Back to the stunning pair of Angels from Blo Norton Hall Chapel – see the first post in the series here.
Before proposing a suitable conservation treatment for the Angels, we needed to better understand how they had been made and the causes of the dramatic flaking of the gilded layers on both Angels.
An initial examination showed that they were covered by several layers of gilding and that the detachment of the surface appeared to be at the same layer on each of the Angels, revealing the mordant of an older gilding layer. The high degree of damage and the shiny/waxy appearance of the uncovered mordant suggested that some change had occurred between the two gilded layers, causing the detachment of the outer gilded layer. It also appeared that the mordant was composed of wax, rather than animal glue, which is a more commonly used gilding technique. Further analysis could help to confirm this, and would give a better understanding of the composition of the successive layers added to the surface, and an idea of the type of gold used – was it pure gold leaf or an alloy?
Angel A was investigated by using a combination of techniques, such as micro-chemical spot testing of samples (flakes which had fallen off the statue), Fourier transform infrared (FTIR) spectroscopy, observation under ultra violet (UV) fluorescent light, thermo-microscopy and analytical studies of two cross-sections under the polarised light microscope (PLM) and scanning electron microscopy with energy dispersive x-ray spectrometry (SEM-EDS). Each of these analytical techniques enables us to characterise the materiality of the object with different degrees of accuracy. Correlating the results of the different analytical techniques would help us determine how the Angels had been made.
In this post we will talk specifically about the polarised light microscope (PLM) and the scanning electron microscopy with energy dispersive x-ray spectrometry (SEM-EDS), which are two amazing techniques that, when combined, allow us to get accurate results.
Two samples, taken from different areas of the statue, were prepared in cross-section. The cross-sections were first embedded in a resin and observed under a polarised light microscope.
The PLM gives precise information about the thickness and the number of layers present in the polychrome surface. Moreover, it is possible to identify the number of gilding layers and so to give the number of gilding campaigns carried out on the statue. The use of the SEM means that the morphology of each layer can be studied under higher magnification than with PLM. Also, the EDS can provide an elemental analysis of the sample and therefore identify and precisely locate the inorganic elements present in each layer.
The investigation revealed the presence of three gilding layers, each composed of five distinctive layers. The gold leaf layer was made of gold alloy of copper and silver. The fillers were made of calcium carbonate, which was occasionally found mixed with clay. Furthermore, the presence of wax in the two mordant gilding was confirmed, mixed with an unknown component. Finally, the use of white lead pigment was identified alongside iron oxide. The difficulty of adhering gold leaf to a wax surface, combined with fluctuating environmental conditions over long periods of time could have been the reason for the extensive detachment at this level.