X-ray Fluorescence

Points in the charter’s surface were analysed with a portable XRF machine. The machine directs X-rays at an object’s surface, effectively knocking electrons out of the inner shells of the atoms present; electrons from the outer shells of the atoms then fill the vacancies left in the inner shells and this results in the emission of radiation – fluorescence – which is different for different chemical elements, allowing the elements present to be identified. These X-rays do not penetrate deeply, so the technique can only identify the elements on the surface of an object. For most materials, this analysis is qualitative – it can only identify which elements are present – but some materials, mainly metallic objects, can be characterised quantitively – the proportions of the elements present are discovered allowing the identification of alloys for example (del Hoyo Melendez, 2017, pp. 257-258).

 

XRF analysis of the text on the charter found iron and sulphur were present. There are hundreds of recipes for iron-gall ink, but iron sulphate is a main component of all of them (Duh et al., 2018, p. 96), so this result indicates the ink is iron-gall ink. If used quantitively, different kinds of iron-gall ink can be distinguished with XRF by measuring the proportions of metal impurities, but this requires a well calibrated setting that was not available at the time (Tonazzini et al., 2019 p. 33).

Copper was found in the original wax seal, the green thread and the metallic thread. A copper based green pigment was probably used to colour the wax and thread. The metallic thread was probably made with a copper alloy, hence its dullness – copper easily develops a dull patina or corrodes in comparison to an unreactive metal like gold which remains shiny.

A reading should have been taken of the blank parchment, as XRF can detect elements like calcium which indicate a liming process (soaking the skin in water and lime to remove hair) was used to produce the parchment (Oubelkacem et al., 2020, p. 176). Other elements can be detected on the parchment such as iron and other metals from tools used during manufacture, and from dirt, so it is important to take a background measurement which can be subtracted from other measurements to prevent false results.

 

X-ray diffraction (XRD) is another technique which uses X-rays to identify the composition of a material but is able to identify specific compounds making it useful for complex materials like manuscripts: portable XRD machines are available which similarly to XRF do not harm the object (Chiari et al., 2016, p. 15). XRD could have characterised the specific compounds in the charter, allowing for more detailed deduction of manufacturing techniques.