| Abstract: | SUMMARIES. The rediscovery of a mixed calcium copper hydroxide as a blue pigment is described. The pigment, named lime blue, lias 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)2-2H2O and calcium copper acetate, CaCu(CH3 COO) 4-6H2O.
CONCLUSIONS. In most mediaeval and renaissance documentary sources, instructions are given for the preparation of synthetic blue pigments based on copper or copper salts. Until now it was assumed that this synthetic lasur, at least for the stable variants, was synthetic azurite, 2CuCO3-Cu(OH)2 [5], which cannot easily be differentiated from natural azurite, so that its detection is difficult and infrequent. Indeed, when the recipes with metallic copper and quicklime or with verdigris, quicklime and water were re-created, the previously unknown pigment lime blue, calcium copper acetate, or calumetite were obtained, depending on the pH value of the solution and the presence of additional components. Neither calcium copper acetate nor calumetite is suitable as a blue pigment because they are either much too green or decompose in wall paintings. The reddish-blue main product, lime blue, is stable in wall paintings and animal glue, but not in linseed oil. Since this lime blue compound has not yet been reported in the chemical literature, it could be detected as a copper- and calcium-containing blue pigment but not correctly identified as CaCu(OH)4H2O. However, with the analytical data for the pigment reported here, it should now be possible to identify any occurrences of this pigment, particularly in wall paintings and decorative schemes. Recipes and entries in trade registers show that synthetic blue pigments from copper or copper salts were important and cheap substitutes for expensive blues like ultramarine or azurite. Even after the introduction of Prussian blue in the early eighteenth century they were still produced, especially for paints on walls. Lime blue is only one of many artificial copper blues. Most of the products resulting from recipes for synthetic blues are not yet characterized by chemists, although some of them crystallize excellently. Further work on this group of pigments should make it possible to rediscover many of these forgotten blues and to identify them on artifacts. |
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