Measure a standard weight of glaze tests, and add a standard
weight/volume of water to each, and mix.
Make tiles of each of original glazes. (Glaze A, B, C, D)
Make tiles of each combination of two, by scooping out equal volumes of each glaze and mixing.. (Glaze AB, AC, AD, BC, BD, and CD)
Combinations of 3 are possible (Such as ABC, ABD etc.), but probably not worthwhile unless researching subtle nuances of shading. Liquid line tests are also possible, by spooning out increasing volumes in different ratios. When done testing I'll often combine the tests that are left and call them ABCDE, but seldom get a useful glaze from that.
If a resulting glaze works out well, combine the recipes from the original formulas, in the ratio that you used them, and try it in a hundred or thousand gram test. It may not be exactly the same, but should be in the ball park. Click here to see table showing combination, posted at bottom of this page
This system works better, the more tests you start with.
The actual formula for the resulting combinations is the number of tests
plus each integer preceding it. So for 5 tests, you get (5+4+3+2+1)
results, or 15 results. The combining process is a bit time consuming,
but nowhere near as tedious as reweighing each combination. I label each
test on the bottom with an underglaze pencil.
Another thing to consider is that the more different types of glaze you use for the test, the more interesting the results. You might combine matte, glossy, opaque, and different colorants, and you will get a wider range of results.
My standard size for glaze tests is 100 grams. Typically formulas are expressed as percentages, often with colorants added above 100. Using a glaze calculation program (I use GlazeChem), I make all my tests add up to 100, including colorants.
To each 100 gram batch (in labelled pint canning jars) I add a standard quantity of water (usually 110 cc). I blend the ingredients in a blender with the stirring unit attached onto the pint jar. (If you try this, keep a gentle pressure on the top of the jar so it does not fly off when the blender starts).
(Note: you can get graduated cyllinders at American Science Surplus or if you have friends in the health industry lots of disposables are measured in cc or ml) I make a note on my glaze recipe if the resulting slurry is unusually thick or thin, and may double dip the tile if it is thin, or to see how double thick glaze works out. For some reason Gerstley Borate often makes a thin slurry at this ratio, so you may have to reduce the water in some glazes with that ingredient, and possibly the percentage of that glaze used in calculating the final ratio..
By adding a standard quantity of water to a standard weight of glaze ingredients, the result tends to be a standard volume of glaze. This is certainly not the exact case, as different ingredients wet differently, but it's probably true within 5 % or so.
I make thrown standing tiles to test on, with 2 inch pie shaped wedges for each glaze test. These tiles show what happens to glaze on upright surfaces, and also I apply common colors as slips to the outside to reveal how the glaze affects those colors, how transparent the glaze is, etc.
Okay, here's a simple example, using simple numbers
|Glaze 58 A||Glaze 58B||Hybrid 58AB|
In 2011 I made a 9 base sample test to try to improve a Cone 9 crystalline glaze which seemed to be boiling at too low a temperature. Here are the results:
At the bottom are the 9 tests, A to I alphabetically. Above them are the combinations, such as AB and AC above the A. You can see that a bad test like B or F will show up diagonally running to the left as also a bad glaze. These tests were made on flat potsherds since I had a few cracked bisque pots around, so the photo looks pretty patchy.
The choices I made for the variations were based on hunches of what might cause the cratering in my current glaze. Unfortunately, although I smeared on these glaze tests very thickly, only a few samples showed any of the cratering.
When you combine two glaze samples, any ingredient not in similar quantities in both glazes will be diluted in half in the resulting combination. Since I wanted to see what effect some of the ingredients had, I hit upon the idea of leaving out the ingredient in one test, and doubling the standard amount in the next test, so when combined there would be a range of responses.
Some interesting qualitative results can be observed from the tests... The high borate frit proved necessary for lowering the firing temperature acceptably (cf. B). All the tests had around 25 % of Zinc Oxide, necessary to produce crystals. Macro crystals grew in most samples where except where some of the base ingredients were doubled. Sample "I" was quite successful in spite of having 10 % kaolin, which has traditionally been regarded as an inhibitor of crystal formation.
Questions? Email Me
Pottery Tips page
My Cone 8-10 Oxidation Glazes Page