Brass and Glass: Crystal Structure Solution and Phase Characterisation of the Corrosion Product Zn₄Cu₃(Zn_1–xCu_x)₆(HCOO)₈(OH)₁₈·6(H₂O)

Abstract

The title compound, Zn4Cu3(Zn1–xCux)6(HCOO)8(OH)18·6(H2O), also denoted as “Zinc C”, a previously unknown corrosion phase of brass in contact with glass, was found on four different historic objects stored in three different collections. Model experiments demonstrated that formaldehyde emitted e.g. from wooden cabinets undergoes a Cannizarro reaction in the alkaline surface film on the historic glass making formate available for the formation of this corrosion phase. The crystal structure of “Zinc C” was solved ab initio from X‐ray powder diffraction data. From the crystal structure and EDX analyses, a phase composition of Zn4Cu3(Zn1–xCux)6(HCOO)8(OH)18·6(H2O) was derived. Zinc C crystallizes in a trigonal unit cell in space group R3(148) with lattice parameters of a = 10.9185(1) Å, c = 27.5058(4) Å, and V = 2839.74(6) Å3. Layers of edge sharing MO6/3 octahedra with voids capped by Zn(HCOO) forming Zn(HCOO)(OH)3/3 tetrahedra are the main motif in the structure. The zinc/copper ratio of the corrosion phase “Zinc C” can vary between 10:3 and 4:9, due to a partial occupational disorder between copper and zinc in the crystal structure.

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

“…Most of the furniture used for storage or display has been wooden and wood, especially oak, and some glues emit formic and acetic acid or its aldehyde precursors. 5 This leads to the formation of acetate and formate salts during the process of glass induced metal corrosion [6][7][8][9] or to a direct attack on calcareous objects. The occurrence of acetate-containing efflorescence phases on calcareous objects has been known as "Byne's disease" since the end of the 19 th century.…”

“…Hence, attempts were made to produce the corrosion phase artificially in order to obtain a pure substance for a detailed characterisation, as this approach already has been successfully applied to historic verdigris pigments 30,31 and metal corrosion phases. 2,7 For this purpose, the solid phase formation in the ternary system Ca (CH 3 COO) 2 4 (HCOO) 2 ·4H 2 O, were synthesised as pure substances. The crystal structures of the solid phases were solved from XRPD data and the substances characterised by vibrational spectroscopy and thermal analysis.…”