Paint and Pigment Analysis
SASAA has been involved in the analysis of pigments in early and medieval wall paintings using a combined X-ray diffraction and scanning electron microscopy (SEM-EDAX) methodology.
Analyses of pigments in wall paintings from a 14th-16th century Byzantine monastery in N Greece has revealed that this type of pictorial technical tradition (see pictures above) relied on the layer-by-layer application of pigments, rather than the mixing thereof, to achieve the right hue. This practice can be examined with the SEM particularly effectively since the chemical identity of each layer can be discerned, on the backscattered image. In the SEM-BS picture (below left), alternating layers of iron oxide (dark grey) are sandwiched between layers of mercury sulphide (bright grey).
Given the ability of the SEM to differentiate paint layers on the basis of chemical composition rather than colour, the method of preparation of the substrate, be plaster, metal or wood, can also be elucidated.
The SEM-SE picture (above right) shows smalt – a cobalt-rich silicate glass. The angular sharp grains of fritted glass adhere onto the surface with the use of an organic binder. A layer of lamp black has been applied on the plaster surface prior to the application of smalt.
In the course of recording and restoring historic buildings, the question often being asked is: what was the original colour of the painted surface?
At SASAA we are able to assist the specialist by providing layer-by-layer elemental analysis.
Layer-by-layer qualitative analysis can clearly differentiate between phases of work corresponding to periods of renovation when potentially new pigments come into use and others go out of fashion. In the above example lead-based white pigments have been substituted by another “white” consisting of barium sulphate and zinc sulphide, known as lithopone.