A major focus since I started making my own glazes has been a good copper red, since that is the type of red I find irresistibly beautiful. Although other approaches can produce attractive reds, they have disadvantages. One way to vivid reds is by adding encapsulated ceramic stains to the glaze. One downside is that stain ingredients include toxic elements such as cadmium or selenium and safety depends on close control of glaze composition and firing protocols. Also, the resulting reds always seem, to be frank, a bit strident. Another path to reds is to use tin oxide with a smidgen of chrome in the glaze. This unlikely combination (tin usually makes white, and chrome green) can make a reliable and attractive red if the glaze composition is otherwise conducive. To my eye, though, this red tends to be so saturated that it seems almost like paint.
When a copper red glaze comes out the way I like, it is actually a combination of three glaze layers. Viewed in cross-section, there is a transparent red layer suspended between two clear glaze layers. This lets light penetrate the glaze and reflect off the underlying body, illuminating the color from within and producing a luminous red I cannot resist.
Why doesn’t everyone make copper reds? Because there are far more ways to fail than to succeed. Let me count the ways …
- If the kiln atmosphere has too air in it, the copper remains as an oxide in the glaze, having little effect and producing a clear or white glaze. If overly oxygen-starved (by adding too little air or closing down the flue damper too far, a livery brownish color results.
- Even my best glazes applied to the wrong clay body lose their appeal – several that look great on porcelain turn grayish maroon on stoneware.
- Glaze thickness can make or break the color – some brilliant reds in a medium coat will lose all color if too thin and go garnet black when thick.
- There are many other factors to fret about as well – timing of reduction cycles and oxidation cycles during ramp to maturity, rate of temperature ramp, 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 visual feast never fails to restore my enthusiasm for further effort.