Copper Oxide Black

Start with 1 gram for every 100 grams of a dry clear or white glaze you already use (your "base glaze"). That's 1%. It's stronger than copper carbonate, so you need less. Make test tiles at 1%, 2% and 3% side by side and pick the shade you like. In an electric kiln, expect green.

Black copper oxide has coarser grains than copper carbonate, and the specks are grains that didn't spread out. Pour the wet glaze through a fine sieve, such as 80-mesh (80 openings per inch), once or twice. Some potters like the specks and skip sieving. If you want no specks at all, copper carbonate is the easier choice.

Copper helps a glaze melt, so more copper makes it runnier. Above about 4 to 5%, glazes can run off the pot. Leave the bottom edge of the pot unglazed, and fire test tiles on a scrap of fired, unglazed clay to catch drips before they reach your kiln shelf.

Yes, copper is a raku favorite. In raku, pots come out of the kiln red-hot and go into a closed container with something that burns, like sawdust or newspaper. With copper glazes, that can give metallic, coppery, red or rainbow-colored surfaces. Results change from firing to firing, and those colors can dull over months as the surface slowly reacts with the air.

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.