Egyptian Blue

[Imperial Dave]

https://www.iflscience.com/egyptian-blue-was-a-color-lost-to-history-finally-we-can-make-it-again-79469

Hopefully of interest...

[Jim Webster]

It is, it looks as if the Egyptians didn't have a set recipe for it either, just 'schools' of craftsmen who sort of had their own variations on a general theme 

[Imperial Dave]

A bit like wargamers really....