I have recently taken an Introduction to Clays class, I’m focusing on the formation of clay and looking at specific types of clay. It has been a great experience to go at my own pace and review the material as needed to grasp its content. Being math and chemistry phobic, I have finally grasped some basics of how the elements interact. Knowing this will serve as a valuable tool to be able to achieve a desired effect in working with a clay body. Therefore, I thought I’d write about something I learned and found useful, which just touches on some different types of clay, and is by no means an exhaustive study.
When planning a project, it is important to be aware that some types of clay can be used alone while others, more than likely, will need to be mixed together with other types of clay to some extent to get one’s desired effect. Clays are a naturally occurring composition of elements derived from rocks, they take on different characteristics depending on how the minerals have been acted upon by natural forces, such as wind, rain, freezing, thawing and other weather conditions which help break down the rocks along with other forces like rivers, glaciers and gravity allowing rocks to fall and break apart. This helps clay particles to separate and settle together to make up various deposits.
A particular type of rock, referred to as a Feldspar, is a primary source of silica and
alumina, of which clay is most usually composed. Clay is divided into two groups, primary and secondary. Primary clay refers to clay that has not moved from the location of the
source rock that has decomposed and thus have the least amount of contamination. This is
referred to as kaolin and that is the primary ingredient in porcelain. The theoretically pure
kaolin formula is Al2O3 * 2SiO2 * 2H2O, which is, to the extent of current knowledge, not found in nature; in nature, we are likely to find other trace elements, which would slightly upset the above formula (Rhodes, 2000, p. 28).
Secondary clays are the ones as described as being transported from their source rock
and mixing with other elements. When silica and alumina are combined with other elements, from what is referred to as alkalies, and when all the clay particles are of the optimal ratios, they can be made pliable with the addition of water. They then can be formed into most any shape if manipulated accordingly. Water allows the clay particles to slide along each other like two flat pieces of glass that has a film of water separating them.
Along with the initial groupings of Primary and Secondary exists subgroups of the secondary clay category. These different types can each perform different functions, and, once again, sometimes different clays are mixed together to get a specific response. The following are some different types of clay and a brief description of some of their characteristics.
The term “ball clay” originates from “an early English clay mining practice of rolling the highly plastic clay into balls weighing 30 to 50 pounds” (Hosterman, 1949, p. C8). Ball clay when damp has the ability to be pressed and made into shapes without cracking. It’s a secondary clay, in that it has been moved by wind or water from its source and settled elsewhere in a deposit, it often contains some organic material. According to Hosterman (1949) “quartz sand or silt and iron oxide minerals are virtually absent from the best grades of ball clay” (p. C8). It is “used in clay bodies to increase plasticity and in glazes to add alumina” and “fires to a grayish or buff color” (Speight, 2003, p.G-1). Its other uses in the ceramic industry include the making of pottery, dinnerware, stoneware, tiles and sanitary ware (Hosterman, 1949, p. C8).
Ball Clay is made up of crystalline kaolinite with small amounts of illite and/or smectite (Hosterman, 1949, p. C1). Illite is defined as “a group name for non-expanding, clay-sized, dioctahedral, micaceous minerals” (USGS). Smectite is defined as “a clay mineral which undergoes reversible expansion on absorbing water” (Lexico). Hosterman (1949) states that “ball clays require 40 and 65 percent water of plasticity to become workable” (p. C8). The most notable characteristics of ball clay include “its plasticity, toughness, high green strength and adhesion” (Hosterman, 1949, p. C8). After firing, ball clay becomes vitreous and dense and the temperatures it melts is between 1,670 and 1,760 degrees Celsius (Hosterman, 1949, p. C8). They are not used by themselves in pottery making due to their characteristic of excessive shrinkage (Rhodes, 2000, p. 68). In fact, ball clays have been known to have shrinkage “as high as 20 percent when fired to maturity” (Rhodes, 2000, p. 68). They are often used as an additive to other clays to achieve “increased plasticity and workability” (Rhodes, 2000, p. 68). If the intended outcome for the clay body color is white, then one should not add above 15 percent of ball clay as anything above that amount tends to turn the fired body “a grey, off-white, or buff color” (Rhodes, 2000, p.68). There are sources of ball clay in the United States, mainly in “Tennessee, Kentucky, Mississippi, Texas, California, and Maryland, with the largest and purest deposits in Tennessee and Kentucky” (Powell, 1995, p. 200).
Fire clays are aptly named as their distinguishing characteristic is their ability to resist high temperatures more so than the other clay types. Clays that resist fusion or deformity up to around 1500 degrees Celsius can be referred to as a fire clay (Rhodes, 2000, p. 68). They are very useful in the making of fire brick and other parts used in “kilns, furnaces, boilers, and melting pots” (Rhodes, 2000, p. 68 & 69). It can be added to stoneware bodies to help increase its refractoriness and enhance the roughness of the body texture (Rhodes, p.68 & p.69).
Stoneware is a secondary clay formed when one clay’s particles became mixed with clay particles of varying sizes which helps it hold its form (aka plasticity) because the clay particles have more surface contact with each other which is helpful in shaping an object. Iron, titanium, and potassium are fluxes in stoneware. It is important to note that iron and titanium may cause the clay to turn brown when fired. Stoneware can vary in its’ characteristic, depending on where it was harvested. Its’ fired color can vary from buff to brown depending on the iron and titanium content. It is often used to make coffee mugs, dinner plates, pitchers and many other types of utilitarian wares. Sometimes it can be used just as found without adding any other ingredients to it (Rhodes, 2000, p. 69). Depending on its composition it can be fired to maturity anywhere between cone 1 to cone 10. (Speight,2003, p.185)
Earthenware clay is a porous secondary clay and is also referred to as a Low-Fire clay. It fires to mature temperatures of cone 010 to cone 1 (Speight, 2003, p. 184). According to Speight (2003), “natural low-fired earthenware varies in color from reddish to yellowish, depending on the amount of iron or lime it contains” (p. 184 & 185). Most clay found in nature might be earthenware clay (Rhodes, 2000, p.69). It is commonly used for making sculpture, flower pots and bricks. (Rhodes, 2000 p.69) (Speight, 2003, p.185).
It’s important to learn some basics about the different kinds of clay, to be able to most effectively choose a material that will help us achieve our desired outcome.
Hosterman, J. W. (1949). Ball Clay and Bentonite Deposits of the Central and Eastern Gulf of Mexico Coastal Plain, United States (No. 1558). US Department of the Interior, Geological Survey; Washington, DC.
Lexico Powered by Oxford. (n.d.). Smectite. Retrieved from https://www.lexico.com/en/definition/smectite
Powell, P. S. (1995, January). Ball clay basics. In Ceramic Engineering and Science Proceedings (Vol. 16, No. 3, pp. 200-206).
Rhodes, D. (2000). Clay and glazes for the potter (3rd rev. ed.). London;Iola, Wis;: Krause Publications.
Speight, C. F., & Toki, J. (2003). Hands in clay . McGraw-Hill Humanities/Social Sciences/Languages.
USGS Coastal and Marine Geology Program. (n.d.). Illite Group. Retrieved from https://pubs.usgs.gov/of/2001/of01-041/htmldocs/clays/illite.htm