Patrick Storme

Improved radiographic methods for the investigation of paintings using laboratory and synchrotron X-ray sources

It is generally known that radiographic inspection of 15–17th century paintings can easily be done with a polychromatic X-ray source using a voltage between 20 kV and 40 kV in combination with classic X-ray films. Unfortunately, the spatial structure of numerous 19th and early 20th century paintings cannot be visualized with conventional radiography due to several reasons such as the use of lead white grounds or low absorbing pigments. Radiographic images are blurred or worse, they do not contain the picture of the painting. During the last decades, many technological innovations have been introduced in the field of radiography but their possibilities in cultural heritage have not been explored in full detail. In our investigation we used phosphor imaging plates, energy dispersive detectors and CCD-cameras in combination with synchrotron radiation and conventional X-ray tubes in order to improve the quality of radiographic images. Several promising techniques that could improve the quality of radiographs of paintings were identified.

The sulfidation process of sterling silver in different corrosive environments: impact of the process on the surface films formed and consequences for the conservation-restoration community

BackgroundPrecious objects made of silver and/or its alloys tarnish and become black when exposed to ambient atmospheres containing moisture and ppb-amounts of H2S. Such objects usually contain small but variable amounts of copper as alloying constituent and this affects the corrosion process due to a preferential oxidation of copper. However the visual appearance of the formed tarnishing layers on different types of silver alloys is very similar. Therefore, conservators-restorers are confronted with the problem that in some cases certain cleaning techniques are very effective while in other similar cases the removal of tarnishing layers is unsatisfactory. Since cleaning experiments are not allowed on genuine objects, many investigations use artificially corroded dummies instead. In order to evaluate the representativity and reproducibility of this often used methodology, differences in morphology, microstructure and composition of the sulfide layers on sterling silver generated by different sulfidation methods were analysed.ResultsSterling silver samples were artificially aged in five different environments. The samples exposed to uncontrolled ambient air at different locations (e.g. residential and laboratory environments) showed different corrosion rates and corrosion colours. Three accelerated ageing methods were executed in a gaseous or liquid environment under controlled conditions. These tests showed different results in morphology, microstructure, composition, thickness and the interface between bulk and corrosion layer. A first accelerated sulfidation procedure in a Na2S solution alternated with exposure to air, resulted in a fast corrosion rate and an even corrosion layer formation with several S-species. A second series of sulfidation in a controlled gas environment of H2S and SO2 developed a thin but uneven corrosion layer, mainly consisting of oxides. A third corrosion methodology used was based on the thioacetamide method. This resulted in an even and relative thick corrosion layer, comparable to the Na2S/aeration sulfidation system. However, the interface between the corrosion layer and the bulk is importantly different, showing severe voids.ConclusionsThe corrosion layers generated by five different experimental sulfidation series on identical prepared sterling silver coupons were clearly different from each other. Analyses demonstrated that the composition and microstructure of the corrosion layers were strongly dependent on the sulfidation method used and copper was found to be an important element present in all sulfide layers analysed. Therefore, artificially corroded sterling silver is not necessarily representative for naturally tarnished historical objects and the extrapolation of the cleaning results obtained on dummies to historical objects must be performed with care.

The study on corrosion of Pb–Sn–Sb alloys in soil

Abstract The paper studies corrosion of selected Pb-Sn-Sb alloys in soil. Corrosion tests were proposed and performed based on the survey of the mechanism of corrosion of archaeological objects – letter types. The composition of tested lead alloys corresponds to the phases in structures of alloys the original types were made from. Three types of soil with different pH were selected as the corrosion environment and one of the set of specimens was exposed additionally to acetic acid vapours to monitor the impact of volatile organic compounds. The results were compared with similar corrosion tests performed at the University of Antwerp. The results implied that it was predominantly the character of the microstructure the lead alloys form that affects corrosion of Pb–Sn–Sb alloys. Intermetallic phases SnSb are formed within the lead matrix, the average composition of which corresponds to 55 % of Sn and 45 % of Sb. The SnSb phase is very hard and resistant; a corrosion microcell is formed when in contact with the matrix, with the lead matrix being preferentially susceptible to corrosion. The content of antimony and tin in the alloy affects formation and the quantity of the SnSb phase.