Understanding Clay Body Formulation, Part 2
August 4, 2009
This is the second in a three-part series. The first installment is available by following the link below.
Moist Clay Performance
A moist clay must be all things to everyone and still be distinctive, with a specific character that the potter can recognize. After all, why buy one clay over another if they all are the same? The clay must produce good handling and firing qualities for a wide range of potters with varying degrees of skill, which is often a difficult balance to achieve. Clay bodies could be formulated that would be “cutting edge” in performance, but such extremes would not make it profitable to produce since only a few skilled potters could enjoy the results.
Clay bodies are formulated for throwing, hand-building, jiggering, Ram pressing or slip casting. Each forming method requires a different ratio of plastic materials to non-plastic materials composed of flux and fillers. Various combinations of clay types, such as fireclay, stoneware clay, earthenware, kaolin, ball clay or bentonite, as well as combinations of two or more clay groups, determine the plastic or non-plastic characteristics of the clay body.
Throwing bodies require the most plasticity, which is derived primarily from ball clay and bentonite and other clays. Hand building, jiggering and Ram press bodies have greater percentages of non-plastic clays, such as fireclay, kaolins and stoneware clays, as well as higher percentages of non-plastic materials like flint, feldspar and pyrophyllite.
It is not unusual to use a throwing body for hand-building in some-but not all-ceramic projects. However, the larger or thicker the hand-built piece, the more specialized the clay body requirement. Casting slips are a world unto themselves and are much more sensitive to the exact ratios of water, clay, filler, flux and deflocculant.
As a general rule, no clay can do everything. That’s why there are many different moist clays offered by ceramics suppliers. Adequately formulated clay bodies should have a firing range of three pyrometric cones. For example, if a clay is rated as c/9, it should function at cone 8, 9, and 10 without deformation or excess shrinkage. Be on guard if a moist clay has a wide range, as it might not be dense and vitreous over its entire range.
While high-temperature stoneware clays can be used for low-temperature pit firing or Raku fast firing due to their open porous nature, low-temperature clays can deform and melt if fired to high temperatures. Most ceramics suppliers offer a range of clays at c/06-04, c/6 and c/9.
Whether the moist clay is fired in an oxidation atmosphere (excess air-to-fuel ratio in combustion), neutral atmosphere (equal amounts of air and fuel) or reduction atmosphere (excess fuel-to-air ratios), the clay body formula has to be compatible to the firing kiln’s atmosphere. Generally, clay bodies designed for reduction firing kiln atmospheres can also be fired in neutral or oxidation, due to possibly high amounts of iron oxide or iron-bearing clays in the clay body not being over fluxed by the neutral or oxidation atmosphere. However, depending on the specific formula, the clay bodies might not look the same. Conversely, clay bodies designed for oxidation atmospheres can possibly be over-fluxed due to their high iron content. Ceramics suppliers should indicate the atmosphere recommendations for each clay body.
Clay bodies can also be developed for wood, salt, soda or Raku firing methods. The wood firing kiln presents several elements to the fired clay not found in other types of atmospheres. Stoking the wood kiln can create intermittent oxidation, neutral and reduction kiln atmospheres-all of which can create random flashing on the exposed clay surface. At temperatures above 2300°F, wood ash begins to flux into an alkaline glaze that alters unglazed and prior-glazed clay surfaces. The clay body must accommodate such wide ranges of kiln atmosphere and variations in wood ash deposits.
In salt and soda firing, a sodium vapor atmosphere is introduced into the kiln and reacts with exposed clay surfaces to create a sodium/alumina/silicate glaze. The clay body must be formulated to develop and an “orange peel” or gloss surface. In some instances, random flashing of the surface is desired.
Raku clay bodies must be able to withstand wide ranges in heating and cooling due to the practice of fast heating and cooling in either an oxidation or reduction carbon trap atmospheres. Additionally, the clay body must accommodate a wide range of fast-fired glazes.
Ceramics suppliers try to offer a wide range of clay body colors in different temperature ranges. Earth tones of dark brown, brown, light tan and cream are often the easiest and least expensive clay body colors to produce since they depend to varying degrees on iron-bearing clays.
Porcelain clay body formulas that require either imported kaolin or domestic kaolins that are a few cents more per pound (on average) than ball clays or stoneware clays are more difficult to develop to achieve plasticity, durability and translucency at c/6 (2232°F) or c/9 (2300°F). Black, green or blue clay body formulas have an even higher level of expense, depending on the coloring oxides or stains used in the clay body.
The next installment in this series will discuss some miscellaneous issues, including clay storage and raw material availability.