In search of structure -High resolution X-ray powder diffraction of metal organic salts for conservation science

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To better understand the production of pigments and the corrosion of materials used by mankind during history, conservation science is in search of the structure of these compounds. Methods like SEM-EDX and Raman microscopy are valuable tools for their investigation, but are limited in their results. Often, these studies deal with metal organic salts like carbonates, formates, and acetates. Some of them have been deliberately manufactured using suitable compounds (e.g. vinegar), others derive from deterioration reactions with the atmosphere (humidity plus carbon dioxide or carbonyl compounds from indoor air pollution). They have the potential to form metal organic frameworks (MOFs). These combine organic and inorganic building blocks to give hybrid structures with plethora of interesting properties, and are, therefore, in the focus of current materials science research. The exact composition of many of these compounds occurring on artifacts is not yet known. In many cases, single crystals suitable for conventional structure determination cannot be obtained, neither from samples, nor by lab growth. The data analysis of modern high resolution X-ray powder diffraction (XRPD) has the potential to overcome this problem, as has been shown by us in two pilot studies of the crystal structure of a frequently occurring sodium copper formate and thecotrichite as efflorescence on calcareous objects. Formulas for them from literature had to be corrected. Therefore, unknown crystalline natural or artificial corrosion products either sampled from objects or synthesized in the lab shall be studied by XRPD: - Many historic glass objects also contain metal parts. Hydrolysis of the glass may cause corrosion in neighbouring metals. Unknown Cu, Zn, and Pb compounds with Na or K (from glass) and small organic anions (from air) will be investigated. - Calcareous artifacts and natural history items stored in wooden cupboards develop various calcium and acetate containing phases, some of them (not in the ICDD) also include formate. - Basic copper acetates were produced as pigment verdigris since more than 2 millennia by artificial corrosion. At least four phases still need structure determination. During the project, a variety of structures and formulas are expected to be solved within 3 years. This will help to better understand the underlying corrosion or production processes and contribute to the overall objective of conservation science: the preservation of artifacts for the benefit of mankind.