Pete Pinnell Hi Alisa- these are great looking tests. I can provide you with a bit of background information if you’re interested. Matte glazes tend to get their character from the choice of alkaline earth flux, and there are four of them: barium oxide, strontium oxide, calcium oxide and magnesium oxide. Of course we can source each of these as a simple carbonate, which makes substitutions and comparisons pretty easy. Generally speaking, the brightest results come from barium, with color intensity diminishing from barium, then strontium, calcium, and with magnesium having the lowest color response (one exception to this rule is cobalt in magnesium, which can make intense purple violets, which I find hideous, but some people like). As for surfaces in a matte glaze, magnesium has the smoothest matte surfaces, with calcium being second smoothest, then strontium and then barium being somewhat abrasive (which is why you probably wouldn’t want barium on a food contact surface even if it was safe, which it is not). Lithium (in the form of lithium carbonate) can be added to any of these matte glazes to get the surface a bit more active and visually broken (texture). Lithium can also intensify color a bit and will tend to lower maturing temperature a bit. As for substituting them, you can’t do a gram per gram sub if you want to get a good comparison, because they have very different weights. One molecule of one alkaline earth will have about the same fluxing power as one molecule of a different alkaline earth, even if they are different sizes. Think about a plum compared with an apple. Each is one “unit” of fruit, but a pound of plums will contain more “units” than a pound of apples, so if you want the same number of molecules then you have to make a molecular substitution. If you made a weight substitution, a pound of plums contains a lot more than a pound of apples, even though they weigh the same. Magnesium carbonate has a molecular weight of 84, calcium carbonate 100, strontium carbonate 148, and barium carbonate 197 (these numbers are all rounded). So, begin with a recipe that calls for barium carbonate. To replace the barium with the same amount of strontium, multiply the barium times .75. To replace the strontium with calcium, multiply the strontium times .65. To replace the calcium with magnesium, multiply the calcium times .84. Leave the amounts of the other ingredients the same. This way you’ll have the same amount of alkaline earth flux (on a molecular basis) in each set of tests. In your first test (pictured at the top), you have one third more flux in the glaze with calcium than in the strontium glaze, because calcium that much smaller of a molecule. That is the cause of some of your variation, not just the choice of flux. BTW, you can always mix fluxes to get different qualities. Calcium and magnesium together give smoother, more predictable results than either alone. I like strontium because it provides a fairly smooth surface with good color variation and a pleasant color response. It’s not as intense as barium, but brighter (and easier to control) than a straight calcium matte. On the other hand, a calcium matte (without magnesium) can give a smooth matte that has an almost teflon-like smoothness. Magnesium+calcium mattes tend to just be white, beige, tan or gray. On the other hand, they can have the kind of smooth, buttery surface that causes people to pick the pot up and start moaning with pleasure at the feel. It’s the kind of thing that causes people to make “baby’s bottom” analogies. I hope this helps. Please post the other tests when you fire them. Thanks for doing this!
This is very helpful information from Pete. My further tests with m.w. subs. for strontium with whiting, proved the point. The tests are posted in glazy.com