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Colourless Powdered Glass as an Additive in Fifteenth- and Sixteenth-Century European Paintings
Selection of data presented in support of the Spring article from Technical Bulletin Volume 33.
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Technical Bulletin Vol. 33, Powdered Glass article
Fifteenth- and sixteenth-century paintings in which colourless powdered glass has been identified in paint samples, together with quantitative SEM–EDX analysis of the particles, normalised and expressed as weight % oxide (not including elements present at values lower than 0.1%). Updates since publication of the article appear in green.
a. Analysis carried out on carbon-coated samples under high vacuum conditions, 200 pA, 25 kV, 60s livetime.
b. Analysis carried out on uncoated samples under variable pressure conditions, 200 pA, 25 kV, 60s livetime, 50 Pa, air as the chamber gas.
c. Particles that seem to be a pure form of silica, containing only small amounts of other elements, were also detected in this sample.
d. The apparent elevated sulphur concentration in this sample is probably not reliable and likely to result either from a sulphur-containing red lake pigment or, due to overlap of the sulphur peak with that from lead, a contribution to the spectrum from lead pigments around the glass.
e. Red lake pigments, especially those based on insect dyestuffs, often contain phosphorus. This could contribute to the apparent concentration in the glass, especially where the particles are small.
f. The binding medium of many of these paintings has been analysed by Gas Chromatography–Mass Spectrometry and identified as a drying oil. These analyses have generally been published in past issues of the National Gallery Technical Bulletin.
g. In this sample the glass particles are very small and are embedded in a matrix of red lake, with a substrate containing alumina. The interaction volume of the electron beam is likely to be larger than the glass particle and the apparent aluminium concentration too high.
h. The quantitative results quoted are from a particle in the priming layer, which does not contain red lake, or from a large particle in red lake paint; the high reported Al content is therefore reliable.
i. A dispersion of a paint sample allowed individual glass particles to be seen in transmitted light under the microscope. They could be seen to have angular edges and showed conchoidal fracture, confirming that they were glass, despite the rather low alkali content seen by EDX analysis, which is a result of severe leaching. See the Appendix in J. Dunkerton and H. Howard, ‘Sebastiano del Piombo’s Raising of Lazarus: A History of Change’, National Gallery Technical Bulletin, 30, 2009, pp. 26–51.
j. Details updated after publication, 2012.
| School | Date Range | Artist, painting title, date and locationf | Areas where powdered glass was found and estimated particle size | glass type | Na2O | MgO | Al2O3 | SiO2 | P2O5e | SO3 | Cl | K2O | CaO | TiO2 | MnO | FeO |
|---|
a. Analysis carried out on carbon-coated samples under high vacuum conditions, 200 pA, 25 kV, 60s livetime.
b. Analysis carried out on uncoated samples under variable pressure conditions, 200 pA, 25 kV, 60s livetime, 50 Pa, air as the chamber gas.
c. Particles that seem to be a pure form of silica, containing only small amounts of other elements, were also detected in this sample.
d. The apparent elevated sulphur concentration in this sample is probably not reliable and likely to result either from a sulphur-containing red lake pigment or, due to overlap of the sulphur peak with that from lead, a contribution to the spectrum from lead pigments around the glass.
e. Red lake pigments, especially those based on insect dyestuffs, often contain phosphorus. This could contribute to the apparent concentration in the glass, especially where the particles are small.
f. The binding medium of many of these paintings has been analysed by Gas Chromatography–Mass Spectrometry and identified as a drying oil. These analyses have generally been published in past issues of the National Gallery Technical Bulletin.
g. In this sample the glass particles are very small and are embedded in a matrix of red lake, with a substrate containing alumina. The interaction volume of the electron beam is likely to be larger than the glass particle and the apparent aluminium concentration too high.
h. The quantitative results quoted are from a particle in the priming layer, which does not contain red lake, or from a large particle in red lake paint; the high reported Al content is therefore reliable.
i. A dispersion of a paint sample allowed individual glass particles to be seen in transmitted light under the microscope. They could be seen to have angular edges and showed conchoidal fracture, confirming that they were glass, despite the rather low alkali content seen by EDX analysis, which is a result of severe leaching. See the Appendix in J. Dunkerton and H. Howard, ‘Sebastiano del Piombo’s Raising of Lazarus: A History of Change’, National Gallery Technical Bulletin, 30, 2009, pp. 26–51.
j. Details updated after publication, 2012.
