Subsurface Analysis of Oil Paintings by Means of Scanning Macro-XRF

From the point of view of a material scientist, easel paintings can be considered as extended and complex multilayered objects, where a variety of materials of different density and physico-chemical properties is applied on top of each other in various layers. When a painting contains only one pictorial (i.e., visual information carrying) layer, the number of strata can be limited to three or four, i.e., (a) the substrate material (e.g., canvas, wood or metal), (b) a ground layer (e.g., a glue-chalk mixture or a glue-gypsum base), (c) the actual pictorial layer and (d) an optional varnish layer. In many paintings, the pictorial layer itself is composed of more than one superimposed level in order to achieve an particular optical effect or because the artists altered or corrected his own design while first creating the work. Quite a few paintings also contain more than one pictorial layer; this is the case for paintings that, immediately upon their creation or several years/decades afterwards, were painted over to re-use the canvas or panel. Usually it concerns paintings from the early (or earlier) period of a painter with which the artist grew dissatisfied in later stages of his career so that a new set of ground+pictorial layers were used to cover up the layers already present. For these reasons, many contemporary art-historians, on the other hand, are interested to obtain information on the covered-up paintings