Sarah Eleni Pinchin
Moisture provides the activation mechanism for significant deterioration to wall paintings. The interchange between an immovable painting and water vapour is one of the most complicated variables to assess and control. To successfully preserve paint materials from environmental pressures it is important to consider the multitude of components contributing to this interaction. Simplified moisture models have been developed to evaluate the response of materials to variation in relative humidity. These models have been used to assess the reaction of many building materials to water vapour, but the literature largely neglects lime plaster÷a fundamental component of most historic wall paintings. Since wall paintings are exposed to variable environments and are also heterogeneous topographically and stratigraphically it is important to understand not only the response of the fabric but also that of the original and added materials, in particular conservation materials.
Conservation materials have contributed significantly to deterioration. Wax coatings, for example, were applied widely as a preservative in the period c.1850- c.1950 but are now subject to removal, their harmful effects having been recognised. The organic coating currently prominently used÷Paraloid B72÷has very different characteristics. However, an understanding of the effects of these materials on the interaction between the painting and water vapour requires further investigation.
Therefore, for the present study contributions to the subject to date were compiled and it was found that sorption and water vapour transmission data for lime plaster and the relevant conservation materials were indeed rare. Following a preliminary investigation, tests were carried out to evaluate the moisture behaviour of painted lime plaster with two organic coatings: wax and Paraloid B72. Two forms of each were tested as they might occur on wall painting: waxed and dewaxed; and Paraloid applied as a varnish (15%) and as a consolidant (2%). Analogue replicas made from a standard 3:1 sand: lime plaster were used, painted with a limewash pigmented with artificial ultramarine. Sorption and water vapour transmission testing was undertaken on sets of four samples for each coating type, and one control set.
The results of the sorption testing revealed a rapid response, with most activity occurring within the first 12 hours. In the sorption experiment the difference between the control and the coatings was not the rate but rather the amount of absorption. This rapid rate of sorption must influence decisions about appropriate RH fluctuation. The water vapour testing produced surprising results, producing a spread of transmission values ranging from the control (the greatest) to waxed (the least, but by no means impermeable).\