When our 1929 kitchen was gutted for a renovation, the contractor uncovered a brick chimney that we decided to keep exposed. The chimney was glazed with Minwax Polycrilic, and within a week, the previously gray mortar had changed color to -- there's no good way to put this -- puke yellow/green. The contractor said he had never seen this kind of reaction before with this product, which means that the 1929 mortar is to blame. Does anyone know (a) what 86-year-old mortar could have in it that contemporary mortar would not, and (b) what kind of glaze or sealer we might be able to use that is so stable it won't react with the mortar? Thanks!