Crystals growing in confined spaces can generate stress and are a major cause of damage in porous materials. The present paper is an experimental study of the isothermal hydration of MgSO 4 • H 2 O, kieserite, in porous glass filters with different in situ techniques, i.e., X-ray diffraction under controlled conditions of temperature and humidity, scanning electron microscopy after cryofixation of samples, and electronic speckle pattern interferometry. Based on the phase diagram of MgSO 4 + H 2 O derived from a careful evaluation of the available thermodynamic data, all experiments were carried out under appropriate conditions in a controlled environment. Hydration of MgSO 4 • H 2 O and formation of MgSO 4 • 6H 2 O (hexahydrite) is a true solid state reaction below the deliquescence humidity of kieserite. This reaction, however, is kinetically hindered. Above the deliquescence humidity, the reaction proceeds via a two stage reaction pathway involving the dissolution of kieserite and the subsequent crystallization of hexahydrite from a highly supersaturated solution. The hydration of kieserite in confined spaces generates substantial stresses, resulting in considerable deformations of the glass filters that were used as porous substrate. The deformation measurements confirm that the deliquescence-recrystallization pathway is more efficient than the direct solid state reaction in generating stress. Finally, theoretical considerations confirm that the stress generated during the hydration is sufficient to damage nearly every building material.
An overview is given of all results from the International Co-operative Programme on Effects on Materials including Historic and Cultural Monuments (ICP Materials), which was launched in 1985. Since then, about twenty different materials have been exposed repeatedly in a network of test sites consisting of more than twenty sites with an extensive environmental characterisation and more than sixty official reports have been issued. Recent results on trends in corrosion, soiling, and pollution show that corrosion of carbon steel, zinc, and limestone is today substantially lower than 25 years ago, but while corrosion of carbon steel has decreased until today, corrosion of zinc and limestone has remained more or less constant since the turn of the century. Unique data are given on measured HNO3concentrations from 2002-2003, 2005-2006, and 2008-2009, and the relative average decrease was about the same from 2002-2003 to 2005-2006 as it was from 2005-2006 to 2008-2009.
Since the mid sixties Cor-Ten 1 steel became one of the most famous materials for outdoor sculptures until today. This material was used by many artists because of its good weather resistance. Due to not always suitable ranges of application many significant problems in the field of conservation appeared.These were surveyed in a project of the study programme 'Object Conservation' at the State Academy of Art and Design Stuttgart with special focus on the material and its protective rust layers, the premises for its protection, different cases of significant damage and their causes, the potential of preventive conservation and practised interventive methods.Although it seems to be in some way a contradiction in itself to coat a weathering steel, this has often been done in hands-on conservation to overcome corrosion problems. Therefore different coating materials already in use for outdoor metal sculptures were tested on Cor-Ten coupons in the framework of the CONSIST project in the Deutsches Bergbau-Museum Bochum (DBM). Cor-Ten B samples were covered with different coating materials such as: microcrystalline waxes, waxes used in technical corrosion protection, an acrylic resin and a polyurethane resin. The sheets were then weathered for 2 months in a climate chamber with a relative high humidity (but also drying cycles) and a climate chamber with a periodic changing relative high rate of SO 2 gas and humidity using standard protocols developed at DBM in earlier projects. The effectiveness of the protective function of each coating materials is compared, consequences for practical applications discussed.
The visual appearance of building structures is an important attribute which reflects the character and identity of a region. Due to the influence of weathering, the surfaces of building stones alter, leading to aesthetic changes of the material surface such as discoloration or darkening. In this study, near-surface regions of weathered Baumberger (BST), Schleeriether (SST), and Obernkirchener Sandstones (OKS) have been analyzed at the microscopic scale in order to investigate the intensity and the extent of visual as well as structural changes and how both can be affected due to the presence of surface treatments with hydrophobing agents. It could be detected that aesthetic changes appeared already after 2 years of outdoor exposure, with the slightest variations on BST surfaces, followed by SST and OKS. The use of hydrophobing agents leads to a reduction in surface darkening in the short term. After long-term weathering, no significant changes are visible, as similar values in total color difference (ΔE*) were measured. Biogenic growth and the formation of black weathering crusts are the main reasons for color alterations in the case of the examined stones. The surface damages occur especially on calcareous (BST) followed by clayey (SST) and quartzitic (OKS) stone surfaces.
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