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Published Jan 20, 2021

glaze testingThe best way to learn about ceramic glazes and glaze materials is to test them. By studying what happens when varying amounts of various ingredients are combined in a glaze and then fired to various temperatures, you start to understand how materials affect each other, and therefore how to troubleshoot when your results are not what you wanted.

But it can be intimidating to delve into glaze chemistry. It is fairly complex. It includes the word ‘chemistry’ in its name. To some—me included—that is an immediate red flag. So in today's post, Greg Daly gives five excellent tips for getting the most out of glaze testing. Read on, and then get out there and test! - Jennifer Poellot Harnetty, editor

Figure 1.1. Greg Daly, bowl, 1986. Glazed with potash feldspar, poured, with iron brushwork. Photo: Stewart Hay.1. Be thorough in your recording of materials, tests and firings. Without a reference to what was in a test firing, all you are left with is a glazed tile, with no information to enable you to explore it further. 2. Closely observe the fired tests; in the firing, many important pieces of information are often overlooked. Don't throw out a test for at least a month, or even up to a year. The test result may not be what you are looking for at the time, but it may turn out to be just right later on. Even sitting with the tests in full natural light, you may see something of interest, right at the edge of a glaze where it thins out. Don't keep tests locked away; I put them on windowsills, on a desk, anywhere I will constantly see them, and I swap them periodically.

3. Keep an open mind at all times. We can develop such tight concepts of what a glaze is and should do that we miss the big picture, as well as the immediate evidence of what we are holding in our hands. As a student researching copper-red glazes, I had a firm idea of what they should look like in color, tone and surface - an ox blood or peach bloom  - having seen them depicted in books. But I was liberated when seeing copper-red glazes en masse at the Victoria & Albert Museum: in a room full of copper reds, the colors were qualitatively different from each other in shade, intensity, surface depth and color. This freed me of my expectations of how a glaze should look. Glazes have families—copper reds, iron, luster glazes, shino, etc.—but within any family there is also diversity.

glaze testing4. Be playful. If it occurs to you to try one material with another, and others say it will not work, do it anyway. You will always learn and extend your understanding. In Chapter 3 [of Developing Glazes] I have outlined a way of testing materials for fun, called "random glazes." Taking any four materials and any four numbers between 5 and 55, you will find you have a workable surface at cones 8 to 10. I have repeated this countless times in workshops. Many potters dismiss this odd way of creating a glaze, but are then surprised when they see the results.

5. Have a quick way of testing. Testing is what puts most people off, so they turn to the last page for recipes (the answers). But when you simplify the process and results come back, there is nothing more rewarding. Systems for ease of testing are covered in Chapter 3 of Greg Daly's Developing Glazes.

This was excerpted from Developing Glazes, which was co-published by Bloomsbury Publishing and The American Ceramic Society.

**First published in 2014.
Topics: Glaze Chemistry