Neighborhood of a Glaze

Like a beautiful shell on the beach, a miracle out of the kiln. Is this glaze reproducible? And what will be the effect of small changes in composition or firing? More, is this glaze unique, or does it have exciting relatives?
Let me tell you of my adventures exploring the near neighbors of one glaze.

‘Neighbor glazes’ are those that might result from an error in weighing, or in the mixing of the glaze; they may be the result of using a new bag of an ‘old material’, a bag whose contents labeled as Custer feldspar was mined a decade earlier than the spar in the just-used-up bag was, and differing from that one's exact composition. Or they result from an intentional tweaking of the glaze by a small amount, for example replacing a potash feldspar with a soda feldspar.

Here I'll describe - and give an example - of a method for exploring the neighborhood of a glaze; a method I call ‘axis testing’. It is a controlled way of tossing off darts into black holes.

To explore is to examine effects without worrying whether the result forms a worthwhile glaze or not. The objective is to isolate parameters, draw conclusions, and attribute an effect to a cause.

For example, increasing the silica in a glaze, or increasing both silica and alumina, which then decreases the proportion of the basic components of the glaze (the fluxes) the first column oxides in the seger formula. The result may, as is commonly assumed, result in a stiffer, underfired glaze. On the other hand, the increase in silica will make the glaze flow off the pot as it would in an alkaline matt.

It's all to the good and I have discovered what an increase of silica does in this glaze.