Unusual crystals in wet collections. Surfacing degradation products in 17th and 18th-century fluid-preserved anatomical preparations

• Anatomical collections in fluid pose several challenges from the conservation viewpoint. • Anatomical collections in fluid are understudied and have not been often investigated using standard analytical techniques. • Ruysch’s fluid-preserved specimens preparation methods were confirmed by evidence from molecular spectroscopy investigation. • Metal carboxylates also form in fluid-preserved anatomical collections as degradation products. This paper presents the analytical investigation of white crystals and sediments coming from five different anatomical preparations by F. Ruysch and his disciple B. S. Albinus, dating from the 17th and 18th centuries. µATR-FTIR and µRaman spectroscopy campaigns revealed the presence of characteristic metal carboxylate spectral features. In particular, calcium stearate and palmitate, formed by reaction of metal ions and fatty acids from bones and adipose tissues, respectively, were the most prominent compounds identified. The mechanism of the formation of this degradation products is outside of the scope of this study. However, this phenomenon can be compared to that occurring in easel painting, where metal carboxylates form due to the interaction of metal-based pigments and oil binders. In addition, the analyses confirmed the presence of some ingredients of Ruysch’s famous anatomical preparations, such as tallow and vermilion, that are mentioned in historical documents but have not been previously investigated analytically.