Copper Carbonate

Start with a clear or white glaze you already use. This is your "base glaze." Weigh 100 grams of the dry base glaze and add 2 grams of copper carbonate. Mix in water until it's about as thick as heavy cream. Pour it through a sieve (a fine mesh screen) to break up lumps, so the color comes out even. Brush or dip it onto a test tile and fire it the way you fire your other work. In an electric kiln, expect green. Then adjust: add more copper for a darker green, or less for a lighter one.

Once fired, both give the same colors. Copper carbonate is finer, so it mixes in evenly with fewer specks. That makes it the easier choice for most glazes, especially if you're just starting. Black copper oxide is stronger and coarser. Use about 0.7 grams of oxide for every 1 gram of carbonate, and expect some dark specks unless you pour the glaze through a fine sieve. If your recipe names one, you can swap in the other at that ratio, then fire a test tile.

A pound is about 454 grams. At 2%, that colors about 50 pounds of dry glaze. At 5%, it colors about 20 pounds. If you're mixing test batches, the 1 lb size will last a long time.

There's usually too much copper. Above about 5%, the glaze can't take it all in, and the extra shows up as a black or metallic surface. A thick coat does the same thing, because it puts more copper in one spot. Try 2 to 4%, apply a thinner coat, and test again. Copper also makes glazes runnier, so leave the bottom edge of the pot unglazed. Set test tiles on a scrap of fired, unglazed clay to catch drips before they reach your kiln shelf.

Not in a normal firing. Copper red needs a reduction firing, where the kiln is short of oxygen, and that usually means a gas kiln. In an electric kiln, copper gives greens. Some potters get red areas in an electric kiln by adding a small amount of very fine silicon carbide to the glaze. Results are hard to predict, and it can make the glaze bubble. Try it on test tiles first.

Yes. At high temperatures, some copper turns to vapor and settles on nearby pots and kiln shelves. In a gas kiln, it can leave pink or red blushes on white or light glazes close by. Leave space around copper-glazed pieces. Keep kiln wash, a protective coating you paint on the shelves, on every shelf.

Be careful. Copper can leach, meaning it dissolves out of the glaze into food and drink. The risk is highest with glossy glazes that have a lot of copper, and with glazes that didn't fully melt. Many potters use a different, copper-free glaze on the inside and keep copper glazes on the outside. If you do put copper on a surface that touches food, use a stable, tested base glaze, and have the finished glaze lab-tested before you sell the work.

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.