Evaluating the suitability of museum storage or display materials for the conservation of metal objects: a study on the conformance between the deposited metal film method and the Oddy test

Storing and exhibiting cultural heritages are the basic social functions of museum. Since it is impossible to store or display objects without using containers, cases, or holders, the equipment quality is of great importance. Evaluating the suitability of the equipment should not only learn the mechanical behavior but also focus on the materials because some of them (such as woods) may do harm to the objects due to contaminants released. A convenient test by the deposited metal film method has been proposed previously in order to evaluate more potential museum materials in limited time. The conformance between this method and the Oddy test, the classic method for evaluating and selecting museum materials, is mainly studied in this work. The two testing systems were compared from several aspects such as sample appearance, corrosion product, surface morphology, and metallic content by naked eye and modern characterization measures like X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The corrosion mechanisms were deduced according to the corrosion products, including Cu → Cu₂O → CuO and Cu → Cu₂O → Cu(OH)₂·H₂O → Cu(HCOO)(OH). The suitability of potential materials for the conservation of metal objects was defined according to the metallic contents of coupons (calculated by atomic ratio) which were classified by the Oddy test. The critical values distinguishing permanently usable from temporarily usable are approximately determined as 70% for copper and 75% for silver, and those distinguishing temporarily usable from unusable are approximately determined as 55% for copper and 60% for silver. The corresponding metal films were classified based on the metallic content standard derived, and then typical appearances of the films assigned to different suitability levels were suggested. Special phenomena, such as the failure in detecting some corrosion products, is attributed to low yield and uneven distribution of ultrafine corrosion products on the films, the covering effect caused by other corrosion products on the copper coupons, and the weakening effect resulted from intensive metal peaks, while some unexpected corrosion conditions on the coupons and the films, are related to the characteristics of general corrosion and pitting corrosion. The results indicate the potential application of the deposited metal film method, giving an optional choice to evaluate and select museum materials with less time. The evaluation methods were preliminarily established from three aspects, such as artificial judgment, metallic content analysis, and corrosion product identification. The artificial judgment is generally applicable, and the other two are useful for verifying the result if possible. More actual cases and further calibration work are essential for further development of the deposited metal film method.