Corrosion of Heritage Objects: Collagen-Like Triple Helix Found in the Calcium Acetate Hemihydrate Crystal Structure
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Updated: Dec 28, 2025

X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects
Published on: June 8, 2016
Sebastian Bette1,2, Jörg Stelzner3, Gerhard Eggert2
1Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569, Stuttgart, Germany.
Abstract:
Helical motifs are common in nature, for example, the DNA double or the collagen triple helix. In the latter proteins, the helical motif originates from glycine, the smallest amino acid, whose molecular confirmation is closely related to acetic acid. The combination of acetic acid with calcium and water, which are also omnipresent in nature, materializing as calcium acetate hemihydrate, was now revealed to exhibit a collagen-like triple helix structure. This calcium salt is observed as efflorescence phase on calcareous heritage objects, like historic Mollusca shells, pottery or marble reliefs. In a model experiment pure calcium acetate hemihydrate was crystallized on the surface of a terracotta vessel. Calcium acetate hemihydrate crystallizes in a surprisingly large unit cell with a volume of 11,794.5(3) Å3 at ambient conditions. Acetate ions bridge neighboring calcium cations forming spiral chains, which are arranged in a triple helix motif.
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