593 Factors affecting the growth of algae on cleaned sandstone buildings in Scotland Maureen E. Young and D.C.M. Urquhart Masonry Conservation Research Group The Robert Gordon University Aberdeen Abstract of buildings and monuments by biological growths, particularly on recently cleaned bmldmgs, is a cause of some concern. Over the past 20 years many of Scotland's building facades have been subjected to stonecleaning. Following cleaning, re-soiling from inorganic sources may take many y ears, however, re-soiling in the form of green algal growths may occur within only a few months. The effects of stonecleaning methods can be to change the physical and chemical properties of sandstones and hence alter the susceptibility of the stone to biological growths. Six different sandstones were chosen for experimental investigation. Samples of these sandstones were subjected to commercially available physical and chemical stonecleaning methods . An outdoors test rig was used to expose sandstone samples to_ natural weathering over a period of 3-4 years. The rate of algal growth on sandstone samples was measured by monitoring colour changes using a chroma meter. The characteristics of treated and untreated sandstones were assessed in terms of surface roughness, pore size distribution and availability of nutrients. While there was no indication that algal growth rates were faster on abrasively cleaned sandstones, some chemical cleaning methods did accelerate algal colonisation of sandstone surfaces. The nature and degree of this effect was found to be dependent on the characteristics of the chemicals and sandstones. Keywords : sandstone, algae, stonecleaning, phosphate, roughness. Introduction Biological growths such as algae, bacteria, fungi, lichens and mosses are common on the exterior of buildings, especially in rural areas. Although microorganisms can damage stone (Bock and Sand, 1993; May et al., 1993; Ortega-Calvo et al., 1993), growths may also be non-destructive and may be considered desirable since they can give a mature appearance to a facade. However, the disfiguring of buildings and monuments by biological growths, particularly on recently cleaned buildings, is a cause of some concern. Over the past 20 years many of Scotland's building facades h ave been subjected to stonecleaning. Following cleaning, re- soiling from inorganic sources may take many years, but re-soiling in the form of green algal growths may occur within only a few months. The degree of algal re-growth has been s uggested to be a direct consequence of stonecleaning intervention (Bluck and Porter, 1991). Stonecleaning methods can alter some properties of sandstones and may alter the susceptibility of the s tone to biological growths. In this paper the colonisation of stonecleaned sandstones by biological growths is investigated in terms of their dependence on certain physical and chemical parameters including the suppl y of nutrients and surface roughness. Methodology To investigate the rate of growth of algae on variously treated sandstones a north and south facing t es t rig (Figure 1) was construct ed to hold a number of sandstone samples. The test rig was located outside, in a courtyard at Garthdee, Aberdeen, Scotland , so that the samples (50x50x15 mm) were exposed to natural urban wea thering. The sands tone used in the experin:ients was chosen to represent a wide range of characteristics typical of building used m Scotl. and (Tab.le 1). Samples consisted of untreated, surface roughened (gnt blasting at 280 kPa (40 psi)), cherruca ll y cleaned (Table 2) and biocide treated (not dealt with in this paper) sands tones. The methodology for stonecleaning of samples followed, as far as possible, methods which would be u sed on a building facade. Samples were he ld at an angle of 60° from the horizontal, with three replicates of each. trpe. The sa ndstone samples were free of any pre-existing algal growths but each stone was m1hally inoculated with 5 ml of a solution containing known algal types (Stz chococcus sp., Chlorococcum sp., Botrydiops is sp. and Tetraspora sp.) isolated from field samples taken from a range of Scottish sandstones.
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593
Factors affecting the growth of algae on cleaned sandstone buildings in Scotland
Maureen E. Young and D.C.M. Urquhart Masonry Conservation Research Group The Robert Gordon University Aberdeen
Abstract Th~ ~isfig~ring of buildings and monuments by biological growths, particularly on recently cleaned bmldmgs, is a cause of some concern. Over the past 20 years many of Scotland's building facades have been subjected to stonecleaning. Following cleaning, re-soiling from inorganic sources may take many years, however, re-soiling in the form of green algal growths may occur within only a few months. The effects of stonecleaning methods can be to change the physical and chemical properties of sandstones and hence alter the susceptibility of the stone to biological growths. Six different sandstones were chosen for experimental investigation. Samples of these sandstones were subjected to commercially available physical and chemical stonecleaning methods. An outdoors test rig was used to expose sandstone samples to_natural weathering over a period of 3-4 years. The rate of algal growth on sandstone samples was measured by monitoring colour changes using a chroma meter. The characteristics of treated and untreated sandstones were assessed in terms of surface roughness, pore size distribution and availability of nutrients. While there was no indication that algal growth rates were faster on abrasively cleaned sandstones, some chemical cleaning methods did accelerate algal colonisation of sandstone surfaces. The nature and degree of this effect was found to be dependent on the characteristics of the chemicals and sandstones. Keywords : sandstone, algae, stonecleaning, phosphate, roughness.
Introduction Biological growths such as algae, bacteria, fungi, lichens and mosses are common on the exterior of buildings, especially in rural areas. Although microorganisms can damage stone (Bock and Sand, 1993; May et al., 1993; Ortega-Calvo et al., 1993), growths may also be non-destructive and may be considered desirable since they can give a mature appearance to a facade. However, the disfiguring of buildings and monuments by biological growths, particularly on recently cleaned buildings, is a cause of some concern. Over the past 20 years many of Scotland's building facades have been subjected to stonecleaning. Following cleaning, re-soiling from inorganic sources may take many years, but re-soiling in the form of green algal growths may occur within only a few months. The degree of algal re-growth has been suggested to be a direct consequence of stonecleaning intervention (Bluck and Porter, 1991). Stonecleaning methods can alter some properties of sandstones and may alter the susceptibility of the stone to biological growths. In this paper the colonisation of stonecleaned sandstones by biological growths is investigated in terms of their dependence on certain physical and chemical parameters including the supply of nutrients and surface roughness.
Methodology To investigate the rate of growth of algae on variously treated sandstones a north and south facing test rig (Figure 1) was constructed to hold a number of sandstone samples. The test rig was located outside, in a courtyard at Garthdee, Aberdeen, Scotland, so that the samples (50x50x15 mm) were exposed to natural urban weathering. The sandstone used in the experin:ients was chosen to represent a wide range of characteristics typical of building sa~dstones used m Scotl.and (Tab.le 1). Samples consisted of untreated, surface roughened (gnt blasting at 280 kPa (40 psi)), cherrucally cleaned (Table 2) and biocide treated (not dealt with in this paper) sandstones. The methodology for stonecleaning of samples followed, as far as possible, methods which would be used on a building facade. Samples were held at an angle of 60° from the horizontal, with three replicates of each. trpe. The sandstone samples were free of any pre-existing algal growths but each stone was m1hally inoculated with 5 ml of a solution containing known algal types (Stzchococcus sp., Chlorococcum sp., Botrydiopsis sp. and Tetraspora sp.) isolated from field samples taken from a range of Scottish
sandstones.
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Algal growth was monitored by measuring changes in the colour of the sandstone samples at intervals using a chroma meter (Minolta Chroma Meter CR-210). Changes in green coloration can be used to monitor algal growth. The colour scale used was L *a*b*. L * is a measure of the brightness level (higher value=brighter), a* and b* are chromaticity co-ordinates, a* for green (-ve) to red (+ve) and b* for blue (-ve) to yellow (+ve). Calculating the change in a* coloration from the initial value can be used to monitor algal growth. The more negative the value, the more intense the green coloration and the greater the amount of algal growth on the surface.
Figure 1 Test rig for sandstone samples.
Table 1 Characteristics of sandstones.
Porosity Specific Mean grain Saturation Calcareous Colour Sandstone (%) gr~vi!Y_~cm3) diameter (µm) coefficient
Blaxter 21.2 2.12 250 0.59 N o buff Cat Castle 14.4 2.25 600 0.66 No buff Clashach 18.1 2.16 200 0.49 Yes buff Corse hill 17.7 2.22 120 0.64 Yes red
Leo ch 6.1 2.52 300 - Yes _grey Locharbriggs 19.6 2.04 350 0.65 No red
Table 2 Composition of chemical stonecleaning agents used on sandstone samples.
Figure 2 shows algal growth patterns on the samples. The most obvious trends were seasonal variations in growth which can be attributed mainly to changes in the availability of moisture. While the overall pattern of algal growth was the same on the north and south-facing sides of the test rig, there was relatively more algal growth on north-facing samples. This can be attributed to the effects of direct sunlight on the south side of the test rig which reduces the period of dampness of southfacing samples relative to samples on the north side and reduces the period of time when algae could actively grow.
Comparison of the data on untreated and surface roughened samples indicated that increased surface roughness (due to abrasive cleaning) did not affect algal colonisation or growth rates on these samples. However, in some cases (especially where the samples were cleaned by Method A) the amount of algal growth on chemically cleaned samples was substantially different to that seen on untreated samples. Figure 3 shows, at time intervals, the p-values (2-tailed, displayed on graph as "1 - p-value") for t-tests comparing the a* colour changes from initial values of untreated and chemically cleaned samples. Values for "1 - p-value" of over 0.95 represent >95% probability of a significant difference in colour between untreated and chemically cleaned samples. It is clear from this data that there was a difference in the response of sandstones cleaned by the two different chemical cleaning methods with respect to algal growth. On samples cleaned using Method B, results indicated either more algal growth on untreated samples or showed no overall consistent pattern. On samples cleaned using Method A, data indicated that there was often significantly more growth on chemically cleaned than on untreated samples. On Corsehill sandstone relatively more algal growth occurred on chemically cleaned samples throughout the experimental period (3-4 years). On Leoch sandstone there was relatively more algal growth on chemically cleaned samples once algae became established (algal growth was relatively slow on this low porosity sandstone). On Cat Castle sandstone increased algal growth on chemically cleaned samples occurred only during the first Yz years of exposure.
Increased algal growth on samples cleaned by Method A could be attributable to two factors . Increased microporosity due to etching by acidic chemicals (e.g. HF acid) might affect the moisture retention of the sandstone. Alternatively, retention of chemical cleaning agents could provide nutrients for algal growth. Previous experiments have shown that significant amounts of cleaning chemicals may be retained in sandstones (MacDonald et al., 1992; MacDonald, 1993). With Method A the amount of phosphate in the cleaning chemicals was much greater than with Method B. Phosphate is an essential nutrient for algae and is normally in limited supply on a stone surface. The total phosphate in the sandstones is shown in Table 3. Very little of this phosphate would be easily available to organisms as it is mainly bound in minerals.
Table 3 Selected oxides(%) in sandstones (analysed by X-ray fluorescence).
Oxide Sandstone (%) Blaxter Cat Castle Clash a ch Corse hill Leoch Locharbriggs
Fe20 3 0.95 0.56 0.36 1.36 3.98 0.73
P20 ; 0.00 0.00 0.08 0.10 1.43 0.00
Results of pore size distribution measurement by mercury porosimetry (Figure 4).showed that for the sandstones and chemical cleaning methods used in this study, there was no s1gruficant change m either total effective porosity or in pore size distribution after chemical cleaning. Changes in pore size distribution could not therefore be responsible for observed differences in the rate of growth of algae on chemically cleaned and untreated sandstone samples.
Preliminary results indicate that substantial amounts of soluble. phosp.hate are present in samples chemically cleaned by Method A. It is hoped that further work will confirm the ex1stance of .elevated phosphate levels in other samples cleaned by Method. A. This would imply that residues of phosphate bearing cleaning chemicals can, at least for hm1ted penods, increase the rate of algal growth on sandstones.
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Figure 2 Mean trends in algal growth (green coloration) on variously treated sandstone samples, as measured by changes in a* coloration.
~ u ·.
·.
Cleaned by Method A
Cat Castle sandstone
·5
·5
·• ·3
·• ·I
02- 12-Mar 20 ....... 1 27· Nov Nov
08·Apr t4·Aug 22· Doc
Corsehill sandstone
·•
02· 12-Mar 20-JlA Nov
27- 08-Apr 14-Aug 22-Nov
Leoch sandstone
· fi ,5
02- 12-Mor 20-Jul 27-Nov Nov
greener
Doc
06-Apr 14-Aug 22-Dec
a* colour
redder
01· 08-S.p 18-Jln 25-M•y Mey
01 - 08-Sep 16-Jan 26-May May
01- 08-Sep 16-Jan 25-May May
Cleaned by Method B
Blaxter sandstone
·5.5
27· Of.Apr 14-Aug 22-Nov Nov
-6 .S
-4.5
· 3.5
2·2.5 8 ·,,,-1 .5
-0. 15
Clashach sandstone
Doc
I I ... Nov
27- 08-Apr 14-Aug 22-Nov Dec
Locharbriggs sandstone
-----untreated: north-facing
...... ., ..... abrasively cleaned: north-facing
--1o-chemlcally cleaned: north-facing
. CJ. ·· untreated: south-facing
·· abrasively cleaned : south-facing
· 6· ·· chemically cleaned: south-facing
.. . · ... fresh sst
01- 09-Sep 18-Jan 25-Mey Mey
I I 01 · 08-Sep 16-Jan 25· May May
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Figure 3 Statistical significance of differences in algal growth between untreated and chemically cleaned samples (see text). North-facing samples only. Dotted line on graphs shows 95% probability of a significant difference between chemically cleaned and untreated samples.
I
I
l
Cleaned by Method A Cleaned by Method B
Cat Castle sandstone
0.9
0.8
0.7
~ 0.6
~ 0.5 Cl.
.'.- 0.4
0.3
0.2
0.1
Corsehill sandstone
0.9
0.8
0.7
~ 0.6
" ? 0.5 a.
0.4
0.3
0.2
0.1
Lench sandstone
0.9
0.8
0.7
~ 0.6
~ 0.5 Cl.
0.4
0.3
0.2
0.1
11
Blaxter sandstone
0.9
0.8
0.7
~ 0.6
l 05
..:. 0.4
0.3
0.2
0.1
.§
Clashach sandstone
0.9
0.8
0.7
~ 0.6
l 05
;_ 0.4
0.3
0.2
0.1
Locharbriggs sandstone
0.9
0.8
0.7
g: 0.6
~ 05 Q.
..:. 0.4
0.3
0.2
0.1
o more growth on untreated samples
•more growth on cleaned samples
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Figure 4 Selected cumulative porosity data for sandstones before and after chemical cleaning.
Cat Castle sandstone Cleaned by Method A Blaxter sandstone
Conclusions Stonecleaning affects the surface and near surface characteristics of sandstones. While growths present at the time of cleaning may be effectively removed, most stonecleaning treatments do not have any long term biocidal effect and in the absence of any preventative treatment, re-establishment of organisms is likely to occur on a fairly short timescale. Some of the effects of stonecleaning may also accelerate colonisation of a facade after cleaning.
Roughness affects the growth of organisms on building facades in a number of ways. It has been shown by other investigators that roughness may affect the colonisation of a substrate by affecting the ease with which spores can settle on or attach to a vertical surface (Darlington, 1981). Rougher surfaces can provide a more sheltered habi tat making it easier for initial colonisation to take place. Roughness can also affect moisture availability (Pietrini et al., 1985). The experimentsdescribed here did not indicate that sandstone roughened by abrasive cleaning was colonised more rapidly than untreated sandstone. It is possible that the degree of roughening caused by cleaning was insufficient to observe any effects on algal colonisation. Additionally these samples were held at 60° and were initially inoculated with algal spores. It is possible that a different effect might be observed on vertical surfaces where spore settling and moisture absorption would be more critical.
Two different chemical cleaning methods were used on the test rig samples and these were shown to have very different effects with respect to algal growth rates. Chemical cleaning by Method A was shown to significantly increase algal colonisation and growth. Investigations of pore size distribution showed no significant differences after cleaning and indicated that algal growth was not affected by changes in porosity due to etching by cleaning agents. This cannot however be taken to imply that chemical cleaning is incapable of causing changes in stone porosity. On more vulnerable calcareous stones, for instance, significant porosity changes might be expected. Porosity might also be affected if more concentrated acidic cleaning chemicals were used or where they were applied for longer periods of time. The effects of repeated cleaning may also be important as etching of minerals will be cumulative.
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Increase algal growth on sandstones chemically cleaned using Method A was most probably due to nutrient retention (phosphoric acid in cleaning chemicals). Assuming that results of ongoing experiments confirm this, algal growth measurements indicated that elevated phosphate levels persisted for longer on Corsehill and Leoch sandstones (>31/2 years) than on Cat Castle (11/2 years). This is consistent with observations made of activity of biocides in the same sandstones (Young et al., 1995) where biocidal chemicals were found to remain active in Corsehill sandstone for longer than in Cat Castle sandstone. This effect was attributed to variations in the degree of adsorption of the biocides by different clay minerals. A similar mechanism may affect retention of nutrients. It is also possible that phosphate retention is affected by the presence of iron oxides in the sandstones as phosphate has a strong affinity for iron. Corsehill and Leoch sandstones have relatively high iron oxide contents (Table 3) relative to Cat Castle sandstone.
The decline in algal growth observed on Cat Castle sandstone after M years implies that elevated levels of algal growth on building facades caused by chemical cleaning should decline after some time. However, depending on the stone type and exposure, the effect may persist for many years and, since the remains of earlier algal growth will remain on the stone, the soiling effects of elevated levels of algal growth will persist even after nutrient levels in the stone have declined to more normal levels.
Acknow ledgernents Part of this work was undertaken in a research project exarrumng algal growth on building sandstones and the effectiveness of biocide treatments. The research programme was funded by Historic Scotland.
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