Detecting emerald green in 19thC book bindings using vis-NIR spectroscopy

Detection and identification of heavy metal-based pigments in 19th-century bookbindings is crucial to avoid human user exposure to toxic substances. Vibrant green bookbindings with arsenical emerald green are particularly problematic due to their friability. A pilot study at St Andrews University tested 800 green bookbindings for arsenic presence using visible near-infrared spectrometry, a technique not previously applied to the detection of heavy metals in bindings. The ASD TerraSpec Halo portable spectrometer that is normally used in geology to identify minerals in rocks, is used here to collect hyperspectral reflectance data between 350 and 2500 nm. Raman spectroscopy and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) are used here to validate hyperspectral test results. The study finds that bookbindings containing emerald green have a distinctive pattern in the visible part of the spectrum that is distinguishable from other green pigments. This finding opens up the possibility for all collecting institutions to test bindings for this toxic compound in a non-destructive, cost-effective and efficient manner. Emerald green containing bookbindings have a distinctive spectral reflectance signature in the visible region of the spectrum.