A multimodal analysis of unusual copper-containing blue and green pigments in the early modern painter’s palette
This thesis presents a multi-scale, multi-modal, and multi-pronged investigation of copper- containing blue pigments in early modern English decorative wall painting dating from 1550-1650. By the late sixteenth century, large-scale decorative wall paintings were found in a wide range of buildings of different status and function. The quality and choice of materials and the way in which colour was used in these paintings was not only an aesthetic choice but a means of displaying social standing. Contemporaneous textual sources including painters’ treatises, accounts, and inventories discuss blue and green verditer, synthetically produced copper pigments with respective chemical formulae Cu3(CO3)2(OH)2 and Cu2CO3(OH)2. Azurite, a naturally occurring basic copper carbonate chemically and structurally analogous to blue verditer (Cu3(CO3)2(OH)2, monoclinic), is found in high status schemes but rarely identified lower down the social scale because of its expense. Study of the materials used in surviving painting schemes provides information about the palette of early modern painters, particularly the possible introduction of synthetically produced copper pigments during this period. Pigment and degradation product identification is also important for interpretation and conservation of surviving schemes. Systematic identification of blue verditer and other copper-based blue and green pigments in this context, however, has not been carried out prior to this work. This thesis investigates the use of synthetic copper-based green and blue pigments from a group of twenty-two samples from decorative wall paintings, a subset of which are from late sixteenth and early seventeenth century middling decorative interiors. The presence of copper-containing blue and green pigments, their identities, and particularly their origin were of significant interest. Laboratory-based optical and electron microscopy and elemental analysis were used to determine the identity and origin of copper-containing pigments. Several samples exhibited unusual or ambiguous morphology and chemistry, precluding identification as blue verditer or azurite. X-ray powder micro-diffraction and micro-X-Ray absorption experiments were carried out on these samples, enabling identification of specific chemistries with high spatial resolution within paint layers. Replications of verditer recipes were carried out and products were identified for the first time by chemically specific methods including X-ray powder diffraction and Raman spectroscopy. The factors influencing reaction outcome were studied in detail. Although blue verditer was identified in many samples studied, with azurite used in one high-status site, several other copper species were also identified in numerous samples. Copper chlorides were identified as both degradation products and (presumably synthetic) pigments. An unusual copper nitrate was also identified in two historical samples, mixed with blue verditer and alone. This nitrate species, rouaite (Cu2(NO3)(OH)3), was also identified as the primary green species produced in synthesis replications. Reactions produced mixtures of mainly rouaite and blue verditer, with proportions varying greatly depending on reaction temperature and, significantly, atmospheric composition. Previously reported reaction control at low temperatures, however, was not replicated; alternative explanations for the prominence of English verditer during this period are put forward. Rouaite’s presence in historical samples and synthesis replications points to its use as a marker for verditer production using the recipes documented in seventeenth century texts and suggests that the widely accepted identity of green verditer should be reconsidered. Furthermore, analysis of the historical corpus suggests that the early modern decorative painter’s palette was more diverse than previously understood where copper blues and greens are concerned; blue verditer, rouaite, and copper chlorides of variable composition were all identified in this work.
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