Cobalt Carbonate

Much less than you'd expect. Start with half a gram for every 100 grams of a dry clear or white base glaze (0.5%). That gives a medium blue. Weigh it on a scale that reads to 0.1 gram, because a small mistake makes a big color difference. Try test tiles at 0.25%, 0.5% and 1% side by side.

A long way. A pound is about 454 grams. At 0.5%, that colors about 200 pounds of dry glaze. That's why we also sell a half-pound size.

Glazes with a lot of magnesium push cobalt toward purple and lavender. The magnesium usually comes from talc or dolomite in the recipe. For a clear, true blue, use a base glaze without much magnesium, then test.

Yes. Mix a little cobalt carbonate with water, with some of your clear glaze stirred in to help it melt, and brush it onto bisqueware (pottery that's been fired once). Then cover it with clear glaze. Keep it thin: heavy cobalt can fire black or metallic and can make the glaze over it bubble or pull away. Try it on a test tile first.

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.