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Although most masonry surfaces will be colonised by various organisms, damage is usually restricted to porous stone and is usually associated with atmospheric pollution. The main pollutant today is sulphur dioxide which dissolves in rainwater to produce sulphurous acid and reacts with the calcium carbonate of limestones to form calcium sulphite. However, sulphite is never found on limestones, but only sulphate which is produced by much more aggressive sulphuric acid. Various chemical explanations have been given for this oxidation from sulphite to sulphate, such as ultra-violet radiation, but it is a fact that sulphate deposits are always associated with the presence of sulphating bacteria, particularly Thiobacillus species. In areas where coal fires are still used extensively and in some industrial areas, nitrous oxide pollution also occurs which should give nitrous acid and nitrites on limestones, but only nitrates are actually found, indicating oxidation to the more aggressive nitric acid. In these circumstances nitrating bacteria, particularly Nitrobacter species, are always found.
 
Urban areas appear to be cleaner since the introduction of the Clean Air Act, but this is only because particulate emissions have been reduced from industrial chimneys and there has been a progressive decline in most areas in the use of coal for domestic heating. Unfortunately sulphur dioxide pollution has become steadily worse, partly through the increasing use of less expensive heavy oil for heating large commercial and industrial premises as this fuel has a relatively high sulphur content, but also partly through a feature of the Clean Air Act which limits only emission concentrations rather than amounts; if an operator is emitting excessive sulphur dioxide concentrations these can be easily reduced by injecting air into the flue, but the total emissions of sulphur dioxide remain unchanged.
Biological Damage to Masonry Structures
Image courtesy: Historic environment Scotland Slime producing algae may result
in slippery surfaces.
Sulphate formed from sulphur dioxide in this way is a source of crystallisation damage, but damage is not confined to limestones. On mortar the sulphate in urban areas may be sufficient to react with the tricalcium aluminate in ordinary Portland cement to cause the expansion and cohesion failure usually known as sulphate attack. However, deterioration problems attributable to bacteria are not confined to urban areas. In rural areas ammonia generated by bacteria from urine in stables and byres can be absorbed on stone walls or asbestos-cement roofs where it is converted by Nitrosomonas species to nitrites and then by Nitrobacter species to nitrates, frequently causing spalling damage.

Bacteria are not the only organisms which colonise damp masonry surfaces. If the surfaces are warm with sufficient light, algae will develop in the water film on the surface, typically producing a bright green coloration, although sometimes dark green, brown and pink colorations occur. Algae often colonise a surface within one or two hours of rainfall, but the algal coloration disappears just as rapidly as the surface dries. Many of the algae are killed by drying but sufficient remain to redevelop and multiply when dampness returns. The humus accumulating on the masonry surface from dead algae and other sources eventually allows mosses, liverworts, grasses and even trees to develop, their root systems often causing serious damage. Organic deposits on the surface also encourage fungi to develop, such as Cladosporium, Phoma, Alternaria and Aureobasidium species; some species are associated particularly with the high nitrogen levels that develop on masonry contaminated by bird droppings.

Serious masonry deterioration is sometimes associated with growth of lichenised fungi or lichens, symbionts of algae growing within fungi, usually Ascomycetes. The fungal hyphae penetrate deeply into stone, exploring fractures but also generating organic acids such as oxalic acid. Oxalates are formed in carbonaceous stones which are usually deposited in or near the thallus or surface growth; eventually these accumulations of phosphates can kill the thallus, leaving a lichen ‘fossil’ of calcium oxalate on the surface of the stone which is sometimes mistaken for lichen growth; repeated applications of biocide sometimes fail to control lichen growth because the growth is, in fact, a dead calcium oxalate fossil formed in this way. If the calcium oxalate is deposited just below the surface, densification can occur which is similar in texture to the calcium sulphate densification that can occur on limestones in urban atmospheres, causing similar spalling damage to the surface of the stone, particularly if it is also microporous and subject to frost or salt crystallisation damage. Where lichens grow on roofs, the oxalic and other lichen acids can cause severe damage to lead, copper, zinc and aluminium roof coverings and gutters; these acids can even cause etching on glass and apparently resistant stones, such as granites.

There are basically three types of lichen, classified according to the shape of the thallus. In the crustose lichens the thallus forms a flat crust on the surface of the stone, the diameter of the thallus giving an accurate indication of the age of the growth; the diameter of the largest growths in millimetres will indicate approximately the years since the stone was installed, a useful feature for identifying original and replacement stones in old masonry. The crustose lichens cause densification of the stone surface on limestones and sandstones, the stone within the centre of the thallus often spalling away to leave bare stone which is then rapidly colonised by the growth. Sensitive species cannot develop in polluted atmospheres, but resistant species become very active in the absence of competition, particularly on limestone, cast stone and concrete surfaces on which acid pollutants are neutralised; Lecanora and Candelariella species are particularly common in these circumstances. Crustose lichens vary greatly in size from minute growths within pores to enormous plates 300mm (12") or more across.

Foliose lichens have thalli like leaves or scales projecting as a group from a point of attachment to the stone. Fruticose lichens also originate from a point in this way but their thalli are branched. Foliose and fruticose lichens are not so common on buildings, except in exposed and relatively unpolluted areas on western coasts, conditions that actually encourage the development of many different lichens. Particular species tend to be associated with particular conditions. Lecanora and Candelariella have previously been mentioned as species which tolerate pollution, particularly when growing on acid neutralising substrates such as limestone, carbonaceous sandstone, asbestos-cement tiles, render and concrete. Calaplaca species are also commonly found on limestones in reasonably unpolluted conditions, whilst Tecidia and Rhizocarpa species are more often found on sandstones.

It will be appreciated from these comments that identification of lichen growths can often indicate both the nature of the substrate on which it is developing and the pollution to which it is subject. Very heavy lichen growth on limestone headstones in a cemetery was found to be causing continuous and rapid stone erosion and spalling damage, each sequence of spalling removing the lichen thallus layer with a thin layer of attached stone, thus exposing a fresh stone surface with lettering still engraved in it but with the detail becoming blurred. Identification of the lichen suggest that it was a species which particularly favoured surfaces with a high nitrogen content and which was usually associated with contamination through bird droppings, although none were present on the headstones and all surfaces were virtually identically affected. The explanation for the abnormal growth was pollution through dust discharges from a neighbouring fertiliser factory.

This abnormally heavy lichen growth was associated with Portland limestone headstones, but adjacent memorials constructed in French Euville limestone with a rather different texture developed instead a heavy coating of black slime fungus. Slime fungi are strange organisms which form a heavy gelatinous coating over the stone in which algae are trapped, giving the coating a colour characteristic of the algae involved. Slime fungi can develop externally or internally on building materials. Green, brown or red slime fungi commonly develop on masonry surfaces in churches in humid areas where the periodic heating results in excessive condensation; this is a common problem in churches in Cornwall.




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