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:

1	Talc
2	Gypsum
3	Calcite
4	Fluorspar
5	Apatite
6	Orthoclase feldspar
7	Quartz
8	Topaz
9	Corundum (pure alumina)
10	Diamond

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:

Ash	20-70%
Feldspar	20-70%
Whiting	5-20%
Flint	15-25%
Clay	5-20%

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.