Using UV-Vis-NIR absorption spectroscopy as a tool for the detection of iron and cobalt in glass: A case-study on HLLA material from the Low Countries

The early modern period is mainly characterised by High Lime-Low Alkali (HLLA) glass, a lime-rich glass made from plant or tree ashes, originating from 14th-century Germany and later used all over Europe. Attempts to link HLLA chemical subgroups to a particular point in time have failed so far. Driven by a request from the archaeological community, our research group has been exploring the feasibility of UV-Vis-NIR absorption spectroscopy as a non-destructive and in-situ applicable analysis technique for more than a decade. The main goal of this research is to provide a better understanding of the subgroups of HLLA glass and their respective dating via non-destructive means. A second topic relates to the role of cobalt as a potential indicator for either the type of applied ashes or for glass recycling. In this paper, we present the UV-Vis-NIR and LA-ICP-MS results of a total of 45 HLLA colourless glass objects or fragments, supplemented with three blue glasses from the 15th to the 17th centuries. First, we observed a correlation between iron and cobalt contents and that these two elements show more elevated concentrations starting from the middle of the 16th century. Different types of sources can be responsible, such as the type of raw materials, the tools employed, and recycling. We hypothesised that both elements were introduced through the wood or plant ashes used as fluxing agent. Then, we noted that the presence of low amounts of cobalt in colourless glass, can be observed through the analysis of the Co2+ absorption bands in the optical spectra down to 18 ppm. Next, we describe an approach to quantify iron and cobalt from the optical spectra and to date HLLA material by means of absorption spectroscopy, a valid, rapid, and non-destructive alternative for laboratory measurements. Finally, we show that the optical parameters (the Ultra-violet Absorption Edge (UVAE), the calculated Fe2+ and total iron contents, and the Co2+ absorption bands characteristics) allow discerning two glass groups: one dating to the end of the 15th-middle of the 16th centuries, and the other to the second half 16th-17th centuries. These results gain valuable information to further resolve the study on recycling, on the influence of the glassmaking tool on the glass batch composition and/or on the selection or treatment of the raw materials in HLLA glass.