Potters face a bewildering set of choices when creating and using glazes.
There are many different surface treatments for pottery and ceramic sculpture, but glazes are by far the most popular and common. Glazes can be shiny, matte, stony, cratered, crackled, glassy smooth, crystalline, and textured in a number of ways. They can be colored in almost any color that can be perceived by the human eye. These variations can be created, effected and affected by formula, method of application (brushing, dipping, spraying, splattering), thickness of coating, the firing schedule and type of fuel used for the kiln.
If all of this leaves you overwhelmed, add in the multitude of choices when it comes to raw materials for creating glazes and it’s easy to see why so many potters choose to use commercially produced products. These tend to be very stable formulations that behave predictably when used as suggested. Many potters use only a couple of base glaze recipes and alter them with different colorants or by changing the ratio of silica to alumina to adjust the gloss or matte of the glaze. I follow that philosophy when creating new glazes. But even that method has its limits. Here is a tale of my own experience.
I needed a new green glaze that was unrelated to the Rowantrees reproduction Evergreen that I already sell. This new glaze needed to have an “antique” character to it, with the green more muted and not as bright. I already had a good, stable recipe for a clear glaze that I had used to make a similarly antique white, blue and yellow. So I decided to try that recipe to make green. Reasonable, no?
Well, no, it wasn’t. Green is usually made by using either copper or chrome. Copper creates a reliable green only in lead-based glazes and in some high-fired glazes. Because all of my glazes are fluxed with boron – and I fire in the medium temperature range – copper created turquoise to blue. So copper was out for making green. That left two choices for me. Either I could use a green stain (more on that later) or chrome. Chrome was the less expensive choice and I already use it for the Evergreen, so I had a supply of it. I only need about 1% chrome to make a strong green color, which is very little as well.
Here’s the thing; every time I made a test batch of my base glaze using chrome to make green, the best I could manage was olive drab. Mind you, it was a pretty glaze that I intend to explore (particularly in combination with other colors), but it wasn’t what I was trying to achieve. Nor was it what my customer was waiting for.
What was going on here? Well, a little research told the story. The base glaze contains both lithium and zinc. Lithium is an alkaline metal and exists at the “reactive” end of the periodic table. In fact, lithium is possibly the most reactive after hydrogen. Zinc and chrome are transitional metals, with chrome being more reactive. When chrome is added to a glaze containing either lithium or zinc the result will tend to brown. So there you have it. I needed a totally new glaze recipe.
Nobody likes coming to that conclusion, but there you have it. I chose to adapt the turquoise recipe, which contains neither of the troublesome fluxes and got a satisfactory green. Sometimes the answer is right in plain sight.
Next time: common glaze defects to watch out for.