A methodology for the identification and classification of weighted silk in museum collections using portable X-ray fluorescence spectroscopy (PXRF).

In the 19th century in Europe and North America, silk was often treated with inorganic salts to increase its weight. Although such silk fabrics, often referred to as "weighted" or "loaded", are considered to degrade rapidly, their presence in heritage collections often remains unclear. So far, the fragility and multilayered structure of garments have prevented swift identification and classification of weighted silks, as often objects cannot be placed in the vertical position for analysis, nor can superimposed layers of fabric be measured selectively. In this article, a non-invasive approach for elemental analysis using portable X-ray fluorescence spectroscopy (PXRF) in combination with a beamstop is proposed, which can be easily employed in a depot setting. This setup was designed and tested on certified standard reference materials containing 13 marker elements (Fe, Sn, P, Si, Al, Pb, Zn, Cr, Cu, Ca, K, Cl, Mg), fabrics reproducing the six prevalent historical tin-, iron- and mixed weighting methods and validated on three historical objects. The obtained results show that PXRF measurements with Zr beamstop and a plastic filter allow not only the identification and differentiation of heavily from lightly weighted and non-treated fabrics, but also the classification of the weighting method on four levels of complexity using a proposed decision tree. As the weighting class is assumed to affect the conservation state of textiles, such monitoring will assist in prioritisation for preservation treatments, deepen understanding on the relative presence of weighted silk in collections and trace their ongoing salt-induced degradation.