From MA-XRF to MA-XRD and MA-FTIR: A Survey of Possibilities and Limitations

Recently, several new (variants of) methods for non-destructive selective imaging of painted works of art at the macroscopic level, based on radiation in the X-ray and infrared range of the electromagnetic spectrum have been developed. Such methods allow to either record depth-selective, element-selective or species-selective images of entire paintings. One can distinguish between camera-based ‘full field’ methods (that record the image data in parallel) and scanning methods (that build up distribution images in a sequential manner by scanning a beam of radiation over the surface of an artefact).

Macroscopic X-ray fluorescence analysis (MA-XRF) is a variant of the general XRF method that is well suited to visualize the elemental distribution of key elements, mostly metals, present in areas of painted artworks of 0.5-1 m² or more. This method is not depth-selective so that projected pigment distributions (present at and/or below the visible paint surface) are obtained, a property that has both advantages and disadvantages. Examples from 15^th to 19^th C. paintings will be used to illustrate this point.

A fundamental limitation of MA-XRF stems from the fact that XRF only provides information that allows distinguishing between different chemical elements (such as iron, copper or lead), but does not permit to see the more subtle difference between, e.g. two different lead-oxide pigments such as minium (Pb₃O₄) and litharge (PbO). By performing macroscopic scans while other signals than X-ray fluorescence emission are recorded, this limitation may be circumvented.

Scanning macroscopic X-ray powder diffraction (MA-XRD) mapping of the distribution of pigments in mockups and original paintings via the use of highly energetic synchrotron radiation has been demonstrated to allow for the highly specific identification and visualization of many pigments, even those containing the same characteristic metals (e.g., Fe in hematite and goethite), provided the angular resolution of the setup is sufficiently high. An additional advantage is that at high energy, absorption of the primary and of the diffracted beams is virtually negligible. Recently, this MA-XRD capability was successfully transferred from synchrotron facilities to the laboratory by making use of a combination of a compact mirror-focussed X-ray source, emitting monochromatic Ag-K_α radiation of 22 keV and a single photon counting diffraction camera. The possibilities and limitations of this approach will be compared to those of MA-XRF.

On the other hand, by means of infrared radiation, either in the NIR or in the MIR ranges, camera-based or scanning based reflection mode imaging can also be performed. The information obtained in this manner is often complementary to that obtained by means of the X-ray based methods.  The combined use of MA-XRF/XRD scanning with NIR/MIR hyperspectral imaging or MA-rFTIR scanning appears to be a very promising new direction for non-invasive imaging of paintings.