Methodology Applied to Historical Walls May Explain Why Moss Gathers and How Paint Blackens

For the correct restoration of deteriorated works of cultural heritage, it helps to know what causes the degradation in the first place. There are studies that characterise these types of damage in detail, but that rarely touch on the origin and development of the deterioration. Chemist Maite Maguregui has come up with an analytical methodology with the intention of filling this need: considering the material of origin of the historical heritage and the product that substituted over time (the decay product), in order to appropriately understand the development of the degradation process.

She presented her PhD thesis at the University of the Basque Country (UPV/EHU), with the title, A new diagnosis protocol to assess the impact of environmental stressors on historical bricks, mortars and wall paintings.

Ms Maguregui characterised the deterioration caused by infiltrating water, atmospheric acid gases and microorganisms, and observed that human intervention has had much to do with all this. Specifically, she applied this methodology to bricks, mortars and wall paintings; amongst others the paintings in dwellings of old Pompeii. Ms Maguregui and her colleagues have published an article in Analytical Chemistry based precisely on the work undertaken in the Roman city and entitled Thermodynamic and spectroscopic speciation to explain the blackening process of hematite formed by atmospheric SO2 impact: the case of Marcus Lucretius House (Pompeii).

Diversity of techniques

This methodology included different analyses. To begin with, use was made of non-invasive spectroscopic techniques in order to characterise both the original compounds and the decay products. Moreover, microdestructive techniques were applied to the quantification of soluble salts, given that the considerable presence of these causes damage to works of cultural and historical heritage. Chemometry was employed for treatment of data and, in order to understand the development of the process of deterioration, chemico-thermodynamic models were used. In the final part, Raman spectroscopy and depth profiling were used, with the aim of determining the distribution of the components, both of the original compound and of the decay products.

With this analysis, it was seen that sulphur dioxide and oxides of nitrogen are amongst the principal causes of deterioration -- compounds that are found in industry, automotion, food industry, and so on. Thus, it is human activity that is responsible for the deterioration of cultural and historical heritage.

Also thanks to this analysis, Ms Maguregui concluded that environmental factors do not negatively affect heritage works in an isolated manner, but each item can deteriorate in diverse ways. In Pompeii, sulphur dioxide had damaged plaster, on which, in turn, moss grew. It was thus concluded in the thesis that there may be a nexus between these two phenomena. In fact, other researchers have already mentioned that gypsum could be a good nutrient for colonising microorganisms. Thus, the sulphation caused by sulphur dioxide on the plaster attracts microorganisms, which is why moss grows and accumulates.

Acceleration of the process

The data gathered with the new methodology enabled the researcher to make hypotheses about how the process takes place and what mechanisms take part in each case. But, besides explaining this at a theoretical level, she has validated it. She copied the supposed process of deterioration in some of the cases, applying it experimentally using a series of techniques that accelerated the development of the process, a series which the researcher, herself, designed.

Concretely Ms Maguregui carried out an experiment on the wall paintings of the Marcus Lucretius House in Pompeii. The zones that originally contained red haematites were blackened, as these had been substituted by magnetite and iron (III) sulphate nonahydrate. According to the researcher, both decay products would have appeared as a result of sulphur dioxide; a hypothesis that she verified thanks to the experimentation accelerating the process. As a result these zones of the wall paintings had blackened due to effect of human activity on the environment.
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References:

Science Daily. 2011. "Methodology Applied to Historical Walls May Explain Why Moss Gathers and How Paint Blackens". Science Daily. Posted: September 13, 2011. Available online: http://www.sciencedaily.com/releases/2011/09/110913092418.htm