Algal growth on building sandstones

Factors affecting growth of algae on building sandstones after stonecleaning

Last updated 29 May 1998
Lichen on sandstone

 

It has been reported anecdotally that following stonecleaning algal growth or re-growth on buildings occurred very rapidly and in greater abundance than was present prior to stonecleaning. This research attempted to discover how both physical (e.g. grit blasting) and chemical stonecleaning methods can influence the rate of algal growth on sandstones.

This research, some of which was conducted within a larger research programme funded by Historic Scotland into the use of biocides on building sandstones, was carried out as a PhD project by Maureen Young. This page carries a summary from the introduction and conclusions to the thesis. The full reference is:

Young, M.E. (1997).
Biological growths and their relationship to the physical and chemical characteristics of sandstones before and after cleaning.
PhD Thesis. The Robert Gordon University, Aberdeen.


 

Research programme

1. Introduction

The work presented here was, in its initial stages, part of a larger research programme funded by Historic Scotland to investigate biological growths and biocide treatments on sandstone buildings in Scotland (Urquhart et al., 1995). Some of the work presented here had an input from other members of the research group and their contributions are acknowledged in the text.

The action of stonecleaning methods (both physical and chemical) on building sandstones can alter their properties and may alter the susceptibility of sandstone to biological growths. In this thesis the colonisation of building sandstones by biological growths is investigated in terms of their dependence on certain physical and chemical parameters of sandstones including the supply of nutrients, surface roughness and moisture availability. It is hypothesised that changes in these characteristics caused by stonecleaning will affect the colonisation of sandstone by organisms. By identifying the important factors which determine the extent of colonisation of a sandstone by biological growths, a predictive model can be constructed regarding the importance and controllability of various factors in the colonisation of building sandstones by algae.

The aims of this research were:

A selection of six different sandstones were chosen for the experimental work. These sandstones covered the broad range of physical and chemical characteristics found in Scottish building sandstones.

A test rig was constructed and situated in a location where the sandstone samples it held were subjected to natural weathering. This allowed the establishment and growth of microorganisms on the sandstones over a period of about 44 months. The sandstone samples included untreated samples and sandstones which had been subjected to chemical and physical stonecleaning or to biocide application. Another set of larger sandstone blocks with established, pre-existing biological growths were used to monitor the effectiveness of a wider range of biocides than could be used on the test rig. In addition to this, a programme of studies of buildings and monuments around Scotland allowed investigation of the distribution of microorganisms (mainly algae) on a variety of building stones in the field. These observations, although necessarily limited in extent, gave an indication of the variations in biological growths throughout Scotland.

Correlation of the colonisation and rate of change in biological growths with the characteristics of the sandstones allowed determination of the important characteristics of sandstones with respect to biological growths. It was possible to establish whether any changes to the sandstones caused by stonecleaning increased the susceptibility of sandstones to biological growths and hence to re-soiling following cleaning. A conceptual, predictive model of biological soiling of building sandstones was developed.

1.1 Biological growths on building stone

The interaction between agents of decay and sandstone is recognised as a complex phenomenon (Eckhardt, 1978; John, 1988; Krumbein and Dyer, 1985; Seaward, 1979). Mechanisms of deterioration and soiling affecting building sandstones are processes which result from the interaction of many phenomena including the physical and chemical characteristics of sandstones, climatic effects, atmospheric and terrestrial pollutants, fluid movement and biological processes (Jones and Wilson, 1985; Mehta et al., 1978; Meinke et al., 1989; Raistrick and Gilbert, 1963; Seaward, 1988).

In addition to being a potential cause of decay in the long term, surface soiling, both biological and non-biological, can be aesthetically disfiguring to stone. The disfiguring of buildings and monuments by biological growths, particularly on recently cleaned buildings, is a cause of some concern. Biological growths are believed to be one of the main factors involved in the soiling of building facades (Grant, 1982; Seaward, 1979).

Biological growths such as algae, bacteria, fungi, lichens and mosses are common on the exterior of buildings, especially in rural areas. They will colonise stonework wherever suitable conditions of moisture, light, temperature and nutrition occur. Although microorganisms can cause damage to building stone, growths may be non-destructive and some, for example lichens, may even be considered desirable since they can give a mature appearance to a facade. In the past, growths have often been encouraged on facades by a number of methods including applying a wash of cow dung and water, urine or skimmed milk (BRS, 1972; BRE, 1992; Richardson, 1973).

1.2 Biological growths as a cause of stone decay

Biological growths are capable of causing disfigurement and damage to building stone, therefore in some cases their presence may be considered undesirable, especially where the stone is exposed to rainfall or conditions of high humidity, since biological growths are most active under such conditions. Many organisms secrete acids and other substances as a result of their metabolic processes which may affect rock-forming minerals and can be destructive to stone (Jones and Wilson, 1985; Krumbein, 1988; Lewis et al., 1985; Ortega-Calvo et al., 1993; Sand and Bock, 1991). Growths can obscure and cause deterioration of inscriptions (Wainwright, 1986) and carvings and some may stain walls (Wee and Lee, 1980). Biological growths can trap dirt and pollutants, leading to accelerated soiling of a building surface and aiding the establishment of higher plants which increase water retention and may lead to blockages of gutters and downpipes (BRE, 1992; Grant, 1982; Grant & Bravery, 1985; Richardson, 1973). Colour changes on stone surfaces have been attributed to biological causes by a number of authors over the years (Knight, 1777 and Liebig, 1853 in Krumbein, 1993) and concern about stone deterioration through biological processes is not a recent phenomenon. Krumbein and Dyer (1985) and Krumbein (1993) note a number of early writings regarding biodeterioration including biblical references to fungi on stone:

... if on inspection he finds the patch on the walls consists of greenish and reddish depressions, apparently going deeper than the surface, he shall go out of the house and put it under quarantine ..... if the patch has spread on the walls, he shall order the infected stones to be pulled out and thrown away outside the city in an unclean place (Leviticus 13: 14).

Recent concern with the possible relationship between biological growths and stone decay dates back to the early years of this century when a number of authors (Bachmann, 1915; Fry, 1924, 1927; Paine et al., 1933; Pochon and Tchan, 1948; Thiel, 1927) suggested that the action of lichens, bacteria and algae might account for the presence of observed stone decay.

1.3 Establishment of organisms on stone

The organisms found on stone may be deposited there from spores or propagules in the atmosphere or by direct contact with organisms in the soil. The conditions required for colonisation of a substrate vary depending on the type of organism and the species. Photosynthetic organisms, which utilise light to fix carbon from atmospheric CO2, can survive with moisture, mineral salts and light. Other organisms require moisture and nutrients but not light. Many organisms can withstand desiccation for relatively long periods but active growth usually requires relatively high moisture levels or high humidity.

In urban areas the diversity and abundance of many biological growths has been greatly reduced by atmospheric pollution. This can result in dominance of a facade by one pollution tolerant species giving a displeasing, monotonous appearance rather than the mosaic of species which can still be found in rural areas. It is possible that if air pollution levels decline in the future, a more diverse fauna will return to the urban landscape.

1.4 The influence of interventions on stone with respect to biological growths

Over the past 20 years many of Scotland's building facades have been subjected to stonecleaning. Stonecleaning is carried out with the aim of removing dark soiling from a stone surface. This soiling is often thought of as non-biological although closer inspection often reveals the presence of green algae. Other organisms, not visible to the naked eye, including fungi and bacteria are also normally present. Following stonecleaning, re-soiling from inorganic material may take many years, however, re-soiling in the form of green algal growth may occur within only a few months. Algal growth is considered to be aesthetically disfiguring and has been suggested to occur as a direct consequence of stonecleaning intervention (Bluck and Porter, 1991). Control of these growths (BRE, 1992; Grant, 1982; John, 1988) is difficult in practice and often ineffective. Biocidal treatments may be used but organisms usually reappear within a few months or years following treatment. Recent legislation, which has resulted in the withdrawal, for safety reasons, of some of the more effective biocidal products, has exacerbated the situation and reinforced the need for a more fundamental understanding of factors involved in the colonisation of building stones and of the mechanisms of action of biocidal treatments at the organism-stone interface.......

2. Discussion of results

2.1 Algal growth and sandstone porosity

Two aspects of porosity may affect biological growths. These are the total effective pore volume, which controls the amount of water which can be held by a sandstone, and the pore size distribution which controls water availability and the space inside the stone which is available for colonisation. The size of an organism will place a lower limit on the pore size in which it can grow. Generally, pores of diameter less than 10 microns are only available to bacterial growth. Fungi and algae, being somewhat larger, are confined to larger pores. However, much algal growth occurs on sandstone surfaces and this surface growth is affected by porosity only in as far as it affects moisture availability and roughness. Algae can also occur below the outer surface where, for photosynthetic organisms, the depth at which growth can occur is dependent on light penetration (Nienow et al., 1988). In such circumstances the pore size distribution will control the organisms which can colonise the sub-surface environment.

Results of mercury porosimetry indicated that of all the sandstones used in these experiments Leoch was exceptional in that most of its porosity was less than 0.1 microns in diameter. Leoch sandstone therefore contained virtually no pore space (<1%) of a dimension which could contain either microorganisms or bio-available water. It was therefore not surprising that Leoch sandstone was slow to be colonised. Of the other sandstone types, for Blaxter, Clashach, Corsehill and Locharbriggs sandstones, >75% of total porosity was over 1 micron in diameter. For Cat Castle sandstone the proportion was slightly less but still approached 75%. This indicated that for all sandstone types, with the exception of Leoch, most of the water held in pores in the stone would be available to microorganisms.

For untreated sandstones there was found to be a fairly good correlation between total effective porosity and the amount of algal growth, with more algal growth occurring on more porous sandstones. This is most likely to be the result of differences in moisture availability since more porous sandstones can retain more moisture than less porous sandstones. The correlation found between porosity and algal growth was not precise and could not be since factors other than porosity influence algal growth. There have also been found to be some problems with comparing algal growth amounts between sandstone types.

2.2 Effects of stonecleaning treatments on algal growth

There is no stonecleaning method which is capable of removing the soiling from a stone without also affecting the stone itself in some way. Abrasive cleaning methods, which work by abrading the soiling layer, inevitably also result in some loss of material from the stone. Chemical cleaning methods work by dissolving the substances present in the soiling layer and by breaking the bonds which bind the soiling to the stone minerals. This inevitably results in some dissolution of the components of the stone and, in addition, liquids applied to a porous stone are unlikely to be completely removed after cleaning and residues of cleaning chemicals may remain.

The aim of stonecleaning is to remove unwanted soiling from a building facade. The common perception of this soiling is that it is simply an accumulation of dirt on the stonework. However, this "dirt" is a complex mixture of materials including soot particles, hydrocarbons, gypsum and other salts, iron-bearing minerals, lead compounds, mineral dust, asphalt and rubber from car tyres (Nord and Tronner, 1993; Nord and Ericsson, 1993). The soiling layer also contains a biological component which may include algae, bacteria, fungi and lichens.

Stonecleaning of a soiled building facade may affect its characteristics as a substrate for organisms in a number of ways. If in no other way, stonecleaning is likely to affect the growth of organisms simply through removal of the soiling itself. Much of the soiling on stone is hydrophobic in nature. As soiling builds up on stone it gradually reduces the access of moisture into the stone. If soiling builds up to a sufficient level it may completely cut off fluid movement into and out of a stone through its face, although moisture may still be able to enter and leave the stone in the vapour phase and moisture movement can still occur internally. As has been shown removal of this hydrophobic soiling layer by abrasive cleaning may increase, perhaps greatly increase, the amount of moisture which can enter a stone. However, MacDonald (1993) has found that some chemical cleaning methods can increase or decrease water absorption rates (although these results were on initially fresh sandstone) making the results of cleaning unpredictable in terms of their effects on moisture absorption rates. Any increase in moisture availability is likely to increase the amount of biological growth and the range of species present on a stone. Increased movement of fluids through a stone may mobilise a previously stable mineral assemblage in the outer layers of the stone altering its suitability for particular organisms. Removal of many of the pollutants (e.g. sulphates) present in the soiling may allow organisms to colonise the stone which were previously inhibited by the toxicity of particular components of the soiling layer. Alternatively, some organisms (e.g. bacteria) may have previously utilised components of the soiling (e.g. hydrocarbons) as nutrients. Removal of a normally dark soiling layer will increase light penetration below the surface of a stone which, in a porous, light-coloured sandstone, may allow colonisation by photosynthetic organisms to greater depths than was previously possible.

Removal of the soiling from a building stone may therefore change the composition of the microbiological community on the building facade with unpredictable effects with respect to stone weathering and decay. However, no less important are other potential effects of cleaning methods on the stone itself as these may also influence biological growths.

2.3 Effects of chemical cleaning on algal growth

On sandstones treated with chemical cleaning Method A there were periods during which more algal growth was observed on treated samples than on non-treated (i.e. untreated and roughened) samples. For chemically cleaned Cat Castle sandstone , there was a sharp peak in algal growth during the winter of 1993-1994 which was not achieved again in following years. On Corsehill sandstone samples there was substantially more algal growth on chemically cleaned samples than on all non-treated samples throughout the duration of the experiment. It is also notable that throughout most of the period of exposure there was more algal growth on south-facing chemically cleaned samples than on north-facing non-treated samples despite the fact that south-facing samples almost always exhibited substantially less algal growth than north-facing samples on other stone types. On Leoch sandstone, although algal growths were slow to become established, when they did occur they showed the highest rates of growth on chemically cleaned surfaces.

On sandstones cleaned using Method B the results indicated either more algal growth on untreated samples or showed no consistently different pattern of algal growth compared to non-treated samples. It was clear that there was a difference in the response of sandstones with respect to algal growth depending on the chemical cleaning method used.

Two aspects of chemical cleaning could have been of importance with respect to encouraging algal growth:

2.4 Porosity changes

While the porosity of a stone can affect the growth of microorganisms, microorganisms can also affect the porosity of a stone both through physically blocking porosity and by actively changing porosity through the production of acidic or chelating secretions or by exerting physical forces on surrounding grains. Other non-biological factors can also influence porosity. These include weathering and decay processes and some stonecleaning methods. It has, for instance, been suggested (Bluck and Porter, 1991) that chemically cleaned sandstones may acquire algal growths relatively rapidly due to dissolution and pitting of mineral grains which increases the microporosity of the stone.

The chemical cleaning methods used in these experiments involved the application of a strong acid (hydrofluoric acid) which is capable of dissolving many of the minerals found in sandstone and a strong alkali (NaOH) which can also remove minerals (e.g. clay minerals) from stone. There was therefore a theoretical possibility that stonecleaning using these methods could result in measurable changes in porosity. However, data on pore size distribution show that, for the six sandstone types used in this study there was no significant change in either total effective porosity or in pore size distribution after chemical cleaning. In none of the graphs of pore size distribution was there any indication of a significant change caused by chemical cleaning. In two cases (Blaxter and Locharbriggs sandstones) the total effective porosity was slightly lower after cleaning. This could be due to residues of cleaning materials blocking pores (both these sandstones were cleaned using the poultice method) or it could simply be due to slight natural variation in porosity between samples as can be seen more obviously in the Clashach sandstone data.

It was possible that chemical cleaning affected the "surface porosity" of the sandstones perhaps by removing vulnerable minerals such as calcite, dolomite or clays. Mercury porosimetry can only measure the internal porosity of a sample, not changes to its outer surface. Changes to the outer surface of a sandstone were investigated by scanning electron microscopy (SEM). Results showed that chemical cleaning (both Methods A & B) can cause changes to the surface characteristics of sandstones with evidence of dissolution of feldspars, dolomite and removal of clays. The evidence of a greater degree of superficial dissolution caused by chemical cleaning Method A is consistent with the higher concentration of hydrofluoric acid present in this mix (14% compared with 2.7% in Method B). However, Method B was capable of removing vulnerable clays from the sandstone surface: smectite was removed but kaolinite appeared unaffected. The evidence from mercury porosimetry suggested that any dissolution or removal of grains was confined to the immediate surface and was unlikely to significantly affect the moisture retention characteristics of the sandstones. It is however possible that changes in the microporosity of the outer surface could affect the ability of organisms to attach to the surface in the initial stages of colonisation.

These results show that in these experiments significant porosity changes did not occur due to chemical cleaning. Porosity changes could not therefore be responsible for differences in the rate of algal growth on chemically cleaned and untreated sandstones. However, if cleaning were to be carried out using acids which were substantially more concentrated or where the dwell time of the acid was longer than was used here, changes in porosity might occur. Additionally, with respect to cleaning of building facades, repeated chemical cleaning may take place over a number of years. Dissolution of minerals by chemicals due to repeated cleaning will be cumulative and could lead to measurable porosity changes in the longer term.

2.5 Chemical residues

It has been shown, in previous research, that chemical cleaning methods can leave substantial residues of applied chemicals in sandstone. In a number of laboratory based cleaning trials, MacDonald (1993) found levels of chemical retention, as a percentage of the amount of material applied to the sandstone, of about 30-85%.

Of the two acidic cleaning solutions used in these experiments those used in Method A contained 28% H3PO4 and Method B contained 1.3% H3PO4 (at working dilutions). Phosphate is an element which is vital to organisms growth and it is normally in limited supply on most stone surfaces. Under normal circumstances the amount of bio-available phosphate would be a limiting condition on algal growth. The amount of phosphate in the sandstones used in the test rig (and in most other sandstones) was low. Phosphate in sandstones is mainly bound in minerals such as apatite (Ca5(PO4)3(OH,F,Cl)) or monazite ((Ce,La,Th)PO4). These minerals are usually only present in trace amounts (<<1%) and the phosphate is generally not readily available to organisms. Phosphate in the form of phosphoric acid (H3PO4), as used in some cleaning chemicals, is in a form which is easily utilised by microorganisms. Of the two cleaning methods, "A" obviously had the potential to leave more residual phosphate in the sandstones following cleaning as it contained approximately 20 times the concentration present in the Method B solution. It was therefore possible that increased phosphate levels in sandstones were responsible for the observed increased algal growth on some chemically cleaned samples.

The data clearly show relatively high levels of water soluble phosphate (63-64 ppm) in Corsehill and Cat Castle sandstone samples cleaned by Method A. The third sandstone type cleaned by Method A (Leoch sandstone) and the sandstones cleaned by Method B (Blaxter, Clashach and Locharbriggs) all showed relatively low levels of water soluble phosphate (2-9 ppm), although these were still substantially elevated above the levels found in untreated sandstones (0.2-0.3 ppm for all sandstone types).

It is likely that the increased algal growth observed on chemically cleaned Corsehill and Cat Castle sandstones was due to the relatively high levels of phosphate left in the sandstones following chemical cleaning. Increased levels of algal growth were also observed on chemically cleaned Leoch sandstone, although the levels of residual phosphate, while elevated above those of untreated sandstone, were not particularly high. However, Leoch sandstone has a much lower porosity than the other sandstones. While it can be assumed that applied chemicals would probably be distributed throughout the other sandstone types (all samples are approximately 15 mm in depth), on Leoch sandstone applied chemicals may have been retained close to the surface where the chemicals were applied. All the phosphate residues have been calculated as a mean concentration assuming equal distribution throughout the sample. For Leoch sandstone this assumption may not hold. If phosphate was retained close to the surface of Leoch sandstone, its concentration at the surface (where algal growth occurred) would be substantially higher than is indicated by assuming equal distribution throughout the sample and this could account for the increased algal growth noted on chemically cleaned Leoch sandstone samples.

Although initially elevated, algal growth declined on chemically cleaned Cat Castle sandstone after about one and a half years until there was little apparent difference compared to untreated samples. This shows that the factor in Method A chemicals promoting algal growth declined over time. Since changes to sandstone porosity would be permanent, this is another indicator that residues of cleaning chemicals rather than porosity changes were responsible for increased algal growth.

Differences in chemical retention between sandstones was also observed with respect to retention of biocides (Young et al., 1995). This effect is attributed to variations in the degree of adsorption of the biocides by clay minerals in the sandstones. Many biocides are cations or organic compounds which can bind to or be adsorbed by 2:1 clay minerals such as smectites and illites. Anions such as phosphate can also be bound to clay minerals but are attached by a different mechanism at different sites. Anions may be exchanged for hydroxide ions at sites on the edges of the crystal lattice and are more strongly bound to 1:1 clays such as kaolinite. The similarity in behaviour of the sandstones with respect to activity of biocides and retention of phosphates cannot therefore be due to the same mechanism of adsorption. Corsehill and Leoch sandstones contained clay types such as smectite, illite and chlorite which can adsorb and retain cations, but Cat Castle sandstone contains mainly kaolinite which should be better at adsorbing anions (e.g. phosphate). Phosphate can bind to other minerals in rock. Phosphate has an affinity for iron and may be bound to iron oxides which are present in relatively high amounts in some sandstones. Corsehill and Leoch sandstones contain relatively high amounts of iron oxides (1.4% and 4.0% Fe2O3 respectively) relative to the other sandstones. Cat Castle sandstone contains only about 0.6% Fe2O3.

While the amount of algal growth on chemically cleaned Cat Castle sandstone was initially increased, it declined to the levels found on untreated sandstone after about 1 1/2 years. On the other hand, increased levels of algal growth on chemically cleaned Corsehill and Leoch sandstones continued until the end of the experiment (about 3 1/2 years). This suggests that while phosphate may be bound to the kaolinite in sandstones it may be more strongly bound to iron oxides and may persist for longer in red (or other relatively iron-rich) sandstones. Some of the phosphate-bearing chemical cleaning agents are recommended for use on iron-rich sandstones as the addition of phosphate is intended to reduce iron mobilisation in the stone. This has important implications with respect to biological re-soiling rates on stonecleaned buildings since the above data suggest that increased levels of algal growth could persist for many years on cleaned stone.

It was not only algal growth which could be encouraged by chemical cleaning. Data on lichen growth on sandstone samples indicated increased lichen growth on sandstones chemically cleaned by Method A relative to non-treated sandstones and Method B cleaned samples. Although a number of species were present, the only genus identified on samples treated by Method A was Xanthoria, a genus characteristically found on nutrient rich sites (Dobson, 1992). Lichen growth did not begin to be observed until relatively late in the experiment and by the time that the first lichen growths were noted (summer 1995) the amount of algal growth on chemically cleaned Cat Castle sandstone was indistinguishable from that on untreated samples. This may indicate that lichens are stimulated by lower amounts of phosphate than algal growth, or perhaps the lichens can extract more strongly bound phosphate by direct contact with, or acid attack on, minerals in which the phosphate is bound.

Chemically cleaned (Method B) Blaxter, Locharbriggs and, to a lesser extent, Clashach sandstones also had levels of phosphate (2-9 ppm) which were higher than normal background levels (0.3 ppm). However, these sandstones did not display increased levels of algal growth on chemically cleaned samples. In many cases treatment with Method B chemicals appeared to decrease the ability of sandstones to support algal growth. Possibly, although these samples originally contained moderately elevated phosphate levels, these were insufficient to stimulate increased algal growth.

There was also no indication of increased lichen growth on these samples relative to untreated samples. Indeed other factors are important as can be seen from the fact that on south-facing Cat Castle sandstone, phosphate residues failed to stimulate significant amounts of algal growth as observed on north-facing samples, perhaps due to the relatively greater importance of moisture levels on the south-facing side of the test rig. Alternatively, phosphate levels may have declined to background levels by the time algal and lichen growths became established on the samples. Another possibility is that some other factor associated with chemical cleaning actively discouraged biological growths, negating the effects of increased phosphate levels. There was evidence of retention of sodium salts in the results of SEM examination of some Method B cleaned sandstones. Although the concentration of NaOH was lower in Method B than in Method A (6% and 25% respectively), in Method B the NaOH remained in contact with the stone for 24 hours in a poultice form compared to 20 minutes in liquid form in Method A. It is possible that residues of the NaOH-bearing poultice were trapped in the sandstones even after washing off. It is possible that salt residues which do not provide nutrients could reduce the amount of algal or lichen growth.

Residues of chemical cleaning agents in sandstones have been studied by MacDonald (1993). He found, using the same cleaning methods as were used here, that substantial amounts of applied chemicals could be retained by the sandstones. Similarly to the results found in the test rig samples, MacDonald (1993) also found much higher phosphate residues in sandstones cleaned by Method A compared to Method B. MacDonald (1993) found higher residues of phosphate than were found in the test rig samples, however his sandstones were much larger in size and might easily have retained more applied chemicals which could penetrate more deeply into the sandstone and migrate back to the surface after wash-off. This suggests that the problem of increased algal growth on chemically cleaned buildings might be even greater than is suggested by the test rig results since the implication is that the levels of phosphate retention in larger stones may be many times greater than in smaller samples.

There was some variation in the amount of algal growth on chemically cleaned sandstones on the north and south-facing sides of the test rig. The greater variability of results on the south-facing samples appeared to indicate that in some cases factors other than nutrition were strongly involved in controlling algal growth on the south-facing side of the test rig. Evidence was presented that differences in moisture availability strongly influence algal growth and it is suggested that in cases where the pattern of algal growth on untreated and chemically cleaned samples on the north side of the test rig was not repeated on the south-facing samples, the difference may be due to the greater degree of importance of moisture limitation with respect to algal growth on the south side of the test rig.

The decline in increased algal growth observed on Cat Castle sandstone after 1 1/2 years implies that increased algal growth on building facades caused by chemical cleaning should also decline over a number of years. However, depending on the stone type and exposure, the effect may persist for many years, especially on iron-rich sandstones and the soiling effects of algal growths may persist for longer than the nutrient effects.......

3. Conclusions

In this thesis I have described those factors which influence biological growths, especially algal growths, on building sandstone and investigated how to monitor these growths and how the effects of stonecleaning or biocide treatments influence them.

It is clear from this research that some stonecleaning methods can affect algal growths on building sandstones. In the circumstances investigated here the degree of surface roughening caused by abrasive cleaning did not appear to cause any increased algal growth. The most important factor was found to be phosphate residues from some chemical cleaning methods. There is wide variation in the amount of phosphoric acid which is used in stonecleaning chemicals. Clearly, higher concentrations have the potential to cause increased algal growth on some buildings for a number of years and on the more vulnerable stone types it would be better to choose a method which contained the least amount of phosphoric acid necessary. It is unclear whether the addition of phosphoric acid to cleaning chemicals is actually necessary and if so, what concentration is required. This question should be further researched.

It is also clear that the performance of biocides varied greatly depending on chemical type, concentration and the nature of the sandstone. There has been virtually no work done on the interaction of biocides with sandstones and this is clearly necessary if improved treatments are to be developed.

Many factors other than those which have been investigated in detail here are involved in controlling algal colonisation and growth on building sandstone. Alteration of these factors would be expected to increase or decrease the amount of biological growth on building stone. Some of these factors, such as climatic conditions (rainfall, humidity, etc.), are uncontrollable. Deposition of atmospheric pollutants may also be generally regarded as an uncontrollable phenomenon with respect to individual buildings, although pollution from locally derived sources may be controllable under some circumstances. Other factors are conditions imposed by the design of the building and are essentially unalterable; these include stone type, facing directions (i.e. north, south, etc.) and surface orientation (i.e. vertical, sloping, horizontal). Some factors which are conditions of building design are alterable (e.g. control of run-off by maintenance of gutters and down-pipes). A number of factors are controllable or alterable. Biological growths on a building facade may be controlled for short periods by application of biocidal washes. If biological growths are largely a consequence of moisture retention caused by shading of a building facade by overhanging vegetation then this may be cut back or removed.

Stonecleaning can also alter some of the characteristics of stone in ways which may affect the growth of algae and other organisms. Removal of soiling may reduce the levels of some toxins (e.g. sulphate) and some nutrients (e.g. hydrocarbons) at the stone surface. Surface roughening caused by abrasive cleaning may affect the susceptibility of a surface to biological growths. Residues from chemical cleaning may increase algal and lichen growth by providing nutrients which are normally in limited supply on stone surfaces.

In conclusion, biological growths including algae, lichens, fungi and bacteria will eventually colonise any suitable surface. Biological growths may be controlled by application of biocides although regular re-application is likely to be necessary every few years and the long term consequences of multiple applications are unclear. Some chemical cleaning methods have been shown to encourage biological growths on building sandstones and may increase growth of algae and lichens for some years following treatment. Any treatment which has the potential increase algal or other growths on a facade should obviously be used with care. Although some biological growths such as algae may be considered aesthetically damaging to a building's appearance they do not necessarily cause significant physical damage to the stone and some growths such as lichens may give a mature appearance to a facade. While under some circumstances it may be appropriate to treat biological growths as harmful to a facade, in many circumstances attempts to control or remove growths could result in more damage to the stone than leaving them in place. Soiling and decay of buildings are natural processes and we can only hope at best to slow these processes, not to halt them.

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Micro-organisms and the weathering of a sandstone monument. In: W.E. Krumbein, ed., Environmental Biogeochemistry and Geomicrobiology. Mich. USA: Ann Arbour Science. pp 675-686.

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