Cornwall Stone (Hammill & Gillespie)

Yes, those are other names for Cornwall stone. Ours is Hammill & Gillespie's version of the English stone. It's close to the original but not identical, so use the same weight and fire a test tile.

Cornwall stone has more silica than feldspar, so it melts a little stiffer. Swapped in for feldspar, it can give a less glossy surface. For more shine, try part feldspar and part Cornwall stone, and test.

Feldspar is a ground rock that does three jobs at once. It helps a glaze melt (it's a flux), it brings silica, the main glass-former, and it brings alumina, which stiffens the melt so the glaze stays on the pot. That's why so many stoneware glazes are built on it, often 20 to 45% of the recipe. Each feldspar has its own mix of sodium and potassium, so swapping one for another changes how the glaze melts. Fire a test tile after any swap.

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.