Cobalt Oxide

Once fired, both give the same blues. Cobalt oxide is about 1.5 times as strong and has coarser grains, so it's more likely to leave blue specks. Cobalt carbonate is easier for even color, especially if you're starting out. Choose the oxide when you want speckles or a recipe names it.

Very little. Start with a quarter to half a gram for every 100 grams of a dry clear or white base glaze (0.25 to 0.5%). Amounts that small are hard to weigh, so mix a bigger batch: for 500 grams of dry glaze at 0.25%, weigh 1.25 grams. Use a scale that reads to 0.1 gram.

They're cobalt grains that didn't break up. Pour the wet glaze through a fine sieve (80-mesh or finer) twice. A ball mill, a rotating jar filled with small ceramic balls, grinds them up completely. Or switch to cobalt carbonate.

Cobalt alone gives a very dark blue, not a true black. Black glazes usually combine cobalt with iron, manganese and sometimes chrome, often 5 to 10% colorant in total. Start from a tested black recipe, and fire test tiles before you glaze a full load.

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.