By Bindia Venugopal
In this x-ray fluorescence imaging research project, we imaged and analysed both mummy cartonnage and phantoms using a portable handheld x-ray fluorescence (pXRF) machine, borrowed from the Institute of Archaeology at UCL. as part of a Deep Imaging project funded by the Arcadia Fund. It was important to characterise the pXRF to suit the application. We found that the pXRF settings in ‘SOIL mode’ reflected our application the best. The pXRF can detect the elemental composition (to parts per million) in the region of interest.
When we imaged cartonnage lent by the Petrie Museum, we found that the blue pigments were made up predominantly of the element Copper (Cu) at ~15,200ppm (1.52%). By investigating the types of pigments used by Ancient Egyptians, we could see that this blue colour most likely originated either from Azurite or Egyptian Blue which have a chemical composition of Copper(II) Carbonate and Calcium Copper(II) Silicate respectively. Similarly, the red pigments were found to be made up of 19,700ppm (1.97%) of Iron (Fe). This suggested the pigment was either Haematite or Umber.
Test phantoms are a great tool to compare the effects of different imaging techniques. For this project, we used four existing test phantoms that were created with known properties by team member Kathryn Piquette – in this case the composition and placement of inks were predetermined. The pigments were iron oxide ink; iron gall ink; carbon ink; and Indian ink. The phantoms were imaged with the pXRF to identify and validate the presence of these inks. During the testing phase, we found that the pXRF could detect Fe in the iron oxide and iron gall, but failed to detect the element carbon in the carbon ink and the Indian ink. This is because the carbon was out of the detection range of the pXRF system. Therefore, any carbon in mummy cartonnage will need to be identified by different methods of imaging.
By imaging the phantoms, we could quantify the concentration of the pigments with respect to the thickness of the sample. Since the placement of the inks varied across the four quadrants of the phantom, it meant that we observed a reduction in concentration for the iron oxide and iron gall phantoms as the number of layers of papyrus above the ink increased. Additional tests with further layers of modern papyrus above the phantom resulted in a conclusion that it took approximately 9 layers of modern papyrus above the ink before which the pXRF could no longer confidently detect the concentration of Fe.
Using the pXRF as an imaging modality could prove useful in many cultural heritage studies due to its portability and non-invasive nature. However, efforts must be made to further calibrate the system so that it can be used specifically for such an application. Moreover, since the pXRF is currently a qualitative technique, in the future, we should look to achieve quantifiable results by devising the optimum methodology for pXRF imaging in cultural heritage.