Yes Ilia, sorry about adding gypsum to the mental mix. I believe they have found some present in the Greek frescoes at Knossos though. It was never used outside.

If the salts are that bad, (((sulfates - : a salt or ester of sulfuric acid - salt is a death sentence for fresco.)) you might be wary of using Blanc Fixe for your whites - its true name is barium sulfate.

Just sending off my new pigment order - going to try a bunch of new fresco colors (to me anyway).