Sheffield Raw Clay, Hammer-milled

Celadons need only a little iron. We don't publish an iron analysis for this clay, so test a range: add 5, 10 and 15% to a clear cone 10 base, fire the tiles in reduction, and pick the color you like.

It's raw clay, dug on our own land and only dried and crushed, with nothing blended in to even it out. Different parts of a clay deposit differ slightly. Test each new lot before a big batch, and buy enough for a whole project at once.

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.

The iron in the clay reacts to the air in the kiln. An electric kiln fires with plenty of oxygen (oxidation), so iron gives lighter, warmer colors. A gas or wood kiln fired with less oxygen (reduction) turns the same iron darker and richer. Fire your test tiles in the kiln you'll actually use.

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.