SWIR Reflectance Imaging Spectroscopy and Raman Spectroscopy Applied to the Investigation of Amber Heritage Objects: Case Study on the Amber Altar of the Lord’s Passion

The chosen object from the National Museum in Kraków collection (the Amber Altar of the Lord’s Passion) was examined by Raman spectroscopy (RS) and reflectance imaging spectroscopy (RIS) in short wavelength infrared (SWIR) spectral range. These studies allowed to identify the type of amber used and to assess the existence of a degraded layer. Based on the analysis of the reflection spectra and using the NMF (Nonnegative Matrix Factorization) algorithm, the amber of the Altar were assigned to two different classes. It may be associated with the use of different technologies for the preparation of amber plates. Moreover, as a result of the PCA (Principal Component Analysis) dimension reduction algorithm, an initial classification of the object’s amber into succinite (both native and German) and/or gedan-succinite and/or glessite was performed. Analysis of Raman spectra obtained by deep profiling method allowed to state the existence of a degraded layer on the surface of the amber figures of Saints Peter and Paul. The change in the spectrum profile depending on the depth of the signal collection indicates, among others, changes in the structure related to the exomethylene group and changes in the CH $$_{2}$$ and CH $$_{3}$$ groups. Thanks to the analysis of reflectance imaging spectroscopy and Raman spectroscopy, it was possible to identify the places of occurrence of amber imitations. The bands distinguished in the Raman spectra indicate the use of an epoxy resin.