@Mark C. - Thanks - I'd be only too happy to use frits, to be honest, if I could get the ones I wanted! I've used frits a lot with earthenware glazes, and am very happy to get them working properly. The real problem in the backwaters of Europe is availability - here in France there is only a small range to choose from, and inevitably not what you are looking for at the time. Hence the question about colemanite, which I can get, and which I would substitute for the more reliable (British) calcium borate frit I am about to run out of.
@Marcia Selsor - Thanks - Gerstley Borate is no more available in my bit of Europe than half the frits I'd like to use!
@Neil Estrick - Thanks - (see my response to Mark C) - I'm good with frits, if only I can get them!
@Glazenerd - Thanks for the link to that thread - I must learn to do a (proper) search on these forums as a first step.
As it happens, overnight I've just tested a glaze (for porcelain, ivory, silky smooth satin) which I didn't think would work as I wanted, but lo, it did. And it has no colemanite, calcium borate, ghastly borate or any other difficult thing in it. That's what I call a win-win situation!
I expected it to craze, it defied me - I don't mind being wrong!
Thanks again to all.