Bentonite HPM-20

Volclay 325 Mesh is our best all-round bentonite for glazes. Volclay 200 Mesh costs the least and works well in clay bodies. HPM-20 is the finest and purest, for porcelain and glazes where grit would show. Hectalite 200 is white and low in iron, for white and clear glazes, and you use less of it.

A little goes a long way. In a glaze, start at 1 to 2% (1 to 2 grams per 100 grams of dry glaze). In a clay body, start at about 2%. With Hectalite, start at 1%. Too much makes glazes crack as they dry and crawl in the kiln (pull back into beads), and it makes clay sticky and slow to dry.

Bentonite swells into a gel the moment it touches water, so a pinch dropped into wet glaze forms rubbery lumps. Always mix it into the dry ingredients first, then add water. If lumps do form, let the mix sit overnight, stir it hard with a drill mixer and pour it through a sieve.

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.