Pigments identification and analysis of the state of conservation of the decorative elements of the castle of Coca (XV-XVI AC, Segovia, Spain) by Raman Spectroscopy

Abstract The castle of Coca (Coca, Segovia, Spain) is one of the summits of the Gothic-Mudejar architecture on the Iberian Peninsula. Among other extraordinary features, this castle presents unique Mudejar decorative elements, which have been partially lost due to this abandonment. In addition, the remaining elements are in danger due to ongoing degradation processes. However, there is no available information about the pigments employed in these decorative elements, nor any analytical study about their state of conservation. This study applies the Raman spectroscopy technique to identify, for the first time, the pigments and ongoing degradation processes on these endangered decorative elements of extraordinary cultural and historical relevance. Red hues were achieved using hematite, whereas the black hues come from the use of carbon black of vegetal origin. Additionally, dispersed carbonaceous particles were also generally found in the walls of the castle, probably related to the use of candles or torches to illuminate the rooms of the castle. Finally, monohydrate and dihydrate calcium oxalates (i.e., whewellite and weddellite, respectively) were detected as degradation products, probably related to biological processes.