then adjust the water as necessary.

A more accurate method is to measure the specific gravity of the glaze with a hydrometer, such as is commonly used to judge the amount of water that has been mixed with milk. When reading the depth the hydrometer sinks, take care that it is really showing the correct density. If the glaze is thick you have to vibrate the bucket repeatedly to make sure the hydrometer sinks in.

Figure 8.6.0.B. Hydrometer made from a glass test tube.

Specific gravity (s.g.) is a measure of the density of a liquid compared to water, which has a standard specific gravity of 1. Glazes will always be heavier than water. The specific gravity is found by weighing a specific volume, say 1000 ml (milliliters). If this weighs 1500 g the s.g. is 1.5. Weighing is more accurate than using a hydrometer.

After you find out the correct amount of water by trial and error, the specific gravity can be measured and future batches of the same glaze made to the same specific gravity.

CAUTION: This is not always a reliable method because the water absorption of your biscuit will vary with its firing temperature. The water will still need to be adjusted by trial and error. Trial application and testing with a fingernail still constitute the most reliable method.

8.7. Old glazes, problems

If you keep wet glazes around for a long time, they will usually have problems with settling or drying up. These glazes can still be used but it will be necessary to adjust the water and to resieve them. If the glaze is extremely thick, it is sometimes best to dry it out completely, crush it and remix it.

Before using a glaze that has set in the bucket for a few days, it should always be sieved through 60 or 100 mesh.

Too much water in the glaze is also a problem. The glaze can be allowed to settle and excess water carefully taken off the top. CAUTION: With soluble glazes, this can remove some of the ingredients and result in a glaze that no longer works correctly. In this case, the water should be allowed to evaporate until the thickness is correct.

Glazes made with raw borax, or incomplete borax frits, will often grow crystals. These cannot be sieved. The glaze should be dried out, the crystals crushed and remixed.

Some glazes will develop mold and begin to smell. Although they can still be used, it is probably better to just throw them out.

8.8. Test your glazes!

The wise potter will never glaze a kilnload with untested glaze. Enough glaze should be kept on hand, so that each new batch can be test-fired in the regular glaze firing before it is used for application.

8.9. Commercial production of glazes

Glazes that are sold commercially are usually in dry powder form. They are made as standard glazes by ball milling, then are dried and packaged.

These glazes are simply mixed with the correct amount of water and sieved before using.

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 9. Glaze application (introduction...) 9.1. Work place, cleaning area 9.2. Application methods 9.3. Density, binders, glaze thickness 9.4. Waxing 9.5. Single-fire glazing 9.6. Handling, drying before firing 9.7. Salt glazing

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

9. Glaze application

Glaze application is a skill that takes some time to learn. In order to get consistent results, it needs to be done carefully and the same way every time. Thin and thick application will give different results, and careless application is always ruinous.

Glazing should be done just before loading the kiln, as glazed pieces that lie around gather dust and get damaged. Some glazes tend to crawl if fired right after glazing. If you have such problems, allow the glazed ware time to dry completely before firing.

9.1. Work place, cleaning area

Before glazing, you should have a neat and clean area to work in. Dust thoroughly and remove small children. The biscuit to be glazed should be organized in one place, with all like items grouped together (cups, bowls, vases etc.). Ware boards are cleaned and arranged, ready to take the glazed ware to the kiln. The glaze should be sieved and checked just before starting the application. Clean water and sponges should be available.

Large items are usually glazed first, as they require a full bucket for even application.

Correct application depends on many different factors:

- Density of the glaze
- Viscosity of the glaze
- Particle size (depending on grinding time)
- Expertise of the worker
- Porosity of the biscuit
- Thickness of the piece
- Dipping time.

Although some of these factors can be controlled accurately in large industries, the small producer will have to depend on experience. Mistakes will be made at first, and it is important to be able to understand what went wrong, so it can be corrected.

9.2. Application methods

The particular method of applying glaze depends on the type of ware -small, big, sculpture, tiles, open forms, closed forms etc.

Generally the inside of an object is glazed before the outside, to prevent handling defects.

Loading systems need to be considered carefully. Most pots are loaded on shelves directly, so the feet must be left unglazed. If foot rings are to be glazed, then each piece must be individually set on special kiln furniture in the kiln.

9.2.1. PAINTING

Glaze is sometimes applied with a brush. This is not recommended because it takes a great deal of skill to obtain an even coat, as well as a lot of time. Painting is used on sculptural objects that cannot be dipped or sprayed. Three to four coats are brushed on, letting each coat dry before applying the next. In order to see each coat, sometimes organic color dye (food coloring) is added.

9.2.2. DIPPING AND POURING

Dipping and pouring constitute the most common method.

Figure 9.2.2.D. Three steps of glazing the inside and outside of a cup in one dip.

The glaze needs to be stirred frequently during application time.

Cups and bowls

Cups can be glazed inside and out in one movement (after some practice). Hold the cup by the foot and dip it at a slant to let glaze inside, while the outside is also coated with glaze. Then quickly pull up and push down. This results in a "fountain" of glaze that covers the entire inside.

Figure 9.2.2.H. Dipping tiles in glaze.

Tiles

To dip tiles, hold them by the edges and dip them in the glaze while moving sideways. This also requires practice!

Double dipping

Applying a second coat of the same or a different glaze over the first is known as double dipping. This often happens inadvertently. When glazing the inside, sometimes there will be runs of glaze on the outside. These should be sponged clean before doing the outside. Larger items are often partly dipped to cover the top, then turned over and dipped again to coat the bottom. This usually results in a line of double glaze, which will look different. If the overlapping area is chosen carefully, it can become a part of the design. Otherwise, it will look like a mistake.

For decorative effects, a pot is sometimes dipped partly in one glaze and then again in a different glaze. This results in a third color where the two overlap.

Waterfall glazing

In the commercial glazing of tiles, the "waterfall" system is used. This consists of a conveyor belt, which carries the tiles under a thin waterfall of glaze that pours over them. The thickness of application is controlled by the speed of the conveyor belt and the amount of glaze flow. Excess glaze runs into a tank, which is again pumped up to the waterfall. These machines are often equipped with automatic cleaners that take excess glaze off the sides of the tiles.

9.2.3. SPRAYING

Spraying is used for items that cannot easily be dipped or poured. It requires an air compressor and a spray gun, as well as a spray booth equipped with an exhaust fan. This is not recommended for the small producer, unless it is required for frequent use or for special decorative effects. Ordinary spray guns for paint can be used, but they wear out quickly because glaze is abrasive. Special spray guns for glaze are equipped with silicon carbide spray heads.

Figure 9.2.2.J. Waterfall glazing of tiles. The tiles run through a curtain of glaze which is continuously recycled with the help of a pump.

Spraying has the disadvantage of wasting a lot of glaze that goes into the air. This is dangerous to inhale, and a spray booth should be provided with an exhaust fan to the outside, as well as having a filter to catch excess glaze. If a great deal of spraying is done, the excess glaze can be collected from the filter and the inside of the booth and reused.

As usual, the inside of the item is glazed first (usually by pouring), and the spraying is done in several even, systematic coats. Each one must be applied before the first one dries, or the glaze may lift off the pot. Each coat should be lightly applied, so that it looks a bit powdery.

It is difficult to judge the correct thickness of glaze and to get it even all over, especially in difficult areas like under handles. In time the glazer will learn to measure the thickness by feeling it with a fingernail.

Airbrush

An airbrush is a very small spray gun that can be adjusted from a pencil-thin spray to a wide pattern. These are not used for glaze application, but are often used for decorative effects-with underglazes and overglazes.

Care of the spray gun

Spray guns are very sensitive. They tend to get clogged, so make sure that your glaze is sieved before putting it in the gun. Clean the spray gun immediately after use by rinsing it out and spraying clean water through it until there is no sign of glaze. Glaze left in the spray gun will corrode it and make it unusable.

Glaze fountain

For glazing the inside of large items a glaze fountain as shown in Fig. 9.2.3.D is helpful. The pot is placed over a nozzle from which an electric pump provides a powerful upward shower of glaze when activated with a switch on the floor.

9.3. Density, binders, glaze thickness

As described above, it is important to have the correct a nouns of water in your glaze. The glaze should always be checked and corrected by test dipping some biscuit before starting and then relying on your experience to judge if the thickness is correct. Checking specific gravity with a hydrometer or by weighing is a good practice but should not be relied on.

It is best not to use binders unless you have no choice. CMC gum is the most satisfactory.

Nonporous biscuit

As previously mentioned, differences in biscuit firing temperature cause differences in porosity and can cause problems in glaze application. Overfired biscuit is especially difficult to glaze, as it will not absorb water. In the making of whiteware, the biscuit temperature is usually higher than the glaze temperature. This results in a semivitrified body that has special glaze application problems. If it is necessary to reglaze pots that have firing defects, they also require special handling.

If you only have a few pieces, they can be heated until almost too hot to handle and then dipped, poured or sprayed (spraying is most satisfactory). The heat will make excess water evaporate.

If glazing vitrified ware is part of your standard production, then it is best to flocculate your glaze. This is the opposite of deflocculation (as used with casting slip) and results in a thick, pudding-like glaze with the normal water content.

9.4. Waxing

In order to keep glaze from being applied to the foot of your pots, it is often more efficient to wax the bottoms as compared to sponging them clean. The coating of wax prevents glaze from sticking. There are two common waxing methods:

Hot wax

Paraffin wax is kept melted in a shallow metal pan over an electric heater or a smoldering charcoal fire (an open fire should not be used as the paraffin may start to burn). It should be hot, but not so hot that it starts to smoke. Before applying the glaze, the foot rings are dipped in the paraffin.

Liquid wax resist

It is much easier to use liquid wax resist, which is a wax emulsion in a water base. It can be thinned with water but after drying cannot be dissolved. This is commercially available in some countries specifically for glaze application. It is also possible to use liquid floor wax.

Liquid wax resist is also used for decoration.

9.5. Single-fire glazing

Single-fire glazing is sometimes called "raw glazing", but this term is confusing as "raw glaze" also is used for unfritted lead or borax glazes. Glaze is applied directly to bone-dry or leather-hard ware and fired once up to the glaze temperature. Not all glazes and bodies are suitable for single firing, and each combination needs to be tested.

Glazes that work on biscuit ware will often also work on bone-dry clay with a small addition of a plastic clay or bentonite. Glazes for leather-hard glazing will need more clay so the glaze layer will shrink along with the clay during drying. The leather-hard method is less practical, since each batch of leather-hard pots must be glazed immediately, causing problems in the work flow.

The advantage of single firing is that it avoids the fuel and extra handling needed for biscuit firing. The main problem with single firing is crawling caused by different shrinkage rates of clay and glaze in the early stages of the firing. Single-fire glazes usually have a high percentage of clay.

Delicate ware cannot usually be single-fired successfully, as it tends to be damaged by the water.

Single-fire glazing needs to be done quickly and carefully, without letting glaze stand inside the pot for a long time. Dipping and pouring can be used, and spraying is also effective.

Firing needs to be done more slowly than usual, so that pots do not explode. The early stages of firing should be done as with biscuit firing.

Single firing is used most often in large tile industries, where it saves fuel.

9.6. Handling, drying before firing

Good glaze application requires careful handling. Many pots are spoiled by fingerprints or glaze that is knocked off during handling. Pots should be allowed to dry before loading in the kiln.

The kiln loader should be responsible for checking each pot as he places it in the kiln. This means inspecting the foot to see if it is clean and rejecting pots with damaged or thick glaze. The loader should constantly clean his hands of glaze dust especially when loading ware with different colored glazes. Otherwise colored fingerprints will mark the pots.

9.7. Salt glazing

In salt glazing, no glaze is actually applied to the pot before firing. The ware is single-fired up to the maturing point of the clay and rock salt is then introduced directly into the firebox. The salt breaks down into sodium and chlorine gas. The sodium combines with silica on the surface of the pot to make a durable glaze and the chlorine goes up the chimney, combining with water in the air to form hydrochloric acid. This is an irritant, as well as causing damage to vegetation and metal structures in the immediate vicinity. Another problem is that the salt erodes the firebricks in the kiln rather fast.

Salt glazing normally is done on stoneware at temperatures above 1100°C. Salt is often mixed with borax to lower the melting point (see also page 19).

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 10. Decoration (introduction...) 10.1. Decoration and design 10.2. Glaze decoration 10.3. Engobe decoration 10.4. Terra sigillata

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

10. Decoration

Decoration is a very big field, which deserves a separate book to cover it in detail. Here we will only discuss some of the main techniques for using glazes and engobes.

10.1. Decoration and design

Good decoration is always related to the design of the pot. It should be used to emphasize and enhance the shape of the pot, rather than being applied randomly.

There are several approaches to decoration;

Banding

Plain or decorative bands of color are painted around the pot, usually by spinning the pot on a banding wheel and applying color with a brush. The bands are placed where they emphasize changes in the curve of the pot, for example, at the rim, the belly, the shoulder.

Area decoration

Decoration is placed inside a defined area, such as a circle. Again this should be done to emphasize the natural curves of the pot.

Overall patterns

These are patterns that are repeated around the pot, often expanding and contracting as the pot does.

Contrasting shapes

These are strongly shaped areas of pattern or color that contrast with the shape of the pot.

10.1.1. MOTIFS, STYLES, LOCAL INPUTS

There are as many motifs and styles of decoration as there are cultures in the world. In traditional cultures, motifs are selected from mythology and familiar designs. Nowadays, with the mixing of cultures around the world, pots are often designed for what can be marketed for export, and design tends to be based on fashion rather than tradition.

The potter selling to tourists will generally choose traditional motifs, since tourists are interested in the culture of the area.

10.2. Glaze decoration

Glaze decoration is done with the glaze itself or with colorants under the glaze or on top of the glaze.

10.2.1. UNDERGLAZE

Underglaze decoration is decoration that is applied under the glaze. It is affected by the transparency and fluidity of the glaze.

Underglazing is usually done under a transparent glaze in order to show it clearly. However, beautiful effects can be obtained under opaque or semiopaque glazes.

A variety of pigments and oxides may be used.

Metallic oxides

The more fusible metallic oxides can be used directly as underglaze pigments, mixed thinly with water. The satisfactory ones are red iron oxide, cobalt carbonate, manganese dioxide and copper carbonate. Designs made with oxides alone will often run with the glaze. Refractory oxides, such as chrome oxide and rutile, can cause crawling.

Oxides mixed with glaze

Metallic oxides can be mixed about 50/50 with glaze, which will prevent the problem of crawling. However, the decoration will usually flow with the glaze and should be designed with this in mind.

Underglaze pigments

These are pigments that are specially prepared by fritting metallic oxides in a base glaze that fires hard but is not fluid. Rather than preparing them yourself, it is usually better to purchase commercial underglazes from a supplier. These are supplied for venous firing temperatures and firing conditions in a wide range of colors. Not all colors can be used under all conditions, and suppliers can usually tell you which are suitable for oxidation and reduction and what type of base glaze will develop the best colors.

10.2.2. ON-GLAZE

On-glaze decoration is applied on top of the unfired glaze. It may be done with a contrasting color of glaze or with metallic oxides or glaze pigments.

Figure 10.2.2.C. Double glazing with the high-surface-tension glaze on top. This draws itself into islands leaving the bottom glaze in irregular patterns.

Application is done by brushing or spraying. Even more than underglaze, on-glaze decoration will tend to flow with the glaze. If distinct patterns are desired, a stiff, viscous glaze will give the best results.

Figure 10.2.2.D. Double glazing with the low-surface-tension glaze on top. This produces a different effect.

Double glazing

Glazes high in surface tension (see page 30) tend to form into small islands on melting. This may cause crawling, but it can also be used as a decorative effect by applying two different glazes on top of each other. The glazes must have different degrees of surface tension. This is achieved by adding clay or talc to one of the glazes. The colors should be contrasting. The best results are obtained with a light-colored glaze at the bottom.

10.2.3. OVERGLAZE

Overglaze decoration is often called "china painting". The pot is glaze-fired as usual and then is decorated with special low-temperature enamels that fire at around 700°C. The enamels are prepared from color pigments and low-temperature frits and are best purchased from commercial suppliers. They are available in every color and have the advantage of firing to true colors, making them suitable for elaborate painting effects. They also stay where they are applied, as there is no chance of the glaze running at this low temperature.

Overglaze is available as powder, which must be mixed with a medium. This is best done by grinding the pigment and medium on a glass plate with a thin palette knife.

Sometimes plain water is used -this works well when filling areas with solid colors. It helps to add some water-soluble glue (white glue) to provide dry strength.

For more elaborate painting, pigment is mixed with special oils. The best is oil of lavender, which is thickened as desired with a thicker oil. The consistency is controlled very much like with oil paint.

Some suppliers have ready-mixed overglaze, which comes in tubes. This can be used directly, without grinding.

Special metallic or iridescent overglazes are known as "luster". These are available commercially as liquid gold, platinum and a variety of mother of pearl colors. They also are fired at 700°C. NOTE: Lusters take on the same surface as the glaze, i.e. a matt glaze will produce a matt luster and a shiny glaze will give a mirror-like effect.

Overglazes are applied by brushing or by spraying.

Overglaze transfers or decals

Most commercially sold decorated dinnerware is decorated with decals (sometimes called transfers), which are made from overglaze that is silkscreen-printed onto decal paper. These are available from suppliers in a range of standard designs and can also be custom-made (in large quantities). They are applied to already glaze-fired ware.

The decal is soaked in water until the design can easily be slid off the paper. The wet paper is placed in the correct location and is carefully slid from under the design, leaving the design adhered to the pot. The design is carefully smoothed, dried and fired like standard overglaze.

10.2.4. REGLAZING, MULTIPLE GLAZING

Reglazing means applying glaze and firing an article that already has been fired once. It is sometimes necessary when glazes do not work correctly the first time -they may be too thin, underfired or not the right color.

Multiple glazing is the process of glazing and firing an article two or more times in order to get special glaze effects that cannot be achieved in one firing. It often involves first glaze firing the pot at a high temperature and then glaze firing with lower temperature glazes, in order to get special colors or textures. For example, a pot may be fired to cone 10, then be refired with cone 06 glazes to get bright colors. It may be fired several times at cone 010 for overglazes and lusters.

Reglazing or multiple glazing makes an article more expensive, but it can also be sold at a much higher price.

Reglazing hints

Because already fired ware is no longer porous, it is difficult to apply enough glaze. It helps to first heat the article (as hot as you can hold in your hand), and spraying is the most effective way to apply more glaze.

For multiple glazing, glaze can be specially prepared by adding cellulose gum (CMC). This thickens the glaze and gives it better handling strength.

10.3. Engobe decoration

Engobe is a specialized type of clay slip that is used for decoration under the glaze. The engobe shows through a transparent or semitransparent glaze and can have the range of color that is possible in glaze.

10.3.1. ADVANTAGE/DISADVANTAGE COMPARED TO GLAZE DECORATION

Engobes stay where they are applied and do not run with the glaze. This makes it possible to do designs with sharp edges or a lot of detail.

Engobes are often used on dark clay bodies in order to provide a bright, white background for glazes.

10.3.2. ENGOBE MAKING, ADJUSTING TO BODY

Engobe is generally prepared as a white base and then colored with appropriate coloring oxides. If you are already using a white clay body, this becomes an engobe simply by thinning it with water. A dark body will require a white engobe formula that fits it correctly.

The main problem with engobe is getting a good fit between engobe and clay body. It must have about the same amount of shrinkage as the body or it will tend to flake off or crack. The engobe should also mature at the same temperature as the clay body in order to provide a strong clay-glaze interface. Engobes can be applied at three different stages:

Leather-hard

This is the best stage for applying engobes, as it permits the widest range of decorating techniques (brushing, incising, inlaying, stencil etc. -see below). The engobe must have enough clay in it to shrink at the same rate as the body.

Bone-dry

Engobes for bone-dry application need to have less shrinkage, so that they adhere to the body.

Biscuit

Engobes for biscuit application are more like underfired glazes.

Some typical engobe recipes are (from. D. Rhodes: Clay and Glazes for the Potter):

Usually engobes designed for plastic clay will not fit on bone dry or biscuit clay and vice versa.

Engobe composition

Engobe is made up of a mixture of plastic and nonplastic ingredients. Recipes for engobes look like those for glazes with a high percentage of refractory ingredients.

For white engobes, the plastic ingredients are china clay and ball clay. The amount of ball clay can be adjusted to get correct shrinkage.

The nonplastic ingredients are feldspar and quartz, and for low-temperature engobes frit is sometimes added to lower the vitrification point.

Small amounts of borax are often added to give better dry strength and to fuse the other ingredients together.

Engobes are often deflocculated like a casting slip, and in fact you can often use a casting slip which fires at the same temperature as your clay body.

10.3.3. COLOR OXIDE ADDITIONS TO ENGOBE

Coloring oxides are added to engobes as a percentage, as with glazes. However, since the color is diluted by the glaze over it, larger amounts are required. You should also remember that the oxide reactions will depend on the type of glaze being applied and on whether oxidation or reduction firing is used. Typical colors and oxide amounts are:

Red iron oxide

Copper oxide or carbonate

Cobalt oxide or carbonate

Chrome oxide

Manganese dioxide

Nickel oxide

Titanium dioxide or rutile

Commercial glaze stains

As with glaze colorants, the most interesting colors are usually obtained by mixing combinations of oxides.

10.3.4. APPLICATION METHODS

A wide variety of decoration techniques can be used with engobe. Leather-hard ware permits the largest variety of techniques and usually has fewer technical problems compared to application on bone-dry or biscuit ware. Work flow for leather-hard engobe decoration is:

- Apply the engobe.
- Biscuit-fire.
- Apply the glaze.
- Glaze-fire.

Dipping, pouring

This is done the same as with glazes. The engobe should be just thick enough to completely cover the clay. Too thick application will often crack, especially on rims. If applied leather-hard, pouring and dipping should be done quickly, so that the pot does not get too soft from absorbing water.

Brushing

Brushing is one of the most satisfactory methods, especially for making bands or areas of engobe. The technique requires some skill in order to get an even coating. The brush should be fully loaded with engobe and should spread it evenly.

Spraying

The engobe must be thin enough to flow through the spray gun. It should be applied in several even coats, taking care to keep a smooth surface and to cover all areas equally.

Scratching or "sgraffito"

To get fine lines, engobe is applied to an area and, after it sets, it is scratched with a sharp tool. This is called "sgraffito", which means "scratching". The clay color shows as a line.

Inlay

Lines are scratched on the leather-hard pot and then filled with engobe. The excess engobe is removed with a metal scraper after it sets, or with sandpaper after the pot is bone-dry. This results in a smooth surface, with the engobe lines contrasting with the clay colon

Stencil

Paper or plastic stencils are placed on the leather-hard pot, and engobe is brushed or sprayed over them. Afterwards the stencil is removed leaving the design of the stencil.

Trailing

Usually called "slip" trailing, the engobe is applied by allowing it to flow from a device with a small opening, which produces raised line decoration. It is easiest to use a rubber bulb (such as an ear syringe available in pharmacies) or plastic containers used for soap or cosmetics. The opening can be made smaller by inserting small metal tubes.

Engobe hints

Engobes will show most clearly under a fully transparent glaze. However, semitransparent or even opaque glazes can give beautiful effects, clouding the engobe colors.

Sophisticated decorators can take advantage of different glazes over the engobe to produce different colors. Complicated effects can result from applying different glazes to different areas of the decorated piece.

10.3.5. ENGOBE PROBLEMS

Often engobe will come off the pot. This almost always is caused by a different shrinkage rate of clay body and engobe and usually happens before firing. In many cases the engobe is applied too thick.

Engobe shrinks more than clay body

In this case, the engobe will develop cracks and will flake off, with the flakes curling away from the ware. The solution is to reduce the amount of plastic clay or substitute raw clay with calcined clay. Deflocculating usually helps.

Engobe shrinks less than clay body

In this case, the engobe will flake off, especially on rims and sharp edges, and the flakes will be flat. The solution is to add more plastic clay or to substitute calcined clay with raw clay.

Flaking after firing

This is caused by differences in firing shrinkage between clay and engobe. Usually adding flux to the engobe will help.

Spit-outs

Application of engobe to biscuit ware sometimes causes the engobe to lift off in small bubbles. This may only show up after glaze firing, but it arises during application. If the biscuit ware is very porous, it absorbs the water in the engobe so fast that air inside the body comes under pressure. When the air is released, it may blow out the engobe layer where the air escapes. The solution is to reduce the absorption by dipping the biscuit in water some time before engobe application.

10.4. Terra sigillata

The technique of coating pottery with terra sigillata was used by Roman and Greek potters and is still used by traditional potters in India and Nepal. It produces a thin, opaque and low gloss finish to pottery.

10.4.1. PREPARING TERRA SIGILLATA

Terra sigillata is made from clay. For temperatures below 1100°C local sedimentary clays are more suitable. The finer the clay particles the better. Such clays normally contain iron and fire to a red colon It is more difficult to produce white-fring terra sigillata from ball clay or kaolin.

Sodium metaphosphate + 0.5%

The best result is obtained when ball milling the clay. Some clay can be prepared without ball milling. After ball milling the batch is transferred to a container and left for 24 hours. The coarse particles will settle and the upper 2/3 of the batch is siphoned off. A bucket with a tap placed 1/3 up is useful for regular production of terra sigillata.

Colors can be made by adding color oxides or pigments. First the terra sigillata is dried and the color oxide is added in amounts similar to what is mentioned for engobes (by dry weight). Then water is added and the batch is again ball-milled for 4 hours. It is then ready for use.

10.4.2. APPLICATION

The terra sigillata should be adjusted to a density of 1.15 to 1.20 for application on leather-hard clay. For dry and biscuit ware more water is added to obtain a density of 1.05 to 1.10. The ware should be clean and dust-free before application.

Application can be done by dipping, brushing and spraying. After drying the gloss can be improved by polishing the surface with a cloth.

10.4.3. ADVANTAGES

The use of terra sigillata makes it possible to produce attractive decorations on low-fired pottery without using glazes. The coating gives a dense, glossy and impervious surface. A very beautiful glossy black can be produced by placing the terra sigillata items in a ridded pot filled with sawdust. This is fired in a normal firing either in a kiln or in a traditional pottery firing. The strong reduction will change the normal red color to black.

The use of terra sigillata coatings as an intermediate layer between body and glaze is reported to reduce crazing and bubbles in the glaze.

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 11. Glaze problems (introduction...) 11.1. Introduction to glaze problems 11.2. Trouble-shooting checklist 11.3. Specific problem explanations

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

11. Glaze problems

Anybody with even a little experience with glazes will realize that problems often arise. Knowing what to do about them requires a lot of experience, and even expert glazers often find it difficult to establish the source of a problem.

11.1. Introduction to glaze problems

It is one thing to develop a nice glaze but quite another to keep it working. One potter may want a glaze that crazes, whereas another wants his glaze to be craze-free. A glaze fault may not mean that the glaze is ugly, just that it reacts differently and does not look like the desired effect.

When a glaze suddenly starts to react differently from what we want, we call it a glaze fault. Solving the problem is seldom easy, and usually several factors are involved. The first thing to check is what changes have occurred since the glaze last worked without problems. The following things should be checked:

1. Was the right recipe used and were the materials weighed out correctly? (Always use batch cards for glaze weighing).

2. Were the right raw materials used? Was there any chance of mistaking one material for another? Was the labeling of materials in stock correct?

3. Have new raw materials or a new frit batch been used since the last fault-free glaze batch was produced? If so, check if the new material is different from the original material in stock.

4. Has the body been changed in any way? For example: new preparation method, change of clay material, change of body recipe, higher or lower biscuit firing?

5. Was the preparation of the glaze done as usual? For example: same ball milling time, same screening, same specific gravity of glaze slip?

6. Was there any change in glaze application and were the products clean and dust-free before application?

7. Were there changes in the kiln setting? Was the glazed ware dry before firing started? Were there any changes in fuel, firing schedule, firing atmosphere (reducing/oxidizing) and was the correct top temperature reached (setting of cones, draw trials) ?

Once we know which conditions have changed, we may already be close to establishing what caused the glaze problem. The following trouble-shooting lists may be helpful to find solutions to the problem.

11.2. Trouble-shooting checklist

Problem: The glaze or part of the glaze settles too fast in the bucket.

Causes:

- High amount of frit in the glaze.
- Glaze materials too coarse.
- Water content of glaze slip too high.
- Ball milling time was too short.
- Too little clay in the slip.
- Metal buckets cause fast settling.

Solutions:

- Add 5-10% plastic clay or 0.5-2% bentonite.
- Reduce amount of frit by removing some of the insoluble materials from the frit recipe and adding these to the glaze batch instead.
- Longer ball milling of glaze materials.
- Add a small amount of vinegar (acetic acid) to the glaze.
- Use plastic buckets or wooden containers.

Problem: Glaze slip is too thin, low viscosity.

Causes:

- Water content too high.
- Alkali materials from frit or feldspar have been dissolved in the water, so the slip is deflocculated.

Solutions:

- Let the glaze stand for a day and decant the clear water off the top. Because some materials may be removed with the water, it is better to allow excess water to evaporate.
- Add flocculant (magnesium sulfate, calcium chloride), but only if the ware has too low a porosity. This is often done in production methods which use high-fired biscuit and lower temperature glaze.

Problem: Glaze layer too thin after dipping.

Causes:

- Water content of glaze slip too high.
- Clay body absorbs too little water.

Solutions:

- Increase density of slip (decrease water).
- Biscuit-fire at a lower temperature.
- Glaze only one side of the article at a time and allow it to dry before glazing the other side.
- Add flocculant to the slip so that glaze layer becomes thicker.

Problem: Glaze layer becomes too thick.

Causes:

- Glaze slip density is too high (too little water).
- The glaze does not contain enough clay materials.
- The biscuit body absorbs the water too fast.
- The glaze slip releases its water too fast.
- Dipping or pouring is done too slowly.

Solutions:

- Reduce density by adding water.
- Add plastic clay, bentonite or cellulose (CMC) binder.
- Biscuit-fire to a higher temperature or moisten the pots before glazing.
- Dip.

Problem: Glaze layer cracks during drying.

Causes:

- Glaze has too high a drying shrinkage due to high content of clay or zinc oxide or due to over-grinding in the ball mill.
- The glaze is applied too thickly.
- Single-fire glaze was applied to biscuit ware.
- In double glazing, the second glaze may tend to crack.
- Glaze is poured over a sprayed glaze.

Solutions:

- Less ball mill grinding of glaze materials.
- Calcine zinc oxide or add it to the frit.
- Replace part of the clay with calcined clay or introduce alumina (Al2O3) as feldspar.
- Reduce viscosity of glaze by adding water.
- Apply single-fire glaze to leather-hard pots.
- When double glazing, reduce clay content of the second glaze.
- When double glazing, apply the second glaze before the first one dries completely.

Problem: Glaze layer does not stick to the body.

Causes:

- Glaze adhesion to body is too low due to:

greasy or dusty body surface
too fast and too thick application
too high body porosity
too fine grinding of glaze
dusty surface from underglaze colors.

- In single-fire glazing, the glaze shrinks less than the moist body.

Solutions:

- Clean the body surface by brushing or sponging.
- Reduce speed of dipping and, if spraying, apply less glaze at a time.
- Biscuit-fire at a higher temperature.
- Add cellulose binder (CMC) when single-fire glazing.
- Add clay, borax, soda ash or glaze to the underglaze colors.
- Reduce grinding time of glaze.

Problem: Glaze dusts off easily after drying.

Causes:

- Too little binding power of the glaze and low adhesion to body.

Solutions:

- Add 2-5% plastic clay or 0.5-1% bentonite.
- Add a binder like cellulose (CMC) or 12% borax.

Problem: Glaze runs.

Causes:

- Firing temperature too high.
- Viscosity of glaze too low.
- Glaze layer too thick.

Solutions:

- Adjust firing temperature.
- Add alumina (clay, feldspar) or silica (quartz, zircon) to the glaze.
- Glaze thinner.

Problem: Glaze does not melt properly.

Causes:

- Firing temperature too low.
- Too much silica or alumina.
- Not enough glass formers (SiO₂ or B₂O₃) and to much CaO, MgO, BaO.
- Glaze materials too coarse.
- Evaporation of fluxes in the firing (extended firing time).

Solutions:

- Fire at higher temperature.
- Slower firing and longer soaking at the end.
- Increase amount of fluxes, reduce content of alumina.
- Longer milling of glaze materials.
- Add the evaporating fluxes to the frit.

Problem: Pinholes or eggshell surface.

This is one of the most common glaze problems and often the most difficult to cure. It is usually caused by gas escaping from the body or glaze, leaving small holes that do not have time to smooth over.

Causes:

- Firing temperature too low, glaze does not have time to melt completely.
- Firing temperature too high, glaze reacts with body, forming gas bubbles.
- Glaze has high surface tension and viscosity, which do not allow gas bubbles to escape.
- Glaze is too thick, not allowing gases to escape.
- Release of gases from body or engobe.
- Early reduction firing forms carbon and sulfates in the body, which cause pinholes when they are later released.
- Body contains organic particles which burn out, leaving small pits.
- Body contains air bubbles from incorrect slip casting.
- Glaze contains zirconium opacifier, which causes large amounts of gas.
- Contamination from ball milling, usually by linings set in lime cement.
- Contaminated water used to mix glazes.
- Dirty biscuit or dust on the glaze.

Solutions:

- Fire to the correct temperature.
- After final glaze temperature is reached, "soak" the kiln by holding at the same temperature for about half an hour.
- Reduce viscosity and surface tension by changing the glaze recipe.
- Less reduction, especially in early stages of firing.
- Thinner glaze application. Higher biscuit firing.
- Body must be prepared to eliminate organic materials (sometimes long aging to decompose the material and repugging will solve the problem).
- Slip must be mixed and poured carefully to eliminate air bubbles.
- Increasing viscosity of zirconium-opacified glazes by addition of clay or talc may reduce the problem.
- Ball mill linings should be fastened with high-alumina cement mortar.
- Use only clean water to mix glazes.
- Biscuit should not be stored too long and should always be cleaned before glazing. Glazed ware should be put in the kiln as soon as possible.

Problem: Glaze crawls.

Normally this is caused by cracking or lifting of the glaze layer before firing (see section 11.2.2. for additional causes and solutions).

Causes:

- The viscosity and surface tension of the melted glaze are too high.
- Too fast firing.
- During firing, the body released gases (steam, carbon, sulfur), which lifted the glaze layer off.
- Drying and sintering shrinkage high.
- Glazed ware was still wet when fired.

Solutions:

- Reduce surface tension and viscosity by firing higher.
- Reduce alumina, magnesia and zircon.
- Increase biscuit temperature.
- Correct what causes cracking and lifting of dry glaze (see above).
- Reduce grinding time of glaze.
- Reduce drying and sintering shrinkage by calcining part of clay and zinc oxide content.
- Dry the glazed ware before firing.
- Add 1-2% borax to the glaze.
- Add clay, borax or frit to underglaze colors.

Problem: Glossy glaze turns matt.

Causes:

- Glaze is absorbed by the body (glaze layer is too thin).
- Flux materials evaporate in firing.
- Sulfates from fuel are deposited on the glaze surface.
- Too much steam in the kiln. Glaze is underfired
- Crystals form due to very slow cooling.
- Glaze slip was not properly mixed.

Solutions:

- Glaze thicker.
- Glazed ware set next to biscuit ware or on new shelves, which may attract volatile fluxes from the glaze. Do not mix glaze and biscuit firing.
- Introduce volatile fluxes in the frit.
- Fire with more draft and less reduction and cool quickly.
- Always stir glaze immediately before application and screen it through at least 60 mesh.

Problem: Matt glaze turns glossy.

Causes:

- Glaze is overfired.
- Too fast cooling.
- Oxides from the body combine with the glaze.

Solutions:

- Slower firing, or longer soaking period at the end of the firing.
- Cool slowly, closing all dampers and fire-boxes.
- Use another matting agent.

Problem: Glaze color changes.

Causes:

- Wrong firing temperature, often over-firing.
- Change in reduction/oxidation atmosphere.
- Impurities in glaze or body.
- Coloring oxides not ground fine enough.
- Color pigment from engobe or underglaze melts into the glaze.

Solutions:

- Better control of firing temperature, oxidation/reduction.
- Check materials for impurities.
- Control the grinding time of coloring oxides.
- Add clay or quartz to underglaze pigments and engobes.

These are problems that only appear right after firing or after the article has been used for some time.

Problem: Crazing of the glaze

Causes:

- Thermal expansion of the glaze is higher than the body, which causes the glaze to contract more in cooling. This may be caused by:
- Too much alkali (soda and potash).
- Too little silica, alumina or zinc oxide.
- Too little cristobalite formation in the body.
- Lack of strong clay/glaze interface.
- Glaze is too thick.
- Fast cooling.

Solutions:

- Apply glaze more thinly.
- Higher glaze firing temperature (to increase cristobalite).
- Higher biscuit firing temperature.
- Reduce amount of soda and potash in glaze by replacing with boron (to decrease thermal expansion of glaze).
- Add additional boron, silica, zinc oxide or calcium carbonate to the glaze (to decrease thermal expansion of glaze).
- Add quartz or talc to the body. Quartz forms cristobalite in the body, which has a high thermal expansion.
- Longer soaking at top temperature, slower cooling.

Causes:

- Moisture expansion (sometimes called delayed crazing). Porous bodies expand when they absorb moisture from the air and force the glaze to craze. This is a common problem with earthenware.

Solutions:

- Fire at higher temperature or add flux to the body, making the body more vitreous.
- Add calcium carbonate, talc or dolomite to the body.
- Reduce the thermal expansion of the glaze.

Problem: Shivering of glaze

This is usually seen as particles of glaze falling off the pot after firing (sometimes after a few weeks). It happens most often on sharp edges, but the entire glaze may shiver or the pot may crack.

Causes:

- Thermal expansion of the glaze is less than the body, which leaves the glaze under strong compression. This may be caused by:
- Too high content of silica, boron or zinc oxide in the glaze.
- Too high content of silica in the body.
- Too low content of soda or potash in the glaze.

Solutions:

- Reduce the amount of quartz in the glaze.
- Reduce the amount of quartz in the body.
- Add more soda or potash to the glaze.

Problem. Lime popping

This is seen as small pieces of glaze popping off the pot, often several weeks or even months after firing. Under each flake of glaze a small white particle can be seen imbedded in the body.

Causes:

- Small pieces of limestone or plaster in the clay body. These slowly absorb moisture from the air and expand, forcing the glaze off the pot.

Solutions:

- Find the source of the lime pieces.
- Replace contaminated material or screen it through 40 mesh.
- Plaster that has gotten into the clay. All contaminated clay must be thrown out, and better care taken in clay mixing. The problem often comes from recycled clay, which has picked up plaster in the forming section.

11.3. Specific problem explanations

As already mentioned, crazing and shivering are caused by differences in thermal expansion/contraction between the glaze and body.

Crazing

Crazing appears as cracks in the glaze. This occurs during cooling, if the glaze contracts more than the clay. Cures for crazing are mentioned above.

Shivering

Shivering is the opposite of crazing and occurs during cooling if the clay contracts more than the glaze. Cures for crazing are mentioned above.

Thermal expansion

All materials expand when heated. This is called thermal expansion. Some materials expand more than others, and the degree of expansion can be measured. Numbers are used as a scale of thermal expansion, and this is called the coefficient of expansion (CE). The glaze layer on a pot has one coefficient of expansion and the body has another.

Glaze-body tensions

After a pot is fired and taken out of the kiln, it will be exposed to a sudden decrease in temperature. The glaze layer and the body will contract, but most often at different rates. Below is shown what happens when 1) glaze contracts more than body and 2) body contracts more than glaze.

This figure shows a body (white) with a glaze on top (black). The glaze and the body have contracted at the same rate. We say: they have the same coefficient of expansion (CE).

Figure

This figure shows a glaze that has a higher coefficient of expansion (CE) than the body.

Figure

The glaze contracted more, so it is shorter and therefore the glaze is under a tensile stress (it is pulled apart). If the body is very thin it will bend as shown. The arrows show the direction of the stress the glaze is under.

Figure

More often the tensile stress is relieved by cracks in the glaze as shown in this figure. This is called crazing. The stress caused by high CE of the glaze may be relieved by crazing as soon as the pot is taken out of the kiln or it may take days, months or years. The longer it takes, the closer is the CE of body and glaze.

Figure

This figure shows a body with higher CE than the glaze. The body contracted more than the glaze. The glaze is under compression, and if the clay is thin it may bend as shown to relieve the pressure. If body contraction is only slightly greater than glaze contraction, nothing will happen.

Figure

If a glaze contracts much less than the body, the compression on the glaze becomes too much and the glaze will start to flake off like this (shivering). This may not happen by itself, but only if something hits the pot. Typically, the rim of a cup will easily chip off.

Figure

High compression of the glaze may also be relieved by cracking of the body.

Moisture swelling

When the body has been exposed to humidity for a long period, water enters the body, which expands slightly (moisture swelling). This expansion causes the glaze to go into tension and it will craze. This kind of crazing is called delayed crazing.

Solutions

As we saw above, crazing and shivering are caused by different rates of contraction and expansion (different CE's). The problems are cured by adjusting the CE of body and glaze, so that the two contract and expand more closely. It is best if the glaze is left under slight compression.

Coefficients of expansion for various materials

Ceramic materials have different coefficients of expansion (CE). The following list shows the relative values for the most common:

Adjusting CE of glaze

From this list we can see that if we replace soda (Na₂O) with boron (B₂O₃) in a glaze we will lower the CE of the whole glaze. This can be done without changing the melting point of the glaze. Addition of silica will lower the glaze's CE but will also raise its melting point.

If shivering occurs, it means the CE of the glaze is too low. Adjusting it means adding soda (Na₂O) and reducing boron (B₂O₃).

Adjusting body

Adjustment of body CE is not done according to the CE of the materials listed above. The contraction rate of the body depends to a much higher degree on the sudden reversible contraction of silica crystals when these change their crystal structure (cristobalite).

Quartz change

Quartz is a crystal form of silica. Quartz is created in the body during firing when the clay crystal changes form and releases some of its silica. When quartz is heated it changes its crystal structure at 573°C. This happens very suddenly and is accompanied by a 1% expansion. On cooling to below 573°C it contracts again.

Figure Figure 11.3.1.B The volume change of quartz is caused by a rearrangement of the bond between the atoms. At 573°C the angle suddenly shifts as shown.

Cristobalite

Cristobalite is another crystal form of silica. It changes its size around 220°C and the volume change is nearly 3%. Cristobalite is created at temperatures above 900°C from silica released from the clay (Al₂O₃ 2SiO₂) or talc (3MgO · 4SiO₂) or from quartz.

Figure 11.3.1.C. The graph shows volume changes of three forms of silica. The two crystal forms change dramatically but silica in glass hardly changes.

Body-glaze contraction

The two graphs below show how the body and its glaze contract during cooling. The graph in Figure 11.3.1.D shows a body that does not contain any cristobalite. At 573°C the body contracts suddenly due to the contraction of quartz, but at this temperature the glaze is still fluid enough to follow the contraction of the body.

Figure 11.3.1.D. Contraction of body and glaze during cooling. Glaze contracts more so it will craze.

Around 500°C the earthenware glaze hardens and from then onwards contracts according to its own CE. In this example the glaze has a higher CE than the body; it contracts more. This leaves the glaze under tensile stress; the glaze is smaller than the body. This will cause the glaze to craze.

Figure 11.3.1.E. The body in this graph contains crisotobalite and shows a sudden contraction at 220°C. This causes an overall higher contraction of body compared to glaze.

The graph in Figure 11.3.1.E shows contraction of a body containing cristobalite. As above, the glaze first follows the quartz contraction, then hardens and starts to contract more than the body. However, at 220°C the cristobalite change causes the body to contract, and at this temperature the glaze is hard so it is left under compression. This compression will prevent the glaze from crazing.

Moisture crazing

After firing, the porous earthenware body will absorb moisture and this causes the body to expand. If the glaze is not under sufficient compression it will craze. Such delayed crazing may occur a long time after firing. The moisture expansion of the body is reduced by making the body more vitreous. Additions of talc or limestone to the body reduce moisture crazing.

Crazing cure

For both types of crazing the cure is:

- Add quartz (or silica), talc or limestone to the body.
- Biscuit-fire to a higher temperature.
- Glaze-fire to a higher temperature.
- Add silica to the glaze.
- In the glaze, replace fluxes with high thermal expansion, like soda (Na₂O) and potash (K₂O), with boron oxide (B₂O₃).

It may seem strange that the cure for crazing is to add silica to both body and glaze. The reason is that adding silica to glaze makes it contract less, but silica added to the body causes the body to contract more.

Crazing test
There are several ways to test how the expansion of glaze and body fit each other. The most simple ones are:

- Rings of clay with a diameter of 5 to 10 cm are made with a small gap and biscuit-The gap is measured, the ring is glazed on its outer surface and refired. After firing the gap is measured to see if the ring has contracted or expanded. If the gap has become greater the glaze will craze.
- Glazed samples are exposed to thermal shocks by repeated heating and cooling.

The thermal shocks can be from boiling water into ice water. The number of cycles the sample can withstand before crazing indicates its craze resistance.

Another method is to heat the sample at first to 100°C then cool it in 20°C water. This is repeated while raising the temperature in steps of 10 or 20 degrees. The higher the heating temperature the sample withstands without crazing the longer it will be able to stay craze-free under normal conditions.

A rough guide is:

Even if a sample survives the thermal shock test it may still craze due to moisture swelling. This can be tested in an autoclave which is simply a pressure cooker that can withstand higher pressures. A pressure cooker can be used instead. The glaze sample is placed in the pressure cooker with some water. It is kept under pressure for a period and then checked for crazing. The time it can withstand pressure without crazing indicates the time it may stay craze-free under normal circumstances. The following rough guide is for testing in an autoclave under a pressure of 3 atmospheres (about 3 bars). If using a pressure with, say, a pressure of 1.5 atmospheres the testing time in the table should be doubled:

All the tests provide only a rough indication of craze resistance. When you do the test you will develop your own procedure' which then should always be followed faithfully. In this way you will be able to compare your crazing test with your previous results.

11.3.2. CRAWLING

Crawling appears as areas of clay that are not covered by the glaze. It may be small areas or, in extreme cases, the glaze may pull up into a pattern of small balls or islands, leaving bare clay in between.

Crawling is caused by:

Poor adhesion of glaze

Dusty or oily biscuit prevents the glaze from sticking to the body. Refractory oxides (chrome, rutile) or underglazes that act as a dust layer prevent the formation of an interface. Adding clay, borax or frit to the underglaze colorants helps.

High surface tension

High surface tension of the glaze in melting pulls it into islands before the clay/glaze interface forms. This is caused by certain oxides, especially magnesia, clay and zinc oxide. The solution is to replace magnesia by other materials, to calcine part of the clay or to use calcined zinc oxide instead of raw.

Cracking of glaze layer

Extensive shrinkage of glaze in drying and early stages of firing, usually caused by too much clay content or by overgrinding the glaze, causes the glaze to crack and separate from the body. A thick glaze layer is more likely to crack.

11.3.3. PINHOLING AND BLISTERING

Pinholes appear as tiny holes in the glaze surface. Blisters look like frozen bubbles or craters. They are a problem in utilitarian ware, as they collect dirt. They may be only on the surface of the glaze or may penetrate to the clay layer.

Figure 11.3.3.A. Pinholing.

Pinholes

During firing gas bubbles are formed in the melted glaze. The bubbles will move to the surface of the fluid glaze and be released.

If you watch any glaze metling, you can actually see this process. Some glazes (especially those containing raw borax) foam and boil until they finally smooth out. When the firing is stopped before the glaze has had time to heal over, a pinhole or crater is left (see Fig. 11.3.3.A). Since overfiring also causes pinholes it is better to keep the maximum temperature for some time (soaking period).

The main sources of the gas are:

- After glazing, a large volume of air exists in the space between the solid glaze materials. The air gathers into bubbles during sintering and melting.
- Release of sulfates and carbon in the body and from some of the glaze materials.
- Air bubbles in the body introduced by improper handling of the casting slip.
- Sulfates and carbon from the fuel may deposit in the body during the initial stages of firing. Above 900°C the gas will be released.

It is important to find out if the problem is in the glaze or in the body. Relatively large pinholes that go all the way to the body are usually caused by small holes in the body that do not accept the glaze-this is most common with slip-cast ware, or with common red clay that contains particles of organic matter, sand or mica.

Problems arise if the glaze starts to cool and solidify while bubbles or craters are still forming.

Detailed causes and solutions are given in section 11.2.3.

11.3.4. COLOR CHANGES

Potters are often plagued by changes in glaze color, either within the same kiln-load or from separate firings.

Often this problem can be traced to glaze preparation. The colors may not be ground finely enough, weighing may be incorrect, raw materials may have changed.

Otherwise, the problem usually is due to more or less reduction than usual. This is one of the most difficult conditions to control in firing and depends completely on the skill of the firemaster.

The problem is worst in glazes that contain color oxides that are sensitive to reduction. The most sensitive is copper, which is green in oxidation and red in reduction; and iron oxide, which is yellow, red to brown in oxidation and mottled red-brown, grey to blue or green in reduction. Other oxides change less.

Heavy reduction will darken the iron in the body, which will affect the glaze, also darkening it. Sometimes a pot will be dark on the reduced side and light on the oxidized side.

Other causes and solutions are given above.

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 12. Developing glazes (introduction...) 12.1. Modifying existing glazes 12.2. Basic equipment 12.3. Testing methods 12.4. Developing a base glaze 12.5. Modifying a base glaze 12.6. Colored glaze

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

12. Developing glazes

Most potters do not know much about glaze chemistry and are usually afraid to develop their own glazes. It is true that glaze chemistry is difficult to understand without a background in chemistry. Still, a simple knowledge of fluxes, stabilizers and glass formers and how they combine to make glaze is useful. It will give even the nontechnical potter an idea of how to approach glaze problems and develop new colors and textures. The reader with more technical background can make use of the Seger formula for a more sophisticated approach to glazes.

Remember that glaze chemistry is something new in the history of ceramics. Before it existed, potters found out how to make glazes by trial and error, without modern methods of analysis or even accurate methods of weighing.

It is also true that once glaze recipes were developed they were closely guarded secrets. A special glaze that no one else could duplicate gave an advantage over the competition. The modern systematic, scientific approach to glaze development has largely made "secret" glazes obsolete because the action of the various glaze ingredients is now well known.

On the other hand, glaze making is very much like cooking. The same recipe may produce very different results when prepared by two different cooks! Standard glaze recipes as published in books often do not work because of differences in raw materials, firing technique etc. Just as a good cook should know how to substitute materials, a good glaze maker can develop an intuitive knowledge of glazes. Like most things, this only comes from hard-won experience.
Potters should not be afraid to experiment with glaze development, although most of them simply do not have the time to take away from production. The following chapters describe standard approaches to glaze development and will be useful to those who want to experiment with glazes.

12.1. Modifying existing glazes

The easiest approach to working with glazes is to start by modifying existing glazes. These may be new recipes from books, or recipes that you are already using. Although you can take a trial-and-error approach, this takes a lot of time and money, and results are largely a matter of luck. It is best to use some of the systematic methods presented below and knowing even a little about the nature of the various glaze materials will help you reach your goal more efficiently.

There are three approaches to modifying glazes:

- You have a goal for a particular surface, color, texture etc.
- You have a glaze problem that needs to be corrected.
- You have no special goal but just want to see what happens when you change the recipe.

12.2. Basic equipment

The minimum equipment required for glaze testing is:

- Accurate balance, either a triple beam balance or a goldsmith's balance with weight. Spring-type scales are not accurate enough for weighing glazes.
- A mortar and pestle for grinding materials. These should be porcelain, so that they do not contaminate tests.
- A 100-mesh sieve. This can be bought ready-made, or you can make your own using brass or stainless steel screen.
- Firing can be done in your regular glaze firing.

12.3. Testing methods

The type of test piece you use is largely a matter of personal preference. In order to show glaze behavior under various circumstances, it should have:

- A horizontal and vertical surface.
- A textured area.
- A hole to tie it. This helps to keep similar tests together for future reference.
- An area for labeling. It is best to write the full recipe of the test on each test tile (with a brush and iron oxide and water, or chrome oxide or engobe etc.) since code numbers often get confused and notebooks get lost.

The first step in understanding your materials is to fire all available materials in small bowls at your standard glaze firing temperature.

A small quantity of each material (ground and sieved through 100 mesh) is placed dry in the bowl. This will show you which ones melt alone, and which ones remain as powder. Most of the materials will not melt but may change in color or may react with the clay. Only the strong fluxes will melt by themselves -other materials that do not melt may be fluxes, but only in combination with other materials.

12.3.2. LINE BLENDING

Line blending is a systematic way of finding out the reactions of two different materials (or mixtures of materials).

The easiest way is to prepare the two materials by grinding, sieving and mixing them with water in two separate containers. To make the line blend, the materials are mixed by volume (using a small spoon) and applied on a test tile, starting with one material alone and adding the other material in equal steps. Since the tests are measured by volume it is important that the same amount of water is added to the two line blend materials.

Glaze half of the test tile twice to show variation in glaze thickness.

Line blend 10 steps

An example of a line blend in 10 steps, which gives the full range of combinations of 2 materials, is shown in the table above.

The most common use of the line blend is to find out the effect of one material in a standard glaze recipe. If material A is the standard glaze recipe, material B could be the standard glaze + a coloring oxide addition of 5-10%.

Line blend 5 steps

Usually, 5 steps will be enough for the first test. In this example material A is the basic glaze, material B is the basic glaze + 10% copper oxide.

Mixing procedure

- First prepare a line blend mixing card like the one above.
- Prepare the two mixtures as usual and add the same amount of water to each.
- Place the two materials in bowls in front of you, glaze A to your left and glaze B to your right.
- In the middle place an empty bowl into which you pour the spoonfuls from the two other bowls according to the number for each test on your line blend card. Keep track of the spoon counting by marking the line blend card.
- Stir the test mixture well.
- Mark a test tile with the mixture's test number (date + serial number).
- Glaze the test tile.
- Discard the remaining glaze and continue with the other line blend mixtures.

Calculation example

In the case above it was easy to calculate the copper oxide addition in each of the tests. When more complex mixtures are used in a line blend the calculation becomes more complicated. Here is an example of mixing two glazes:

After firing, test number D turned out to be the most interesting. We now want to test a larger amount of this and the recipe is calculated in this way (see table next page):

Materials in glaze A were multiplied by 3 and those in glaze B by 2.

The sums of each material were then divided by 5 and the final recipe is:

The recipe is based on a line blend test measured by spoonfuls. That is not very accurate so, before going any further, the test result should be retested by weighing the dry materials.

12.3.3. TRIAXIAL BLENDING

Triaxial blending is a method of testing varying amounts of three different materials or colors.

Figure 12.3.3.B. A triaxial blending chart system with 10 steps. Composition of a test at an intersection is found by following the lines to the periphery of the triangle.

Each corner of the triangle represents 100% of the material. Each side of the triangle is the line blend of the materials at its ends, and the intersections inside the triangle represent combinations of all three materials. So the result is three line blends, plus all the combinations. Fig. 12.3.3.B is an example of a biaxial system with 66 tests.

The system is better explained by an example. You may have a basic opaque glaze and you want to see how it responds to 3 different coloring oxides: cobalt oxide, copper oxide and iron oxide. In this case we use a simple biaxial blend with only 21 tests as shown in Fig. 12.3.3.C.

Figure 12.3.3.C. triaxial system with 5 steps. The number at each point refers to the test number on the triaxial blending card.

The procedure is:

- Prepare a biaxial blending card as shown.
- Prepare 3 mixtures of basic glaze with oxide additions:

A glaze + 5% cobalt oxide
B glaze + 10% iron oxide
C glaze + 10% copper oxide

- Add same amount of water, screen 100 mesh.
- Place 3 bowls with the mixtures in front of you: B on the left, C on the right, A in the center. Right in front of you place an empty bowl.
- Have all test tiles numbered and arranged in sequence near by.
- Collect teaspoonfuls of each mixture; A,B,C according to the numbers on the biaxial card. Mark each time you have finished collecting from each bowl.
- The mixture is collected in the empty bowl.
- Stir the mixture, pick the test tile with the right biaxial blend number.
- Glaze the test tile.

Figure 12.3.3.D. Arrangement of bowls for triaxial mixing.

Getting the right number of spoonfuls into the collection bowl for each test takes a lot of concentration. A mixing card as shown below helps you to keep track of your progress with the spoon counting.

If you want to know the recipe of one of the tests, say number 14, you calculate this in the same way as for line blends:

Once you get used to working with biaxial blends, you will be able to read the percentage directly from the triangular chart.

This biaxial blend was based on only 21 variations. Out of these only 6 were blendings of all three mixtures; the rest were simply line blends involving only two mixtures. A larger biaxial blend system would produce more intermixing of all three materials, but also a lot of extra work. However, you could use a system with 10 steps on each side as shown in Fig. 12.3.3.B, but leaving out the line blends A-B, A-C and B-C, and only blend the 36 tests in the center of the triangle.

12.3.4. KEEPING RECORDS

The key to experimenting with glazes is keeping accurate records and labeling them in such a way that the actual tests can be compared with your notebook.

As mentioned above, it is best to write the entire recipe on the test tile itself, along with the date of testing. This is possible with a simple test like adding coloring oxides to your basic glaze. For more complicated tests and line blend or biaxial blend tests, you will have to rely on the test number on the tile. Mark the date, the number and, if you do more than one test a day, add a serial number. In your notebook, the date and recipe are also written.

Make it a habit to take notes of the fired results immediately after unloading the kiln. Write down firing conditions, location of test tile in the kiln, and your impression of the glaze. Is it well melted, running, pinholes or tendency to crawl? Use a whole sheet of paper for each test or test series. Finally write down your conclusion like "make I kg test batch", "test again with 5% increase of clay". You could make a standard record form like the one in Fig. 12.3.4.B.

Testing is costly and the records help you to avoid unnecessary tests. When planning your next test, first take a look at your earlier results and compare them with your notes. When deciding which materials to add or which to decrease you may check with the oxide list below (page 122). As long as you work with one particular problem or line of research keep the test tiles close by for easy reference. Once you have finished the research you can store all the tests together by hanging them on a string in chronological order.

Figure 12.3.4.B Example of glaze test record form.

12.4. Developing a base glaze

The first step in developing a new glaze is to develop a base glaze, which is simply the combination of materials that melts at the desired temperature (without addition of colorants). Here, we describe an approach to making base glazes without knowing anything about the chemical composition of the materials.

Since all glazes require flux, stabilizer and glass former, these three materials are the starting point. There are a large number of fluxes available (divided into primary and secondary' fluxes), but the stabilizer is usually china clay (kaolin) and the glass former is usually quartz (silica). The main differences are between low temperature and high temperature glazes. Below only the main materials (not chemicals) are mentioned. This list is a rough guide only.

Low temperature (900-1100°C)

Low temperature glazes require more flux and stronger flux than high temperature ones.

High temperature (1100-1300°C)

In the appendix there is a selection of glazes that can be used as a starting point for developing new glazes.

12.4.1. SELECTION OF MATERIALS

Materials necessarily have to be selected from what is available in your area, as most potters do not have access to suppliers with everything on hand.

When selecting materials to use in glazes, a general rule is to use materials that supply more than one oxide. For example, if magnesia (MgO) and silica (SiO₂) are both required, it is better to use talc (3MgO 4SiO₂) than magnesium carbonate and quartz. This is because the elements are already combined and contribute to a better glaze melt.

The biggest trouble with glazes is not to develop a nice new glaze but to keep it nice. Most materials vary from batch to batch and some materials may not be in regular supply. Therefore try to base your basic glaze on materials that you can rely on. Chemical stores often have ceramic oxides, but in a chemically pure form that is always very expensive. Instead look for the natural mineral containing the same oxide.

12.4.2. USING GENERAL RECIPES

There are hundreds of books on ceramics, most of which have recipes for glazes. These are limited in their usefulness, as often the raw materials are not available or are different from what you have in your country. Most of the time, these recipes do not work as expected and require modification. Without knowing the chemical analysis of materials, it is still possible to develop good glazes, using standard ones as a starting point and then modifying them systematically using the methods below.