The technology of Baroque oil-based wall paintings in Malta : materials and implications for conservation

Abstract

Oil-based painting on stone is the most common technique used in an architectural context in Malta since the 17th century. It is characterized by having oil paint applied directly to a very porous stone support, which in Malta is the ubiquitous Globigerina Limestone. The combination of these two very different materials, having remarkably different porosities and affinities for water, creates a sharp interface between the substrate and paint film that is prone to severe salt- driven deterioration. Salt-related deterioration is indeed generally severe in these paintings and has often determined the focus of their condition assessment. This thesis provides for the first time a holistic overview of the technology of Baroque oil-based wall paintings in Malta building on data acquired in the past, and new data generated by the study of eight wall painting schemes, investigated using a range of non-invasive and invasive techniques. The technology is seen in the context of southern European technical treatises and literature related to canvas paintings. The research has a specific focus on targeted degradation phenomena inherent to the ageing of oil painting, specifically those related to lead-based pigments, smalt and arsenic-based pigments. These phenomena have been extensively studied in easel paintings but are often overlooked in wall paintings. This thesis reveals that the technique used in oil-based wall painting in Malta is more complex and sophisticated than what has been hitherto acknowledged, even in the case of less prestigious commissions including ornamental wall paintings. One example concerns the limited analysis of binding media carried out as part of this research, which indicates that in two of the eight cases investigated, the paintings are not simply oil paintings, but their binder also includes egg. Painting materials also show greater variety and sophistication in their combination than what has hitherto been reported, especially in continental Europe. Such sophistication calls for the same level of detail that has been used to investigate easel paintings. The thesis also discusses the important implications that paint degradation has on the design of conservation treatments, especially cleaning, besides the obvious repercussions it has on the appearance of the paintings and their appreciation. The deterioration linked to salt activity remains a major concern for oil-based wall paintings, but this research has demonstrated that surface phenomena, such as efflorescences and crusts, are not simply the result of salt activity linked to ions originating from masonry materials or introduced by the environment or through restoration materials. Surface accretions and crusts can also be the result of oil paint degradation processes. Moreover, ions generated by the degradation of oil paints were found to play a significant role in the formation of salt efflorescence. For example, lead chlorides were frequently detected within the paint stratigraphy, as well as in surface crusts, often in combination with other salts. The impact of these salts, alone or in salt mixtures, remains to date unexplored. It is, however, clear that analytical methods targeting selected ions, or anions (e.g. ion chromatography, salt strips), commonly used in the field, are not sufficient to characterize the origin and distribution of salts in these paintings, and to attain a precise diagnosis. This research should be seen as a first milestone for understanding the technology of oil-based wall paintings in Malta, and their specific conservation needs. The holistic approach advocated here paves the way for future research, inevitably multidisciplinary in character.