Cobalt absorption bands for the differentiation of historical Na and Ca/K rich glass

The main purpose of this publication is to define sensing parameters by means of optical spectroscopy to study and characterize ancient glass. Because during different times different flux agents were used, a chemical individuation of these elements provides a first dating classification. Normally, X-ray-based chemical methods are applied to this kind of investigation. On the basis of the Ligand-field theory, the absorption band positions of a transition metal ion undergo a change when the ion is placed in different chemical environments. A total of 25 cobalt coloured Roman, post-medieval and industrial glasses were analyzed by means of SEM-energy dispersive X-ray spectroscopy to group them according to their chemical composition. Optical spectroscopy was applied to build a method to study glass with this low-cost and non-destructive technique. It is proved that the cobalt absorption band situated around 535.5 nm for sodium-rich glasses (Roman and industrial material) is shifted towards 526.5 nm for calcium-potassium-rich glasses (post-medieval window glass). This feature was employed as a marker in the case study of the Renaissance window of the Church Wardens from the Church of Our Lady in Bruges, Belgium to distinguish the post-medieval material from the 19th and 20th century restoration material. Copyright (C) 2011 John Wiley & Sons, Ltd.