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Raw material variability is a fact of life for many production potters. While high quality, consistent materials can be obtained from suppliers that cater to pottery makers, such materials are not widely available in all areas. In some cases, high-quality materials cost more than their lower-grade counterparts, making them difficult to justify for artists and small pottery producers. Fireclay, for instance, which is frequently used by potters, is a refractory, coarse-particle-size clay that can contain carbonaceous materials such as lignite, peat and coal, as well as other tramp materials. It can also contain silica, iron and manganese particles, and the latter two metallic oxides can deform and disrupt the surfaces of fired glazes, appearing as brown or black blemishes. All of these “bad” qualities can be refined out of the fireclay, but for the vast majority of fireclay users—steel mills, casting foundries and brick manufacturers—the clay works well in their products in its unrefined form. Since potters represent less than 1⁄10% of the raw material market in the U.S., it does not make economic sense for mines or raw material processors to refine a product for such a small market share.
The same principle also applies to many other materials used in making and decorating pottery. In essence, “You get what you pay for.” It therefore often falls to the potter to understand the potential irregularities in their raw materials, know how to recognize them, and know how to compensate for them in body and glaze formulations.
Particle SizeClays can range in particle size from 100 microns to 0.1 micron, so an important aspect of any clay or raw material is its particle size distribution—what percentages of the particles are fine, medium or coarse. At some point in the mining or processing stages, the particle size distribution of a clay or raw material can “shift” or change radically. While such changes are infrequent, the results when they do occur can considerably alter the finished ware. For example, if a ball clay is used in a clay body formula, and a shipment of ball clay comes through with a higher percentage of “fine” particles, excessive shrinkage or cracking can occur in the drying or firing stages of the ware.
Particle size can also be crucial in determining how a glaze applies to the ware and how it eventually fires on the ware. For example, whiting, a calcium carbonate, acts as a secondary flux in high temperature glazes. Since whiting is a generic name for a raw material that is produced in several different mesh sizes, and all of the types look deceptively like white powders, using a different type of whiting or obtaining it from a different supplier can alter the fired glaze. A smaller particle size raw material can cause increased melting in a glaze due to the greater surface area of the material coming into contact with the heat work of the kiln, while a higher percentage of coarse particles can cause the glaze to settle or sink in the glaze bucket. Clear, transparent glazes can also appear semi-opaque or white due to high percentages of large particle material that have not gone into a complete melt.
Keep in mind that all clays and raw materials look like white, off-white or colored powders, and it is impossible to visually determine particle size with the naked eye. When ordering raw materials from a ceramic supplier, always specify the exact mesh size to ensure a consistent result. Particle size can also be tested regularly and charted on a graph to provide a “fingerprint” of an individual material or clay. This type of testing is often carried out at the mine. Potters who do not have the resources to test their clays for particle size can usually obtain this information from a typical data sheet provided by the mine or their ceramic supplier.
Chemical CompositionThe chemical composition of a ceramic raw material or clay influences the material’s handling characteristics, forming qualities, fired texture and color. Higher than normal amounts of silica (fine grained quartz) found in clay can cause quartz inversion cracking in clay bodies if they are heated or cooled too fast in the 1063F region. In clay bodies fired over cone 8 (2280F), any quartz present in the clay body starts to change to cristobalite. If enough cristobalite develops in the clay body, it can go through a cristobalite inversion and cause cracking if the body is cooled too quickly in the 392F region.1 All of these events can be magnified if the chemical composition of the clay has changed from one clay bag to the next.
Factors that can radically change a glaze are associated with the loss on ignition (weight loss after burning off a certain component) of raw materials. Chemical water plays the largest component, with carbon and sulfates contributing to lesser percentages of loss on ignition. Excessive shrinkage of high mechanical or chemical water concentrations found in raw materials such as Gerstley borate, borax and colemanite can cause the raw glaze to fly off the pot and onto kiln shelves during the first part of the kiln firing. Such a violent reaction is more likely to happen if the kiln is heated too quickly from 212 to 1100F.
In slip casting clay bodies, sulfate and soluble salts can greatly influence the amount of deflocculant required to achieve the proper flow rate or viscosity in a liquid casting slip. Over time, excessive sulfate growth in slip casting bodies neutralizes the deflocculant, causing a jelly-like, thick casting slip that cannot be poured into molds. In throwing and handbuilding, a clay body’s soluble salts can interfere with the glaze and also leave a residue of white crystals on unglazed areas of the fired ware.
Unfortunately, the chemical composition of a given clay or raw material can be difficult to determine without sophisticated testing equipment. For most potters, the best way to verify that the chemical composition is correct is to test fire any new batches of raw materials and clays. Since this can be a time-consuming process, potters should order raw materials before they are needed in the production process. Additionally, the new materials should be slowly incorporated into test batches to avoid having an entire kiln load of pottery completely dependent on them.
Crude ColorEach clay blend has a characteristic crude color, such as tan, off white, white, buff, brown, etc. A change in crude color in a new batch of clay can indicate a mistaken shipment of a different brand of clay. However, crude color does not always indicate fired color. For example, crude color comes from carbon in some clays, but some very dark crude color clays actually have a low carbon content, and some of these clays fire out white.
The crude color of other raw materials is often hard to interpret, since most ceramic raw materials are white or shades of off-white. However, as with clays, a sudden change in a raw material color can indicate a mistaken shipment of material. It is always best to test each batch of clay or raw material before committing it to full production.
Any time a raw material has to be re-bagged to another container there is also the possibility of error. For this reason, it is always best to buy full bags (50- or 100-lb bag sizes) of the clays and raw materials that are commonly used in clay body and glaze formulas. While this might seem like it can lead to a large raw material inventory, keep in mind that ten to twelve of the same raw materials are generally found in over 80% of glaze formulas.
Organic LevelThe organic level of clays can fluctuate depending on the specific clay pit or mine from which the clay is obtained. While minor changes in organic materials are not generally severe, an organic content that is much higher than normal cannot be oxidized or burned off in the 662 to 1292F range that is typical of a pottery kiln firing cycle.
If carbonaceous materials are not removed from the clay in the first part of the firing, glass formation in the clay can trap gases, causing bloating or bubbles within the clay body. If the organic materials contain iron, carbon in the clay can react with the lack of oxygen to form CO, which reduces the iron to a strong fluxing agent. The reaction can cause a black core in the clay body. High levels of organic material trapped in the clay body can also cause the glaze to blister as the material turns to gas and is released into the molten glaze.
Unfortunately, a high organic level cannot be easily determined before the clay is fired. Firing test batches of new clay before committing to a full-scale production run can ensure some degree of reliability in the production process.