Mixed up some tests for the teapots this evening. Glaze tests are small batches (usually 100 grams plus colourants) of whatever recipe you’re planning on testing. You mix up all your minerals, add them to water, sieve the whole shebang a few times to get a homogenous mix, dip a small bisqued tile in your new sludge, and fire the tile to see what the heck the recipe looks like when *you* mix it up (because, thanks to the wonderfully varied natures of mineral deposits and tap water, the exact mix of moleclues is never the same from one studio to the next, and what the other guy says is a varied green/brown glaze breaking blue over edges could, for you, come out a plain muddy mess breaking muddy).
I’m playing with two of the recipes I came up with from the glaze chemistry course a few months ago.
The first one is mostly good (that particular tile there is a chrome/tin mix that came out a nice pale lavender colour, and there’s another test tile with nickel that came out almost lime green), but I want to see how it behaves over a clear glaze. I’m hoping it’ll change the texture a little–if not, I’ll have to fiddle with fluxes and refractories and try to make it melt more smoothly–and I also want to see if there’s a colour difference.
The second one is my favourite in terms of colour and feel (soooo glossy and smooth), but it started developing cracks the second it was out of the kiln. This is a sign that the glaze and the clay don’t have a good “fit”–they are expanding and contracting with temperature and humidity changes, but not at the same rate. The stress caused by the different expansion rates causes crazing. All pottery crazes with time, especially clear glazes, but it’s considered bad technique to have something craze this quickly. I added some silica to the recipe, which should reduce the crazing and, as a happy side effect, reduce the epic runniness of the glaze. I’m also layering this one over clear and, just for shits and giggles, doing an extra tile to see what we get with chrome and tin as colourants instead of the original copper carbonate. I’m hoping for pink or purple.
 Unless of course you *want* that look, like with crackle or crystalline glazes. But I don’t.
 Runny glazes are prone to dripping or, in bad cases, sticking their pots to the kiln shelf. Sometimes, chisels and angle grinders are involved in trying to save the kiln shelf (the pot is usually a lost cause). It’s bad.