A major focus since I started making my own glazes has been copper red glazes. I find copper reds much more attractive than reds made by more reliable approaches. And, though more consistent, those other methods have their own problems.
Using ceramic stains can make vivid hues, but the stains depend on highly toxic elements like cadmium or selenium and are of questionable stability. Also, to me, the resulting reds are overly strident. Another approach uses chrome and tin to make a reliable and attractive hue, but with a density that makes it seem almost like paint.
A good copper red displays a softer and almost luminous color. Viewed in cross-section, it consists of a transparent red layer suspended between two clear glaze layers. Light can penetrate the glaze and reflect off the underlying body, illuminating the color from within and producing a vibrant red I cannot resist.
The problem? Making copper reds is hard. If the firing atmosphere has too much oxygen, the copper has no effect, leaving the glaze clear or white. Too oxygen-starved produces a murky livery brown. Using the wrong clay body can turn even a good red vile. My best reds on porcelain often make for a grayish maroon on buff stoneware. The glaze thickness can make or break the color as well – a brilliant red in a medium coating will be clear if too thin or garnet black if too thick. Other factors to obsess about include timing of reduction cycles, ramp rate to temperature, rate of cooling, and holding periods during the cooling cycle. Finally, every firing is influenced by temperature, wind strength and direction, and humidity.
When all factors somehow balance, though, the resulting visual feast never fails to restore my enthusiasm for further effort.