Preparation of a paint sample for analysis by Fourier transform infrared microspectroscopy (FTIR)
The Painting MaterialsTo 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.
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.
Scientist Ken Sutherland examining a paint sample from the chest using the Museum’s scanning electron microscope (SEM)
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)
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)