Why Test Glazes
A lot of potters find a few glazes that they like, and then stick with them. Over time, their work can become a bit stale...they fall into a rut. Experimenting with various glazes and glaze combinations can keep your work fresh and intriguing, both to you and those who see and use your pots.
Even just making slight changes, such as layering a glaze over instead of under another specific glaze, can result in dramatic changes. Some glazes change significantly depending on how thickly it is applied, and if it is brushed on or dipped. Other changes can be tried in colorants, colorant percentages, fluxes, other modifiers, temperature, and
Test Colorants in a Base Glaze
Adding colorants to a base glaze, especially one that is clear, can be an interesting exercise and can give some quite useful results. Various colorants have different native strengths. For example, 2% chrome oxide may give a fairly mild green color, while 2% of cobalt oxide may be almost overwhelmingly blue.
Begin by making a batch with a tiny amount of a colorant and then continue with other batches of the base glaze in which you increase the percentage of colorant in regular increments. Continue setting up batches until you reach a percentage that you think will be too strong. For example, you could run a series of tests starting with an addition of 0.5% red iron oxide, then increase to 1%, increase to 2%, and so on up to 10 or 20%.
Testing is usually done on some type of test tile. There are several types; you may find that one or more work best for you. The two types I prefer are test bowls for most testing, and flat slabs for line blend testing.
Test bowls work well because they are guaranteed to catch a glaze that becomes too fluid. They also show you how the glaze acts on both vertical and flat surfaces. Test tiles can also be L-shaped, L-shapes with depression in the foot to catch glaze, small flat tiles, bell shapes, and so on. For line blends, a flat slab scored into sections are both convenient and give a great deal of information at a glance once completed.
Line blends are basically a systematic way to test and chart results at the same time. They can be a simple line test with only one component being tested, such as our example above with the red iron oxide. They can also be more complex, where two or more components are tested for their interactions, as well as their individual effects.
Record Your Test Results
Make certain you keep careful records of your tests as you conduct them. As you get your tiles ready, be sure to mark the test tile or line blend section so that you know what is being tested on them. Set up your records as you do the tiles, so that your recording system matches and to avoid forgetting a tile or section.
After the firing, record all your results in a log or potter's diary. Make note of what worked well, but also of what didn't. In addition, note any results that you think might bear further investigation.
Through testing and recording your results, you will build up your knowledge and feel for glazes and glaze components faster and with more surety than you would be able to otherwise. It is well worth the effort.
Test Glaze Layering
Layering one glaze over or under another glaze can sometimes result in unexpected effects. This is especially helpful if you are using commercial glazes, which are often rather bland in their visual appeal.