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

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.

Glass-Induced Lead Corrosion of Heritage Objects: Structural Characterization of K(OH)·2PbCO3.

The investigation of the corrosion of a lid made from a tin-lead alloy of a 200 years old beer jug induced by the degradation of the potash based glass revealed SnO, Cerussite (PbCO3) and K(OH)·2PbCO3 as main corrosion product. A model experiment, simulating the corrosion of lead at room temperature confirmed the formation of K(OH)·2PbCO3 as a corrosion product in alkaline, potassium containing medium. For detailed characterization K(OH)·2PbCO3 was prepared by hydrothermal synthesis, as well. K(OH)·2PbCO3 crystallizes in space group P63/mmc with lattice parameters of a = 5.3389(1) Å and c = 13.9295(5) Å. The structure consists of Pb(OH)1/2(CO3)6/9[CO3]3/91/2- layers and intercalated K+ and exhibits a close relationship to the crystal structure of hydrocerussite (Pb(OH)2·2PbCO3), also known as "lead white". A novel structure family, Mn+(OH)n·2PbCO3 (with n = 1,2), was identified by structure solution of K(OH)·2PbCO3, which can be assigned to a 2H-type subspecies and detailed comparison to Pb(OH)2·2PbCO3, which represents a 3R-type subspecies.

Solid-state structure of a degradation product frequently observed on historic metal objects.

In the course of the investigation of glass-induced metal corrosion processes, a microcrystalline sodium copper formate hydroxide oxide hydrate, Cu4Na4O(HCOO)8(H2O)4(OH)2, was detected on a series of antique works of art, and its crystal structure was determined ab initio from high-resolution laboratory X-ray powder diffraction data using the method of charge flipping, simulated annealing, and difference-Fourier analysis (P42/n, a = 8.425 109(97) Å, c = 17.479 62(29) Å, V = 1240.747(35) Å(3), Z = 8). In the crystal structure, the metal cations are interconnected in a two-dimensional metal-organic framework via the oxygen atoms of the formate, hydroxide, and oxide anions. Doublets of face-sharing square pyramidal Cu(2+) polyhedra are linked via a single, central oxide oxygen atom to give a paddle-wheel arrangement, while the Na(+) cations are organized in Na2O11 moieties with highly disordered, edge-sharing octahedral coordination. In addition, hydrogen bonding plays an important role in stabilizing the crystal structure.

Morphological Study of Bio-Based Polymers in the Consolidation of Waterlogged Wooden Objects.

The removal of water from archaeological wooden objects for display or storage is of great importance to their long-term conservation. Any mechanical instability caused during drying can induce warping or cracking of the wood cells, leading to irreparable damage of the object. Drying of an object is commonly carried out in one of three ways: (i) air-drying with controlled temperature and relative humidity, (ii) drying-out of a non-aqueous solvent or (iii) freeze-drying. Recently, there has been great interest in the replacement of the standard, but limited, polyethylene glycol with biopolymers for wood conservation; however, their behaviour and action within the wood is not completely understood. Three polysaccharides-low-molar-mass (Mw) chitosan (Mw ca. 60,000 g/mol), medium-molar-mass alginate (Mw ca. 100,000 g/mol) and cellulose nanocrystals (CNCs)-are investigated in relation to their drying behaviour. The method of drying reveals a significant difference in the morphology of these biopolymers both ex situ and within the wood cells. Here, the effect these differences in structuration have on the coating of the wood cells and the biological and thermal stability of the wood are examined, as well as the role of the environment in the formation of specific structures. The role these factors play in the selection of appropriate consolidants and drying methods for the conservation of waterlogged archaeological wooden objects is also investigated. The results show that both alginate and chitosan are promising wood consolidants from a structural perspective and both improve the thermal stability of the lignin component of archaeological wood. However, further modification would be necessary to improve the biocidal activity of alginate before it could be introduced into wooden objects. CNCs did not prove to be sufficiently suitable for wood conservation as a result of the analyses performed here.

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

During the systematic investigation of the ternary system Ca(CH3COO)2-Ca(HCOO)2-H2O at room temperature, the congruent crystallisation of two solid calcium acetate formates was observed: the hitherto unknown Ca3(CH3COO)4(HCOO)2·4H2O and the poorly characterised Ca(CH3COO)(HCOO)·H2O. The latter is a frequently observed efflorescence phase found on calcareous objects and it could be also identified as a corrosion phase in a natural history collection of birds' eggs. Elemental and thermal analyses were employed to determine the phase compositions and by Raman and IR spectroscopy the presence of acetate and formate anions in both solids was confirmed. Laboratory X-ray powder diffraction data were used to solve the crystal structures. Ca3(CH3COO)4(HCOO)2·4H2O crystallises in a primitive tetragonal unit cell with space group P41212 and lattice parameters of a = 6.8655(1) Å and c = 45.5454(6) Å, while Ca(CH3COO)(HCOO)·H2O crystallises in a primitive monoclinic unit cell with space group P21/c and lattice parameters of a = 9.2729(1) Å, b = 6.8002(1) Å, c = 11.2219(2) Å and ß = 121.232(1)°. Calcium carboxylate zig-zag chains [Ca(µ2-RCOO)+]n (R = CH3 or H) are the main motif of both crystal structures. In Ca3(CH3COO)4(HCOO)2·4H2O these chains are exclusively composed of acetate anions, whereas in Ca(CH3COO)(HCOO)·H2O only formate anions are situated in the chains. The remaining places in the 7-8 fold coordination sphere of the calcium cations are filled by water molecules and additional carboxylate anions that interconnect neighbouring chains, which eventually leads to layered motifs in both structures.

On verdigris, part II: synthesis of the 2-1-5 phase, Cu3(CH3COO)4(OH)2·5H2O, by long-term crystallisation from aqueous solution at room temperature.

Long-term crystallisation from aqueous copper(ii)-acetate solution after the addition of ammonia at 25 °C led to the formation of a hitherto poorly characterised phase in the verdigris pigment system Cu(CH3COO)2-Cu(OH)2-H2O. Laboratory X-ray powder diffraction (XRPD) was successfully employed to solve the crystal structure. The structure solution reveals a phase composition of the Cu3(CH3COO)4(OH)2·5H2O ≡ 2-1-5 phase, which was also confirmed by thermal analysis. The 2-1-5 phase crystallises in space group P21/c (14) with lattice parameters of a = 12.4835(2) Å, b = 14.4246(2) Å, c = 10.7333(1) Å and ß = 102.871(1)°. The crystal structure consists of Cu2(CH3COO)2(CH3COO)1/2(OH)4/3H2O1/6+ dimers that are interconnected by Cu(CH3COO)(CH3COO)1/2(OH)2/31/6- squares forming chains running in the c-direction. Non-coordinating hydrate water molecules are intercalated inbetween the chains and mediate the inter-chain interaction. IR and Raman spectroscopy techniques were also employed to confirm selected aspects of the determined crystal structure. The magnetic properties of the 2-1-5 phase decompose into two independent subsystems: a strongly antiferromagnetically spin exchange coupled magnetic Cu-Cu dimer and a significantly weaker coupled Cu monomer. The light blue colour of the sample originates from a reflectance maximum at 488 nm and significantly differs from the known verdigris phases. An investigation of several historic verdigris pigment samples revealed that this phase occurs both as a minor and a major component. Hence, our reference data for the title compound will help to improve the understanding of the multiphase mixtures occurring in historic verdigris samples.

On verdigris, part I: synthesis, crystal structure solution and characterisation of the 1-2-0 phase (Cu3(CH3COO)2(OH)4).

Known synthesis approaches for basic copper(ii) acetates, the main components of historic verdrigis pigments were reinvestigated and revealed to be partially irreproducible. A modification of the reaction conditions led to the successful and reproducible synthesis of the 1-2-0 phase (Cu3(CH3COO)2(OH)4 = 1Cu(CH3COO)2·2Cu(OH)2·0H2O). The phase composition was derived from elemental and thermal analysis and confirmed by the crystal structure solution using synchrotron X-ray powder diffraction (XRPD) data. The 1-2-0 phase crystallises in space group Pbca with lattice parameters of a = 20.9742(1) Å, b = 7.2076(1) Å, and c = 13.1220(1) Å. The crystal structure consists of Cu2(CH3-COO)2(OH)4/3(OH)2/21/3- dimers, which are interconnected by corner sharing Cu(OH)2/3(OH)2/21/3+ squares forming layers perpendicular to the a-axis. The deep blue color of the solid originates from a reflectance maximum at 472 nm and from an absorbance maximum at 676 nm that is comparable with other historic blue pigments like azurite or Egyptian blue. IR- and Raman-spectroscopic properties of the solid were investigated as well, which demonstrated that the obtained product is identical with a previously synthesised verdigris phase that was obtained by applying historical procedures. Therefore, our reference data for the title compound will help to improve the understanding of the multiphase mixtures occurring in historic verdigris samples. The magnetic properties of the 1-2-0 phase were also investigated. At low temperatures the magnetic susceptibility is well described by a spin-1/2 Heisenberg chain with uniform antiferromagnetic nearest-neighbour spin exchange coupling of only one of three Cu magnetic moments. Due to the very strong antiferromagnetic coupling of the Cu2(CH3-COO)2(OH)4/3(OH)2/21/3- dimers their contribution to magnetism becomes relevant above ∼140 K, which results in the presence of two distinct temperature regions where Curie-Weiss behaviour of the magnetic susceptibility with different Curie constants and Weiss temperatures is found.

X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects.

The crystal structure determination and refinement process of corrosion products on historic art objects using laboratory high-resolution X-ray powder diffraction (XRPD) is presented in detail via two case studies. The first material under investigation was sodium copper formate hydroxide oxide hydrate, Cu4Na4O(HCOO)8(OH)2∙4H2O (sample 1) which forms on soda glass/copper alloy composite historic objects (e.g., enamels) in museum collections, exposed to formaldehyde and formic acid emitted from wooden storage cabinets, adhesives, etc. This degradation phenomenon has recently been characterized as "glass induced metal corrosion". For the second case study, thecotrichite, Ca3(CH3COO)3Cl(NO3)2∙6H2O (sample 2), was chosen, which is an efflorescent salt forming needlelike crystallites on tiles and limestone objects which are stored in wooden cabinets and display cases. In this case, the wood acts as source for acetic acid which reacts with soluble chloride and nitrate salts from the artifact or its environment. The knowledge of the geometrical structure helps conservation science to better understand production and decay reactions and to allow for full quantitative analysis in the frequent case of mixtures.