Efflorescence on calcareous objects in museums: crystallisation, phase characterisation and crystal structures of calcium acetate formate phases

Abstract

By congruent crystallisation from aqueous solution the efflorescence phase Ca(CH3COO)(HCOO)·H2O found on calcareous heritage objects and the hitherto unknown Ca3(CH3COO)4(HCOO)2·4H2O were obtained, characterised and their crystal structures solved.

The formation of formates: a review of metal formates on heritage objects

“…As a degradation product, it does not seem to be rare [48]. The formula was confirmed by our crystal structure determination from XRPD [52]. Unpublished reports from the British Museum laboratory also detected this compound on Greek and Roman ceramics [53] and marble [54], and we found it on a vulture eggshell in the Natural History Museum in Stuttgart.…”

“…Unpublished reports from the British Museum laboratory also detected this compound on Greek and Roman ceramics [53] and marble [54], and we found it on a vulture eggshell in the Natural History Museum in Stuttgart. In lab experiments [52], the compound forms from equimolar solutions of calcium acetate and formate. If the acetate to formate ratio is increased to 2:1, the new compound Ca 3 (CH 3 COO) 4 (HCOO) 2 •4H 2 O precipitates at even higher acetate ratios together with Ca(CH 3 COO) 2 •H 2 O [52].…”

“…In lab experiments [52], the compound forms from equimolar solutions of calcium acetate and formate. If the acetate to formate ratio is increased to 2:1, the new compound Ca 3 (CH 3 COO) 4 (HCOO) 2 •4H 2 O precipitates at even higher acetate ratios together with Ca(CH 3 COO) 2 •H 2 O [52]. The compound has so far not been reported on heritage objects that only show Tennent's salt together with calcium acetate monohydrate [51,52].…”

Corrosion of Heritage Objects: Collagen‐Like Triple Helix Found in the Calcium Acetate Hemihydrate Crystal Structure

“…The acetate originates from wooden furniture used for storage or display as wood emits considerable amounts of formic and acetic acid or its aldehyde precursors . Accordingly, calcium acetates, mixed acetate formates, and acetate nitrates were identified as efflorescence salts. Their crystal structures contain characteristic calcium carboxylate zig‐zag chain motifs which also enable the intercalation of additional water molecules or chloride ions in between these chains .…”

Korrosion von Kulturgut: Entdeckung einer kollagenartigen Tripelhelix in der Kristallstruktur von Calciumacetat‐Hemihydrat

“…deren Aldehyd‐Vorstufen abgeben . Dementsprechend wurden Calciumacetate, gemischte Acetat‐Formiate und Acetat‐Nitrate als Ausblühungssalze identifiziert. Deren Kristallstrukturen enthalten charakteristische Motive von Calciumcarboxylat‐Zickzack‐Ketten, die auch die Einlagerung zusätzlicher Wassermoleküle oder Chloridionen zwischen den Ketten erlauben .…”