(Not necessarily in order of importance)
- Fully vitrified with an absorption rate of <3% (mine is 1.25%-2.0%)
- No significant glaze flaws (crazing, shivering, crawling, pinholing)
- Stable glazes (no leaching)
- No hazardous materials (lead, cadmium, antimony, etc)
- Good form and construction
I once had someone insist that "all clay contains lead" and "handmade pottery can never be used for food because the clay itself contains lead". Umm, well, this lady was ultimately WRONG!!! In fact, the clay itself contains no lead whatsoever.
A "pure clay" consists of silica, alumina, and water. However, most potters do not work with pure clay (it requires an impractically high firing temperature and has a consistency that is more difficult to form), but instead utilize clay bodies that have had additional materials added to improve the workability of the clay for a particular application. Different firing methods and different finished intent possess different clay body requirements. The potter also usually has a preference for certain properties such as color or plasticity.
A typical naturally occurring red clay consists of 57.02% silica, 19.15% alumina, 6.7% iron oxide, 3.08% magnesium oxide, 4.26% calcium, 2.38% sodium, 2.03% potassium, 3.45% water, and .91% titanium oxide (Clay and Glazes for the Potter, by Daniel Rhodes). See? No lead!
Many potters today, including myself at times, work with a clay body categorized as a stoneware clay body. A typical stoneware clay body may consist of 20-80% of a naturally occurring clay such as the one listed above along with 10-40% additional naturally occurring clays including a more pure clay (called kaolin or china clay). There may also be 10-20% feldspar added to act as a flux and lower the vitrification point of the clay body. Again, you will see, there is no lead involved!
I mentioned that particular properties affect the clay body choice. What are some of these properties? Well, in the case of stoneware, one of the properties is obvious by its name...desire for it to be stone-like, a hard, preferably impenetrable surface after firing to maturity. Also, it is important that the clay body be able to receive finishing techniques such as glazes without rejection or unacceptable flaws. Although not directly relevant to the consumer, the potter also needs to consider how well the clay body withstands the desired forming techniques and what firing methods are available. And, yes, convenience of obtaining the clay is also a consideration.
Ok, now what were my preferences that led me to choose the clay body that I work with? Well, first of all, let me tell you that I actually have two different stoneware clay bodies and one raku clay body that I prefer. For now, I will stick with discussing the stoneware ones.
I opt to utilize a commercial clay body rather than digging up my own or even mixing a custom mix. I'm not so adventurous as to spend tremendous amounts of time and energy digging up my own clay and then going through the extensive process of preparing and testing it. Also, there are enough commercial clay bodies designed by others with more knowledge than I have, that it is not necessary for me to design my own. I did however do a lot of research to find the clay bodies that I felt would best serve my purposes.
Intended results and available firing method were primary considerations. When I first began designing my own artwork in clay, I worked strictly in raku. It was exciting, but due to its porous nature did not meet all my considerations. I wanted to create vases and such that were water-tight and of a harder, more durable nature. My only available kiln was an electric kiln. Although I could go into even more detail, suffice it to say that working in the electric kiln left me with two options: earthenware (which, like raku, remains somewhat porous) or mid-fire stoneware (commonly called Cone 6 electric stoneware). With the proper clay body fired in the proper manner, I could achieve stoneware with the hard, durable, stone-like vitrification (non-porous, glasslike qualities) in my electric kiln.