“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, mistakes persist, 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, allows for a simple and straightforward differentiation of these materials. Our technique enables rapid lithological identification of complex composite sculptural ensembles. We illustrate this through two case studies: The 15th century Saint Catherine of Alexandria from Kortrijk, attributed to André Beauneveu, one of the most prominent artists of the late Middle Ages, was supposedly made of alabaster, but is in fact made of marble and restored with alabaster replacement parts. The tomb of Prince-Bishop Julius Echter in Würzburg Cathedral is an example of the variety of materials used for such monuments in the 17th century. Here we highlight a previously undocumented but extensive use of multi-coloured alabaster.