April Fools…
Just a quick note for April (okay, it’s later than April, but I’m trying to get caught up with my one-post-a-month plan - foolish me…) to talk about how one can be fooled by limit formulas. Limit formulas are best used as merely advice about glazes, not rules that must be followed. There are lots of very useful glazes which fall outside of the so-called ’standard’ or ‘food-safe’ limits, and will still be perfectly good glazes and quite possibly even food safe. Food safety can best be determined by having the glazes tested by a dependable lab, if you have questions about a particular glaze. If you’re making sculptural ceramics, many of the more interesting glazes for sculptural texture and color fall outside the more industrially-based limit formulas.
One other aspect of glaze calculation that can fool you is the calculated thermal expansion. For a range of gloss glazes which don’t contain lithium or strontium, the calculated thermal expansion can often be very close to the real measured thermal expansion. For everything else, the calculated thermal expansion is at best an indicator of the likely range of thermal expansion and how it might change with a change of ingredients. For matte glazes this is because matte glazes contain crystalline materials (those little tiny crystals in the glaze are what makes the glaze matte as it cools and the crystals form), and crystalline materials have much more unpredictable thermal expansion. So take calculated thermal expansion with a bit of skepticism, but it is still quite useful when you’re trying to adjust a glaze to stop crazing, if nothing more than giving you an idea of what a change in materials might do to change thermal expansion.