Tennessee #10 Ball Clay

Terra sigillata is a very fine clay coating that's polished to a soft sheen. Stir Tennessee #10 into plenty of water with a little deflocculant, let it settle for a day or more, and siphon off the thin middle layer. Brush that onto bone-dry pots and polish with a soft cloth or plastic wrap. Fire low; high firing dulls the shine.

Ball clay is a very fine, sticky clay, mostly mined in Kentucky and Tennessee. A little in a clay body makes it easier to throw and shape, and stronger before it's fired. In a glaze, it helps keep the other powders from settling. Because it's so fine, it shrinks a lot, so most clay bodies keep it to about 10 to 30%. Raw ball clay is often gray from natural carbon, old plant matter that burns away in the kiln.

If your recipe names one, buy that one: a recipe works best with the clay it was written for. Starting from scratch:

For glazes: Old Hickory #1 Glaze Clay.

For porcelain and white bodies: C&C, Jackson, HTP or M-23. Tennessee #10 fires especially white.

For slip casting: KT1-4 or 54-S.

For an all-round stoneware body: OM #4 or Foundry Hill Cream.

For salt, soda and wood firing: XX Sagger.

Both are clays. Kaolin, also known as china clay, is purer and fires whiter, but it isn't very plastic on its own: a coil of it cracks when you bend it. Ball clay is finer and stickier, so it adds plasticity (how far wet clay bends before it cracks) and strength, but it fires a little darker and shrinks more. Porcelain is mostly kaolin, with a little ball clay or bentonite added so it can be thrown.

For shrinkage, roll a slab of the wet clay and scratch a line exactly 10 cm (100 mm) long. Let it dry, fire it, and measure the line again. Each millimeter it lost is 1% shrinkage. For absorption, which tells you how dense the fired clay is, weigh a fired bar, boil it for 5 hours (or soak it for 24), pat it dry and weigh it again. Absorption % = (wet weight − dry weight) ÷ dry weight × 100. Lower numbers mean a denser, stronger clay.

Glaze and clay recipes go by weight, not by volume, and they don't count water. Each ingredient is a share of the dry total. 2% means 2 grams for every 100 grams of dry ingredients. For a 500-gram batch, that's 10 grams. Colors and opacifiers are often listed as extras on top of a recipe that adds up to 100, written like "+2" or "add 2%." Weigh those the same way: 2 grams for every 100 grams of the base recipe. A digital scale that reads to 0.1 gram makes small amounts easy.

A test tile is a small piece of the clay you use, glazed and fired just like your real work. Raw materials vary from bag to bag, and results also change with your clay, how thick you apply the glaze and how your kiln fires. A test tile is the only way to know what you'll get before you glaze a whole batch of pots. Write what's on each tile on the back (an underglaze pencil works), and keep notes so you can repeat what works.

Yes, whenever you handle the dry powder. Fine dust is the main hazard with most raw materials, and some can harm your lungs over time. Wear a NIOSH-approved respirator (N95 or better) while you weigh and mix. A basic dust mask that isn't N95-rated won't protect you. Once a material is mixed with water, dust is much less of a problem, but wipe up spills with a wet sponge before they dry, and mop instead of sweeping. Don't eat or drink while you work, and wash your hands afterward. Check each product's Safety section for anything extra that material needs.