Ceramic colorants

Iron is commonly used as a colorant in its red iron oxide form as (Fe₂O₃). Red iron oxide is commonly used to produce earthy reds and browns. It is the metal responsible for making earthenwares red. Iron is also another tricky colorant because of its ability to yield different colors under different circumstances. At low percentages (.5-1%) and in the presence of potassium, iron will become light blue or light blue-green in reduction (as is seen in traditional celadons). In the presence of barium, iron may become yellow green. When used in combination with calcium, red iron oxide can become pale yellow or amber in oxidation or green in reduction. Common percentages for red iron oxide range from (4 up to 10%).

Copper's carbonate form (CuCO₃) is commonly used to produce greens, turquoise, and copper reds. If need be, copper oxide (CuO) can be substituted but has a larger particle size and glazes should be adjusted to generally half the amount called for. In barium based glazes greenish blues often result from copper. Alkaline feldspar glazes with copper fired in reduction atmospheres will often yield ox blood or copper red glazes discovered by the Chinese. When fired above cone 8 copper can become unstable and will often fume off of a glaze in vapor form.

• Clay and Glazes: Revised edition. Chappell, James. Watson-Guptill Publications, New York. 1991.
• Glazes for the Craft Potter. Fraser, Harry. Watson Guptill Publications, New York. 1974.
• Clay and Glaze for the Potter. Rhodes, Daniel. Krause Publications. 2000. Expanded and Revised by Robin Hopper.