Colorants
- Although not a commonly known fact, many oxides and coloring oxides are seldom completely pure
- Cobalt compounds often contain nickel impurities
- Zinc oxide may contain some lead oxide
- Materials vary from batch to batch or in the case of the potter, from bag to bag
- Variation can be especially large when it comes to colorants
A common example is rutile. Potters may buy a 50 pound bag of rutile that may last several years. When they run to their local ceramic store to purchase another bag, they may find to their surprise, when they run that new colorant it changes their glaze entirely. Rutile is made up of varying amounts of iron oxide and titanium dioxide. Depending on batch to batch you may get more titanium dixodie or iron oxide. For this reason if a ceramic artist knows that they need a certain material to be consistent they may stockpile that material so they have a consistent source.
Knowing as much as possible about the properties of colorants, sources, and variances is great way to control outcome of your glazes and minimize surprises.