Colemanite is a natural calcium borate and one of the few ways to get boron into a glaze without using a frit. It appears throughout older recipes and has largely been replaced, because its composition varies between deposits and it is partially soluble, which makes results hard to repeat.
Why boron matters
Boron melts glass at temperatures nothing else reaches usefully, and it contributes to the glass itself as well as fluxing it. That is what makes bright, well-melted low-fire glazes possible, and it is why every earthenware glaze shelf is effectively a boron shelf.
The problems with the raw material
Natural colemanite varies in composition from source to source, and it is partially soluble, so some of the boron dissolves into the glaze water and migrates as the glaze dries. Both mean a recipe that worked with one bag may behave differently with the next, which is a poor foundation for repeatable production.
What replaced it
Borosilicate frits, in which the boron is already melted into an insoluble glass of known and consistent composition. Substituting a frit for colemanite in an old recipe is a calculation rather than a swap, because the frit brings its own silica, alumina and alkalis with it, and published analyses are what make it possible.
If you are using it
Buy from one source and expect variation between batches, test each new bag on a tile before committing work to it, and treat any recipe result as specific to that material. The usual powder precautions apply, and it is worth keeping the bag sealed because absorbed moisture changes both weight and behaviour.
Questions people ask about colemanite
Why do modern recipes not use colemanite?
Its composition varies between deposits and it is partially soluble, so results are hard to repeat. Frits give the same boron consistently.
Can I substitute a frit for colemanite?
Yes, by calculation rather than by weight, because the frit brings its own silica, alumina and alkalis.
What does boron actually do?
It melts glass at low temperatures and contributes to the glass itself, which is what makes bright earthenware glazes possible.