This collaboration between the Museo del Prado (Madrid) and the partnership between Technical Art History and the Imaging Spectroscopy and Analysis Centre (ISAAC) at the University of Glasgow) is focused on dry-process artificial arsenic sulphide pigments (‘AASPs’) and their use in painting and polychrome sculpture. This topic is of great interest since fragments of yellow and/or red powders and cakes derived by roasting, melting and sublimation were recently identified in a small number of 16th and 17th century works of art from Germany, Italy, The Netherlands and Spain.
Although dry-process artificial arsenic sulphides have rarely been identified as a pigment in painting and polychromy of Western Europe, its use has been often suspected on works of art from the 16th century onwards, in particular Italian and Spanish paintings. There are some clear indications that artificial arsenic sulphide pigments have been used by Venetian Renaissance painters including Titian, Tintoretto and Veronese, who are famous for their use of arsenic sulphide pigments, in a striking fashion in order to produce the glowing orange and yellow tones in their works. The use of the artificial varieties is supported by contemporary documentary sources such as Lomazzo’s Trattato dell’arte della pittura (1584) where l’oropimento arso (burnt orpiment) is recommended as a pigment. Lomazzo referred to orpiment, which he described as ‘the colour of gold,’ as ‘the alchemy of Venetian painters’. The first definite references to the dry-process preparation from natural orpiment by sublimation appear to have been made by Vannoccio Biringuccio in his De la Pirotechnia (1540)
The Museo del Prado is famous for its outstanding collection of Italian and Spanish paintings from the middle of the 16th to the early 17th century including the Venetian School, which is one of the great strengths of the Prado’s collection. This includes an exceptional group of masterpieces by Titian, Tintoretto and Veronese who are the main artists studied in this particular collaboration. Another strength of the Prado collections are the early 17th century Spanish naturalist works with various outstanding paintings by such artists as Juan Bautista Maíno (1569-1649), who is suspected to have used artificial arsenic sulphide pigments in one of his works (Pentecost, dated 1612-14).
The identification of these highly complex pigments requires the use of multi-technique analysis including optical microscopy, polarised light microscopy (PLM), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX) as well as Raman micro-spectroscopy. A detailed examination of the crystalline and x-ray amorphous artificial equivalents of natural orpiment (As2S3), realgar (As4S4) and members of the β-As4S4 – As8S9 series revealed the use of at least 8 different types of artificial arsenic sulphide pigments. Raman analysis proved to be a powerful tool, particularly for differentiation between realgar (As4S4), pararealgar, and members of the β-As4S4-As8S9 series, which includes the χ-phase of almost identical chemical composition, as well as the distinction of X-ray amorphous arsenic sulphide glasses. Discussion of samples taken from the selected group of paintings focused upon in this study via the Museo del Prado and Technical Art History (University of Glasgow) will promote joint reflection throughout this project. This will ensure the best practice for multi-technique analysis; assessment of analytical data and pigment alteration issues.
Our partners in this project team are Dr Günter Grundmann (formerly Technical University Munich, Chair for Engineering Geology), Dr María Dolores Gayo García (Museo del Prado), and Maite Jover de Celis (Museo del Prado).