September 27, 2002, Newsletter Issue #109: A brief guide to oxides

Tip of the Week

Chromium and nickel:
They are not predictable throughout all temperature ranges, and should be handled carefully as they are toxic.
If you make your own glazes, you already have seen many oxides used in glaze recipes.
It is probably best to stick with recipes.

Cobalt Carbonate and Cobalt Oxide. This is the most powerful color and produces various shades of blue. It can be harsh used by itself, so is sometimes mixed with iron, manganese, magnesium or copper to create more subtle colors. Like iron oxide, it can be added to glaze, or can be applied to the clay surface and fired to stoneware temperatures. By itself it will tend to create a dark slate metallic finish.
It can be mixed with manganese and iron to produce rich black slips.

Copper Carbonate and Copper Oxide. (Note: Copper should not be used in soluble glazes for food and drink containers; in addition to not being safe, it will leach into tea for example, making your tea have a coppery taste.) Not very effective used on it’s own. Better to add to or cover with glaze. In an electric kiln it will create a variety of green shades.
In alkaline glazes, it will create turquoises. It may achieve red colors in a reduction kiln.

Iron Oxide. This comes in many varieties and can provide a wide variety of colors under different firing conditions. Types of iron oxide are red iron oxide, black iron oxide, and yellow ochre. At earthenware temperatures, up to 4 percent oxide will produce amber and honey glazes.
At stoneware temperatures it can be applied directly to stain the clay surface. It is often used in this way to highlight textured surfaces. Can also be added to glazes.

Manganese Dioxide and Manganese Carbonate. In glazes creates colors such as mauve, purple and brown depending on the other ingredients present. By itself it produces an attractive brown with tiny metallic specks at stoneware temperatures (dioxide is specklier than carbonate).
It is often mixed with other oxides such as cobalt to create purples and iron to produce rich browns.

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