The rediscovery of a mixed calcium copper hydroxide as a blue pigment is described. The pigment, named lime blue, has not yet been reported in the chemical literature. The formula CaCu(OH)4·H2O was found from a single-crystal X-ray structural analysis. The pigment is simple to prepare according to historical recipes for the production of synthetic blue pigments. The recipes are based on an original that is included in the ‘Mappae Clavicula’ and probably of southern English origin. Lime blue is only stable in aqueous binders and in wall paintings. To date, no evidence of the pigment has been found. Further pigments detected in the products of the historical recipes for lime blue are calumetite, Cu(Cl,OH)22·H2O and calcium copper acetate, CaCu(CH3COO)4·6H2O.
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Krekel et al. (2003) studied this question.
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