Effects of atmospheric NOx on biocalcarenite coated with different conservation products

One of the most important factors of calcareous stone decay is the chemical attack caused by atmospheric pollutants dissolved in water, in particular acidic gases such as SOx, NOx and CO2. The treatment of stone artefacts with hydrophobic products reduces the water uptake of the rock and is, therefore, considered a protective treatment. The aims of the present study were the observation of the effects of NOx on a biocalcarenite (Lecce stone) and the evaluation of the performance of protective products (Paraloid B72 and a fluoroelastomer) normally used in stone conservation. A system for the simulation of accelerated ageing has been assembled and the Lecce stone samples, treated with polymeric products, have been exposed to NO2 (nearly 1 ppm) during thermo hygrometric cycles. The nitrite and nitrate salts formed on the specimens have been determined by ion chromatography. Both the polymeric products tested have given high hydrophobic properties to the stone, even if they db not assure satisfactory protection toward NO2 exposure. Actually, their protective efficacy, or ability to reduce the acidic attack of NO2, quickly decreases when the ageing period increases. Paraloid B72 seems more effective than fluoroelastomer because, on the samples treated with the latter, higher amounts of nitrites and nitrates have always been detected. In contrast, non-treated samples - used as reference - have shown a constant increase in the amount of nitrates and nitrites during ageing. However, the surface analyses of the stone samples, by environmental SEM and mu-laser profilometry, did not show any change in superficial morphology after the ageing, suggesting that NO2 acts inside the pores. (C) 2007 Elsevier Ltd. All rights reserved.