Even when you’ve worked in ceramics for a while, can you choose a good clay body for your studio? It takes some thought. Do you want stoneware, earthenware, porcelain? High fire, low fire? What about grog? Paperclay? We looked at some factors to consider when you choose, and at some good sources.
Choose a Good Clay Body: Types of Clay
Stoneware is a plastic clay body with coarser platelets and particles. The finished work is not brittle and can be put in the kitchen oven or dishwasher with no ill effects, if properly fired and vitrified. This type of clay shrinks less that porcelain. Stoneware is a an excellent body for functional pieces or artistic work like sculpture. It can be grogged to make it even sturdier for building large pieces. Stoneware is great for handbuilding, and throwers like it, too. For throwing, non-grogged versions work better. Stoneware is less expensive than porcelain, more forgiving, and easier to keep hydrated.
Porcelain is fine grained. Because it contains a high percentage of kaolin and is formulated to remove impurities like iron or copper, porcelain is usually off-white to very white. Like stoneware, the finished work is durable and non-porous. Traditional formulations of porcelain are known to be less plastic and less stretchy than most stonewares.
Porcelain has a reputation for being less forgiving as you work it, and more difficult to control. You may find it cracks more easily if not handled very carefully, but there are new formulations that reduce this issue. Standard Ceramic clay distributor has a couple of versions they describe as “very plastic.” Certain porcelains can be worked into pieces so thin that light passes through the fired work. Porcelain is not the first choice of many handbuilders.
Earthenware is the go-to low fire group of clay bodies widely available at local resources or through national distribution companies. This is clay that has travelled, via water or wind, from one location to another as it formed. Subjecting the clay to weather and water motion helps refine and formulate it. As the particles move along, they pick up and mix with elements like iron or copper, giving the characteristic range of warm orange, red, and brown colors.
Finished earthenware work is more porous than stoneware or porcelain and so makes a good option for creating work that needs porosity. Think about garden planters, for example. Earthenware is coarse and particulate, though the particles tend to mix into the clay readily as you work it. This gives a nice strength for sculpture or ornate pieces, while providing a surface that can be readily smoothed, polished, or burnished to a velvety finish. Earthenware is the most forgiving clay. You can mend small flaws in any state up to bone dry by wetting the clay and re-compressing parts that need mending.
Choose a Good Clay Body:Temperature and Shrinkage
In general, we talk about three firing ranges for clay.
Low-Fire—Cone 06 to Cone 3 (1850 °F – 2135°F) Southwestern (Sedona) Red Clay, and Earthenware are good examples.
Mid-Range—Cone 4 to Cone 7 (2160 °F – 2290°F) Good examples exist in stoneware, with colors ranging from buff or off-white to deep coffee brown.
High-Fire—Cone 8 to Cone 10 (2315 °F – 2380°F) Most porcelains are high fire, though you can find a wide array of stonewares formulated for these temperatures.
At any of these temperatures, firing the clay changes it’s makeup, reducing some of the components, like water, that make up the clay body, and causing the work to shrink.
For example, organic and carbon materials burn off. That’s carbon, lignite, certain acids, and so forth. Water evaporates, closing down the spaces between platelets . Clay contains water described as mechanical water and chemical water.
Mechanical water is the water you feel and see that lives between clay particles. Some of it dries off before you bisque fire; some dries further as you preheat in the kiln.
If we understand chemical water, we understand ceramics. Let me distill it for you (pun intended) with thanks to DigitalFire for their explanations of clay chemistry.
Chemical water is water bound inside crystal particles like silica, kaolin, calcium, and quartz. These crystals lose their moisture at a wide range of temperatures, and the crystals can contain as much as 45% of their weight in water. This water has to be released before your clay body turns into ceramic work.
If you consider that various clay bodies have varying amounts of different crystals, you will understand why shrinkage rate differs from clay to clay. So when you choose a good clay body, read about or ask about specific shrinkage rate for the clay you’re considering.
If you process your own hand-dug clay, you have to test it by creating a piece of work, measuring the dimensions, firing, and measuring again. Now you can calculate the shrinkage.
Choose a Good Clay Body: The Bottom Line
If you prefer to create thin-walled pieces. or you rely upon a lot of throwing, learning to work porcelain properly will likely give you the most satisfaction. Recognize that the clay, glazes, and other preparations will cost more. Porcelain is beautiful, but not a utilitarian choice.
For home studio potters who like flexibility, earthenware is a good choice. The colors are pleasing. I know potters who fire earthenware in their backyard barbecue grill with awesome results (subscribe and read the story we did on backyard firing).
Earthenware pots can be used in the oven, on the stove top, or even for campfire cooking. Know without doubt that when you exceed the stated firing maximum temperature, earthenware will melt and turn glassy black, rather like obsidian rock. Your pot will be ruined. Your kiln will be damaged. You will release toxic gases around your neighborhood.
Stoneware is the most reliable, least shrinking type of clay, whether you sculpt or
handbuild functional items. It’s versatile. You can buy stoneware clay bodies that vitrify anywhere from cone 4 or 5 to cone 10. Ask your distributor and read the label to make sure of the specific firing temperature. Melted clay pots are no fun.
Stoneware can be very smooth and buttery for throwing, molding, or handbuilding. Add grog for a slightly stronger version, or add more grog to make an extremely sturdy, yet plastic version for large pots or tall sculptures.
As for temperature—most school studios and community studios choose cone 10. That’s because it would be really hard to overfire and melt cone 10 clay. Cone 10 fires beautifully in reduction or oxidation. On the other hand, many home-based artists, who fire their own pottery, choose mid-range firing.
Mid-range costs less in fuel for the kiln. The clay and glazes are cheaper and more readily available. The firing cycles are often shorter. When fired to maturity, mid-range clay bodies are vitreous with low porosity and are food safe.
Potters who like to dig or mix their own clay bodies may like earthenware.
We suggest studying a few before you choose a good clay body. Talk with other potters. Buy small quantities of a few bodies, or see if you can swap some with another artist. Make tiles or teabowls and test fire a number until you’re happy with results. Remember that most potters settle on more than one body, depending on their current projects.
A Word About Paperclay
Paperclay can be a porcelain, earthenware, or stoneware. The clay is mixed with varying ratios of paper pulp or cellulose insulation material, then
wedged, and used as you would any clay body.
Paperclay is incredibly forgiving. Errors can be fixed, pieces can be re-attached in any drying stage. Wet paperclay is stronger than the other types. Therefore, you can create intricate work and joinings not possible without the paper support.
Artist Cory McCrory’s teapot houses are outstanding examples. Her waving draperies and cantilevered levels would be almost impossible without the paper component to her clay body. Subscribe to our premium content, and read about making paper clay.
Choosing a Good Clay Body: Sources
There are clay distributors all over. Almost certainly, you can explore within an hour or two of most places and find a distributor. There are plenty of places to buy online, but shipping cost is a concern.
Clay is mostly sold in no less than 25 or 50 pound lugs. Fifty is becoming the standard.
We’ve found deals on shipping.
Amazon carries well-known Amaco wet clays. Most 25-pound bags cost between $20 and $30 and the buyer pays $6 shipping. Not a bad deal.
FreeFreightPotterySupply ships Laguna and Coyote Clays free on orders over $50. Minimum purchase is 50 pounds. Prices are just under 80 cents per pound. Discounts on larger quantities.
Sheffield Pottery ships 50 pounds free. Clay priced in the low $40 range, depending upon clay body.
Blick Art Materials has some clay, too. They almost always ship free with minimum price requirement or coupon from DickBlick website.