Preventive conservation of amber: some preliminary investigations. Pyrite Decay in Amber: Deterioration of Collections and Conservation Guidelines

Abstract Fossilised plant resin (amber) collections represent invaluable sources of information about the past. However, amber is particularly sensitive to exposure to light and oxygen, as well as changing temperature and humidity, rendering its preservation a major challenge. Here we reveal significant degradation of raw and carved amber specimens from part of the Early Cretaceous amber collection of Rábago-El Soplao (Cantabria, Spain), extracted more than a decade ago. These amber specimens, which originated from resin namely formed under subaerial conditions and so generally lack bioinclusions, are of remarkable importance from the palaeoecological, taphonomic, and physicochemical standpoint. Deterioration in amber pieces is due to pyrite decay resulting from the oxidation and hydration of iron sulphides in sedimentary rock remains intimately associated with the amber. We structurally, chemically and morphologically characterise hydrated-sulphate efflorescences found adhered on or included in amber as thin veins or nodules. Clays from sedimentary rock remains associated with the amber play a dual role: they react with acid derived from the dissolution of previous pyrite or sulphates to provide Al, Si, and K that are incorporated into the new sulphates, and they reinforce the alteration of pyrite by hygroscopically capturing water. We also provide guidelines for preventing and remedying pyrite disease in amber specimens, including treatment for new specimens entering collections, optimal physicochemical conditions for storage, and conservation for mildly-altered specimens. Our results urge putting measures in place to guarantee the integrity of the raw and polished amber pieces from the Rábago-El Soplao collection, and are more broadly relevant for the conservation of amber in palaeontological, mineralogical, gemological, and archaeological contexts.