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Preparation of a paint sample for analysis by Fourier transform infrared microspectroscopy (FTIR)

The Painting Materials

To identify the materials used for the decoration of the chest, microscopic paint samples were analyzed in the Museum’s scientific laboratory using a range of instrumental and microscopy techniques. Some of the samples were prepared as cross-sections, by embedding them in cubes of a transparent resin. Once the resin is cured, the samples are cut and polished, and examined under a light microscope to reveal the sequence of layers. The same cross-sections can also be analyzed in the scanning electron microscope using an energy dispersive spectrometer. This instrument reveals the elemental composition of the sample – in the form of an x-ray spectrum – which helps to identify the pigments used. Other analytical techniques used to characterize the paint materials included Fourier transform infrared microspectroscopy and gas chromatography mass spectrometry.

Scientist Ken Sutherland examining a paint sample from the chest using the Museum’s scanning electron microscope (SEM)
The materials identified in the earliest paint layers were consistent with fifteenth-century painting practice, supporting the interpretation that remnants of the original painted decoration were present on the surface. A cross-section from an area of yellow paint in the coat-of-arms on the lid shows a typical sequence of layers: two preparation or “ground” layers were applied to the outer wooden surfaces of the chest before painting. The first was a coarse ground based on calcium carbonate, which would have served to fill and even out irregularities in the wood. The second layer was an application of fine gypsum, a form of calcium sulfate, to provide a smooth surface for painting. Both layers were applied in a protein-based medium, probably animal glue.

Cross-section sample from yellow paint in the coat of arms on the lid (left) and backscattered electron image of the same sample (right), showing the coarse calcite ground (1), fine gypsum ground (2) and paint layer (3)

Double-layered grounds are common in early Italian panel paintings, and their preparation is described in contemporary artists’ treatises. However, the use of calcium carbonate is unusual in Italy, where gypsum, or gesso, was more commonly used for both the coarse and fine ground layers. Examination of the calcium carbonate in the lower ground layer of the chest by scanning electron microscopy revealed irregular, angular particles characteristic of powdered marble. While not commonly described as a material for the preparation of panels for painting, marble dust did find use in Italian painting technique – particularly fresco – and so its use in place of coarse gesso is perhaps not surprising, and may represent a local or workshop practice.


Energy dispersive x-ray spectrum from a yellow paint sample: the presence of arsenic (As) and sulfur (S) are consistent with the use of the pigment orpiment (arsenic sulfide)

On top of the preparation layers in this sample are particles of orpiment, a mineral pigment based on arsenic sulfide. Orpiment is a bright, lustrous yellow pigment, and was noted in contemporary sources for its resemblance to gold. It was used – fittingly – for the rings and the background of the coat of arms. This and other pigments identified on the chest are typical for the fifteenth century. Madder, a red colorant derived from the roots of certain species of plants, was used for the background color of the chest and in the red lozenges on the banding. Lead white, a synthetic form of basic lead carbonate, was used in areas such as the banding and scrolls. Azurite, a copper-based mineral pigment, was used for details such as the diamonds on the rings. Vermilion, red lead, red earth and charcoal black pigments were identified in other parts of the painted decoration. The pigments were applied using egg as a binder, which is also typical for early Italian painting.

 

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