I am often asked to describe the clay that we use — and I am asked to clarify the difference between various clay types. All of this gets somewhat confusing for the average person because identical terms are often used to describe entirely different things. Marketing — is the all powerful reason that technical ceramic terms are not always used in accordance with their dictionary definitions. Clay Types – Clays are naturally occurring alumina silicate compounds that have a curious capacity which is called plasticity – the ability to be easily molded or formed without cracking. The plate-like particles of clay are separated by lubricating water which together with surface tension creates the quality of plasticity or extensibility.
Clays may be kiln fired to vitrification — fused to a glassy matrix — where they become usefully hard and are essentially transformed into stone. Once fired there is no going back — short of the eons long natural weather process that eventually erodes everything. OK — so what kind of clays are found in the natural environment.
Earthenware Clay — Red clay flower pots and chimney bricks – are made virtually everywhere and get their distinctive color and lower firing range primarily from various forms of iron oxide which is a flux and colorant at kiln temperatures. With a high level of iron, earthenware begins to vitrify at approximately 1000 degree centigrade. Items made from this clay are relatively inexpensive due to the availability of the material and the lower firing temperature.
Generally speaking — wares made from earthenware are not as a group as vitreous or durable as other higher temperature clay products. However — with proper body formulation earthenwares can be made both vitreous and strong and can be adjusted to fire over a range of temperatures.
Fire Clay — This clay type – though somewhat less common — is frequently used to make brick for fireplace linings or as a component in refractory cement. Fireclay and related Stoneware clays are generally lower in iron content and as a result vitrify or mature at a higher temperature — beginning at approximately 1200 centigrade.
Kaolin — This clay type is the purest form of naturally occurring clay. The word Kaolin originally derived from the Chinese who have forgotten more than we will ever learn about clay and clay working.
One of the more common Kaolins is called Edgar Plastic Kaolin or EPK which is mined in Florida and used extensively in the pharmaceutical industry — Kaopectate, etc — as well as the applied clay products industry.
Virtually all of the major bath fixture manufacturers use clay bodies based on Kaolins together with another related clay called Ball Clay. Kaolin based bodies — like Porcelain — can be made to vitrify at different temperatures – with the range beginning around 1250 centigrade.
China Clays — Now this is where it gets interesting. You have no doubt heard of China or maybe even China Clay — as in fine China plates etc. China is a term coined by Europeans who were unable to copy the Porcelain produced in China because they did not have the naturally occurring plastic Kaolin — or China Clay — deposits the Chinese were fortunate to have.
Additionally — Europeans did not understand how the Chinese got their kilns so hot. In the spirit of friendly competition the Chinese thought it would be fun to see how long it took everyone else to figure this out. Actually they probably hoped that no one would. With a bit of persistence and a lot of idle tinkering — the western mind found they could mix something they had a lot of – bone ash (yes – from animal bones) to get something that sort of resembled Chinese Porcelain.
Eureka — they called it China or Bone China and began to convince people it was something really new and special that everyone should want and have. So you will begin to see — this is what happens to technical ceramic terms — they get hyperbolized by the marketers. Vitreous China — Bone China — Sanitary Ware — Iron Stone are all marketing terms that have gradually found themselves in common use and this has created a certain amount of confusing regarding the meaning of the terms.
What is Pottery?
Pottery is simply a common term used to describe a functional item made from clay. It is frequently used to refer to items that are not very durable — such as red clay flower pots that are never intended to be vitreous or durable. It can also be widely applied to items that are very high quality and equally durable.
I could be described as a Potter making Pottery Sinks or alternatively called a Ceramic Artisan or even Artist if your mind — unlike mine — finds that description attractive.