Weyl, in Coloured Glasses (pg. 257-258), and Volf, in A Chemical Approach to Glass (pg. 538), describe making a pure "ultramarine sulfur blue". They both point out that this is done in "borate melts". So my questions are a)has anyone here ever tried this? And b) Is a "borate melt" a batch that contains borax/boric acid or is this implying that it's only attainable in borosilicates? Im going to be doing some sulphur melts/experiments in the next month and thought I'd give the sulphur blue a try, if anyone has any experience to share, I'm all ears. Thanks!