Effect of acid rain on sandstone: The Royal Palace and the Riddarholm church,Stockholm

The deterioration of two kinds of sandstone is discussed for two 18:th century buildings in central Stockholm: the Royal Palace, and the Royal Carolean Burial Chapel (Karolinska gravkoret) annexed to the mediaeval Riddarholm church. The facades of calcitic Gotland sandstone show many signs of serious decay, such as gypsum formation, pulverized surface, exfoliation, discolouration, and salt efflorescence. The socles are built of the more resistant quartzitic Roslagen sandstone, displaying some discolouration, cracks, and slight exfoliation. In total about 300 samples have been analysed. The surface concentration of sulphur is highest at ground level and at rain-sheltered positions. Chemical and sulphur isotope data indicate that the stone decay to a large part may be attributed to anthropogenic sources like acid deposition and car traffic.     

Corrosion of archaeological bronze artefacts in acidic soil

Corrosion of bronze in soil is a well-known phenomenon. In particular, archaeological artefacts which may remain in the soil for thousands of years are subject to severe corrosion. However, bronze objects excavated 50–100 years ago seem to be less corroded than those found today. Therefore, recent pollution of the soil is suspected to accelerate the corrosion. An interdisciplinary project has been started in Sweden to search for correlations between the degree of bronze corrosion, corrosion products, general archaeological and environmental conditions, and parameters characterizing the soil chemically.From three archaeological sites near Stockholm (Birka, Fresta, and Valsta), 33 bronze artefacts and related samples of soil have been investigated. All corrosion products and the metal core (if any) were analysed by SEM/EDS and XRD. Metal oxides, carbonates, sulphates, chlorides and phosphates have been identified. Each soil sample has been geologically classified, and a number of chemical analyses have been undertaken: pH in water and KCl, resistivity, loss on ignition, exchangeable acidity, chloride, phosphate, sulphur contents, acidsoluble cations extracted in two different ways, etc. About 8000 data have been compiled in an EXCEL data base. A statistical evaluation including multivariate modelling and analysis utilizing the SIMCA-S system, has been undertaken. The results so far obtained are only tentative but suggest that high concentrations of soot, sulphur or phosphate in the soil may have accelerated the corrosion of the investigated bronze objects. The influence of low pH values, though, is less clear.     

Atmospheric Bronze and Copper Corrosion as an Environmental Indicator. A Study Based on Chemical and Sulphur Isotope Data

Corrosion products have been takenfrom 130 copper or bronze outdoor objects all overEurope. Their chemical composition and crystalsymmetry have been determined by means of scanning electron microscopy (SEM/EDS) and X-ray powderdiffraction. Data on location, sampling, objectcharacteristics, general environment and air pollutionlevel; type, colour and chemical composition of thecorrosion layers have been obtained and evaluated by multivariate statistical analysis. The resultsverify that the highest air pollution levels are usually associated with the occurrence of thick,black or dark grey corrosion layers on copper orbronze objects, preferentially containing soot, ironoxide hydroxides, and antlerite,Cu₃(SO₄)(OH)₄. Pale green corrosionusually contains brochantite,Cu₄(SO₄)(OH)₆, and is rather associatedwith less polluted areas. Atacamite, a copper hydroxide chloride with the chemical formula Cu₂Cl(OH)₃, is preferentially observed in coastal regions.In addition, sulphur isotope analyses have beenperformed on eleven corrosion samples from citycenters. The δ³⁴S values are typically inthe region from +4 to +6‰ relative to the sulphurisotope standard CDT (Canyon Diablo Troilite) with amean value of 4.7±1.2 (1σ), therebyindicating that the sulphur in the corrosion layers,in the form of brochantite or antlerite, mainlyoriginates from a similar source despite geographicvariation, most likely sulphur contained in air pollutants.