“And Hence Have Been a Thousand Mistakes”. Marble or Alabaster? Resolving an Old Problem of Material Identification with Ultra-Portable Near-Infrared Spectroscopy

Gypsum alabaster as material for European sculpture emerged in the 12th century and soon rivalled marble due to its accessibility, ease of sculpting, and aesthetic qualities. Lack of clear terminology and the visual similarity of the two materials have led to a considerable amount of confusion and deliberate misnomers. Despite attempts, since early modern times, to make a clear physical and chemical distinction between both materials, the confusion persists, even in modern collections. Here we present a non-invasive, cost-effective, reliable technique to differentiate the two, using an ultra-portable near-infrared spectrometer. The characteristic NIR spectrum of gypsum alabaster over the range of 900-1700 nm strongly contrasting with the near-featureless spectra of marble and limestone, allows for a simple and straightforward differentiation of these materials. Distinction of marble and anhydrite alabaster is less evident but appears possible due to the frequent partial hydration of natural anhydrites, leading to weak but characteristic features in the 1200 nm and 1400-1500 nm ranges. As demonstrate our two cases, the 15th century Kortrijk Saint Catherine of Alexandria and the 17th century funeral monument of Prince-Bishop Julius Echter in the Würzburg Cathedral, our technique enables rapid lithological identification of complex composite sculptural ensembles.