I test glazes, lots of glazes! A glaze for me is not just the color that one sees, but the ingredients that constitute the body of the glaze, that is the reflectivity/refraction of the surface (glossy, semi-gloss, matte), the transparency/translucency of the glaze, and the durable fit of the glaze (how well it holds up to freeze/thaw, the lemon juice acid test, and its resistance to crazing and cracking).
It cannot be overstated just how important it is to develop a robust and reliable method for taking notes and tracking the incremental changes in a formula or recipe. Notes, dates, observations, and firing sequences should be made in a bound journal. Serendipitous results can be just as exciting as planned and expected results!
The glaze tile is a critical tool for documenting the appearance of a fired glaze. Tiles can be rolled, thrown, or extruded. For myself, I use an extruded tile that provides horizontal top, an undercut, a couple of horizontal ridges, and a couple of horizontal grooves, variations in surface and texture you might find on any pot, all to test the glaze movement during the melting/flow of firing. Each tile is stamped my logo, and pressed with a sequential number; the same sequential number appears in a testing journal. One should be able to look up a glaze test in the journal and locate the corresponding tile, or conversely, from the number on the tile, look up the test notes in the journal.
There is so much research available describing the relationships between glaze ingredients in various firing atmospheres, and at various temperatures. Find a combination that works best for you! After reading John Hasselberth and Ron Roy’s book “Mastering Cone 6 Glazes”, my eyes were open to the need to look at a glaze analysis, not just the recipe. The analysis provides so much insight into the relationships of the elements in the glaze. So, now I use the glaze analysis software called GlazeMaster™3, also by John Hasselberth.
Each new glaze is tested first with the original ingredients and no added coloring oxides. Who knows, a new base glaze may be lurking! Subsequent tests include various coloring oxides; often a base works well with one coloring oxide, but not another. Using GlazeMaster™3, and keeping the unity formula as close to the original as possible, I like to switch out ingredients, one feldspar for another, one ball clay for EPK, or one boron-silicate for another, just to see happens! So, forge ahead and test, test, test, and then test again!
P.S. Use distilled water, it’s like a constant in the experiment! And incorporate slow cooling if you don’t already!