National Gallery Technical Bulletin Volume 33, 2012
Colourless Powdered Glass as an Additive in Fifteenth- and Sixteenth-Century European Paintings
References in historic documentary sources to the use of colourless powdered glass as a paint additive have long been noted and discussed. Mary Philadelphia Merrifield included a substantial consideration of this subject in the introduction to her 1849 book ‘Original Treatises, dating from the XIIth to XVIIIth centuries on the Arts of Painting’ (note 1). Other significant contributions include that by Van de Graaf, in 1962, which focused on the difficulties in interpretation of the terms that might refer to glass in the documentary sources (note 2), and Kirby Talley’s comments on its possible function in his 1981 review of the English technical literature before 1700 (note 3). Until relatively recently, rather few scattered published examples where it had been observed in paint samples existed, generally in individual paintings and polychrome sculpture (note 4), although it had been reported to have been used in many early works by Gainsborough (note 5).
It is only in the last few years that it has come to be appreciated that colourless powdered glass was used extensively by artists all over Europe during the fifteenth and sixteenth centuries (note 6). The first paintings of this period in the National Gallery in which it was possible to establish with certainty, through analysis, that it was glass that was present in the samples rather than some other silicaceous material were those by Raphael. In ‘The Ansidei Madonna’ (NG 1171) the particles are particularly large, making it easy to see their characteristic angular shape, especially when the cross-sections of paint samples are viewed under ultraviolet light and by backscattered electron imaging in the scanning electron microscope (SEM).The particle size also made it possible to extract an individual particle from a paint sample from ‘The Mond Crucifixion’ (NG 3943) to view, in transmitted light under the microscope, the characteristic conchoidal fracture and stress lines of glass. Energy-dispersive X-ray analysis (EDX) of individual particles in the SEM showed a series of elements that are typical of glass. Silicon and oxygen are the major elements present, together with significant amounts of sodium, potassium and calcium and more minor quantities of magnesium, aluminium, phosphorus, titanium, manganese and iron.
These findings on paintings by Raphael were first presented in 2003 at a symposium on the materials and technique of paintings by Pietro Perugino, organised by the EU-funded project LabsTech. Perugino also regularly added colourless powdered glass to his paint, as can be seen not only from the National Gallery contributions to the symposium postprints, but also those from Martin and Rioux and from Seccaroni (note 7). In 2004, a workshop on Raphael’s working practices, organised by the follow on EU project Eu-ARTECH, expanded considerably the number of known occurrences of powdered glass in paintings by Raphael and other artists working in the same period (note 8). The circumstances of these conferences, held as part of cooperative European projects, led to unusually close interaction between authors, both before and after each conference, and an awareness among them of the possibility that powdered glass might be present. A large proportion of the published occurrences are therefore in works by these artists, a circumstance that needs to be borne in mind when considering the patterns of use of this material that they suggest. Quantitative elemental analysis of the glass particles, presented for the first time at the Raphael conference, allows them to be securely distinguished from the other silicaceous materials often encountered in paint, and also showed that in Italian works the composition is of the soda-ash type typical of Italy, made using marine plant ash as the flux. Most of the glass that had been found in northern European paintings at this time was instead made with wood or fern ash, following the same general geographical trends that were known from analyses of archaeological vessel glass. More examples in German paintings published by Lutzenberger et al. have since shown that soda ash glass, probably imported, can also be found in works from these areas (note 9).
This article will bring together and review the work of these earlier publications, and also significantly extend the number of examples of the use of colourless powdered glass in National Gallery paintings of the fifteenth and sixteenth centuries to more than 70. This allows a more expansive discussion of the patterns of use, and begins to provide a statistically significant basis for consideration of the relationship of the type of glass with the geographical area in which the work was made.