Byne’s decay on microscopic calcareous shells: case study of Alcide d’Orbigny’s foraminifera collections stored in Paris and La Rochelle, France

The foraminifera collection of micropalaeontology pioneer Alcide Dessalines d’Orbigny (1802-1857), housed at the National Museum of Natural History (MNHN), Paris, was recently diagnosed with Byne’s decay. The condition report presented here establishes that at least half of the foraminifera is affected, with numerous types seriously damaged. A comparison with specimens from the Museum of La Rochelle, also collected by d’Orbigny, and the study of the MNHN archives indicate that the decay is related to the type of mounting and to the poor environmental conditions of the storage. The emergence of the salts appears linked with a pollution of Volatile Organic Compounds (VOCs) emitted by the wood-derivative materials introduced during the XIXth century (paper and cork) and unappropriated temperature and humidity fluctuations. The study of the damage highlights a correlation between foraminifera sampling site and their current condition, recent to sub-recent specimens collected in marine environment showing more intense degradations than fossil ones. The nature of foraminifera test seems to have also an influence on the degradation, porcelaneous foraminifera being more sensitive to salt crystallizations than hyaline ones. Non-invasive Raman spectrometry enabled to identify degradation products. Calcium acetate or mix acetate-formate salts, that are the most common Byne degradation products, were not detected. Instead, the two polymorphs of calcium formate [the orthorhombic α-Ca(HCOO)2 and the metastable tetragonal β-Ca(HCOO)2] are predominantly present, occasionally with magnesium formate dihydrate [Mg(HCOO)2∙2H2O] and calcium lactate pentahydrate [Ca(CH3CH-OH-COO)2∙5H2O]. The glass tubes that were exposed to the VOCs are also damaged by crystallizations of sodium formate anhydrate [NaHCOO(II)]. Deeper investigations must be undertaken to better understand the occurrence of these unusual degradation phases and the non-detection of acetates.