Corrosion of ancient marble monuments by rain: Evaluation of pre-industrial recession rates by laboratory simulations

Atmospheric Environment Vol. 271], No. 3, pp. 339-351, 1993.

0957 1272/93 $6.00+0.00 © 1993 Pergamon Press Ltd

Printed in Great Britain.

CORROSION OF ANCIENT MARBLE MONUMENTS BY RAIN: EVALUATION OF PRE-INDUSTRIAL RECESSION RATES BY LABORATORY SIMULATIONS* FEDERICO GUIDOBALDIand ANNA MARIA MECCHI Centro CNR Conservazione Opere d'Arte, Rome, Italy (First received 20 April 1991 and in final form 1 April 1993)

Abstract--By a multiple series of laboratory simulations of rain runoff on marble tiles it was possible to obtain a semi-quantitative evaluation of pre-industrial recession rates for marble surfaces exposed to weathering. The approximate value of 0.03-0.05 mm per century was established for partially sheltered vertical surfaces. The lower value refers to climatic conditions typical of Rome whereas the higher value relates to more severeclimatic conditions (heavierrainfalls and/or lower rain intensities).Even higher values, that is, 0.15-0.25 mm per century (always to be intended as approximate) can be obtained for fully exposed and inclined marble surfaces. These data were compared to the recession rates obtained by other scholars, especially relating to tombstone studies: a certain agreement was found only for the lower values reported. On this basis a revision of the methodologies and an interpretation of the divergenciesare proposed.

1974) and, in some cases, even later (Parrini and Soverini, 1984; Crnkovic, 1985; Martin and Barber, It is only recently that some data regarding recession 1987; Cheng et al., 1987) were over-simplified and rates of marble surfaces exposed to the open air, have therefore not comparable to the real phenomenon. been published. However, the majority of these refer to A research programme was started in 1981 at the the global effect of wet and dry deposition and do not Centro CNR 'Conservazione Opere CArte', in Rome, allow us to separate the contribution of rainfall and using a multi-parametric experimental simulation air-pollution to deterioration. which allowed us to establish a closer link between the For this and other reasons some scholars have laboratory results and the real phenomenon. affirmed that it is very difficult, if not impossible, to An important experimental advantage of the proestablish a reliable recession rate for the marble posed method, which will be described in the following surfaces of monuments (Charola, 1988), others have paragraph, is the possibility of relating the effect of the critically examined the results directly obtained variation of each parameter, which characterizes the (Dragovich, 1987; Reddy, 1988; Mirwald et al., 1988) rainfall-marble interaction, to the calcium concentraand others have reviewed previous evaluations, exam- tion in the simulated runoff. ined the problems and suggested more positive apThis can immediately provide a comparison beproaches (Livingston and Baer, 1985; Husar et al., tween the damage induced by the variation of one 1985; Gibbs, 1986). In reality, the majority of the specific parameter and another and, therefore, the experiments are carried out on newly quarried or calcium concentration itself can become an index of ancient materials exposed to the present polluted weathering. atmosphere and therefore no information is given The main advantage, however, of this method is about the past, if not through theoretic extrapolations that it allows us to study separately the effects of both or by identifying the pre-industrial with actual rural "clean" and polluted rain. This means that we can, on atmospheres. the one hand, compare the difference in corrosivity These difficulties can only be overcome by the use of between rain having various degrees of pollution and, laboratory simulations that have the additional ad- on the other hand, evaluate the corrosive effect of vantage of allowing us to dissect the mechanism of "clean" rain. As this was certainly the predominating, marble corrosion by rainfall in many single elements, if not the only, aggressive agent in very ancient each one relating to the variation of a single para- atmospheres, the recession rate caused by "clean" rain meter. This kind of approach has already been pro- only, can be assumed to be very close to the global posed (Pauly, 1973; Guidobaldi, 1981; Livingston, recession rate for the pre-industrial past. 1982) but the experimental simulations carried out It is clear that our simulation method has some before the 1980s (Vinot, 1971; Thomson and White, disadvantages and we will mention here only the most important. First, it only takes into consideration the *Paper presented at the International Conference on primary corrosive effects and not the secondary ones, Acidic Deposition. Glasgow, September 1990. such as the crystallization of soluble or semi-soluble INTRODUCTION

339

340

F. GU1DOBALD!and A. M. MECCHI

salts, the intercrystalline ionic substitutions etc., and secondly, it only refers to new quarry materials that are more resistant to the aggressive atmospheric agents than ancient ones, which have a higher porosity and many existing structural alterations. These and other limitations in the methodology inevitably correspond to a decrease in the precision of the evaluations, but we have to take into account that an exact determination of the weathering rate, as observed in previous studies (Guidobaldi, 1981, 1987) is impossible to obtain, even using other experimental methods. Anyway, even though our absolute evaluations can never be considered more than approximate, the relative ones remain valid and provide a lot of information about the rainfall-marble interaction.

SIMULATION APPARATUS The experimental method and related apparatus have already been described in a previous work (Guidobaldi, 1981) but we think that a brief description here will be useful. A tubular glass vessel equipped with five ground-glass joints, under which can be inserted five standard medical needles, was used for dripping the simulated rain solution. A large (25 •) container, the position of which can be vertically regulated (so varying the hydrostatic pressure on the needles), is the simulated rain reservoir. A moveable support with small marble tiles (15 × 5 × 1 cm) is placed underneath the needles in such a way that simulated rain falls on the tiles and runs down to be collected and then stored in small airtight glass bottles (Sovirel). The calcium concentration in the runoff solution is determined by AA spectrophotometry. The parameters, which can be varied independently even if only discontinuously, are reported in Table 1 together with the symbols eventually adopted and the values to which parameters were fixed when kept constant.

In the simulations that we are concerned with here only eight of these parameters have been applied (numbers 1-8). The others (9-12) were always kept constant. As our aim was to obtain data for the monuments in Rome, a quantity of 100 mm of rainfall per year was established as a standard condition, which does not correspond to the total annual rainfall between 500-800mm, but to the percentage of this that is inclined sufficiently to wet a partially roofed vertical marble surface. If we consider a totally exposed marble surface, the recession rate must be increased at least five times. To adapt the simulations to other climatic conditions we only need to multiply the recession rate data by the value (expressed in dm) of the annual rain that hits the surface. The parameters mentioned (numbers 1 8) have also been varied in pairs so that we could study the synergic effects and the eventual interferences. Only some of the results of those "cross-variations" are reported in Figs 1 8 (especially when acidity is involved). We must add that the data utilized here, having been obtained in various experimental phases and published separately between 1981 and 1989 (Guidobaldi, 1981; Guidobaldi and Mecchi, 1985; Guidobaldi et al., 1988; Guidobaldi et al., 1989) sometimes refer to slightly different standard conditions and so they are not necessarily homogeneous. To overcome this problem we either had to repeat some of the simulations or to re-use other unpublished data or to interpolate two different series of measurements. The modifications were made, above all, on the data pertaining to the surface roughness, the frequency, the temperature and the acidity.

RESULTS AND DISCUSSION The corrosion data corresponding to the variation of the single parameters or pairs of them are re-

Table 1 Parameter 1. Quantity of rainfall 2. Rainfall intensity 3. Frequency of precipitations 4. Temperature 5. Rain acidity 6. Chemical composition of simulated rain (N) 7. Additional runoff 8. Average surface roughness of tile 9. Size of drops (dia.) 10. Angle of rainfall l 1. Material 12. Efficacy of superficial treatments

Symbol

Constant value

Q 1 F T pH C

100 mm 6.7 mm min- 1 1 25 C 5.6 or 4 0 (distilled water)

AR RA

100 %* ca 5 l~m ca 2 mm

45 ° Carrara marble (no treatment)

* The area of the tile directly hit by the drops is normally about 10 cm 2, but this can be increased to 15 cm 2 if we include the edge of the tile that the drop runs over; an increase of the runoffis obtained by sliding the tile lower so that the solution can be made to flow on an additional area of the tile on which dripping does not occur.

Corrosion of ancient monuments by rain

341

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500 Rainfall

mrn

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•

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0.01 •

1'o

pH5.5

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20

Rain Intensity

mrn/min

Fig. 2. Recession rate of marble surfaces as a function of rainfall intensity at two different pH values.

presented in Figs 1-8. The original data, expressed as calcium concentration "(as they were published previously), are presented here as recession rates in m m per century so that they can be compared with other published results. In Fig. 1 one can see as expected, a linear dependence of the recession rate on the rainfall quantity. In Fig. 2, also as expected, an increase in the corrosion corresponds to the decrease in rainfall intensity. Even if the intensities were exceedingly high, our simulations allowed us to draw a curve that can be extrapolated towards the lower intensities.

The choice of the rather high intensities is justified by their suitability to the experiment: it has been established, in effect, that lower intensities cause an incomplete runoff of the solution on the tile surface. We must admit, however, that this parameter is the most difficult to reproduce in our simulation because the most c o m m o n real rainfall has an intensity that does not correspond to a runoff because the drops are too small and too few and so they are almost totally absorbed by the material. In this way, no runoff can be collected and no calcium concentration can be detected.

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Fig. 4. (a) Recession rate of marble surfaces as a function of time with a simulated rainfall intermittence of 10 min at two different pH values. (b) Recession rate of marble surfaces as a function of time with a simulated rainfall intermittence of 24 h at two different pH values.

Corrosion of ancient monuments by rain

343

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Temperature

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Fig. 6. Recession rate of marble surfaces as a function of rain temperature at two different pH values. This limitation is, at least in part, overcome both by extrapolation from the measured concentrations corresponding to higher values and by considering that the calcium concentration does not reach, or even approach, in our experimental conditions, the solubility limit of calcium carbonate in equilibrium with atmospheric carbon dioxide, about 60 ppm (Miller, 1952). Our experiments and other research (Bawden, 1988; Reddy and Sherwood, 1985) indicate that only the calcite (and not calcium sulphate) supplies the calcium in solution. This does not normally exceed 8 or 9 ppm which corresponds to 20 or 24 ppm of calcium carbonate.

In Fig. 3 the notable increase in corrosion that seems to correspond to the increased runoff must be, in part, redimensioned. It is most unlikely, in fact, that the surface subjected to washing is much larger than that subjected to dripping. In effect, a one-to-one relationship between the two surfaces (additional runoff = 100 %) already seems rather high and we must keep in mind that the additional runoff relates to the geometry of the surface and is thus a constant for a specific monument. Nevertheless, this parameter should not be overlooked and further research is needed to establish a more correct and analytical evaluation of it. In our simulations, we need, above all, to distinguish between the calcium carbonate dis-

344

F. GUIDOBALDIand A. M. MECCHI

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Fig. 7. (a) Recession rate of marble surfaces as a function of sodium nitrate concentration in the simulated rainfall. (b) Recession rate of marble surfaces as a function of sodium sulphate concentration in the simulated rainfall at two different pH values.

solved by the single runoff from that dissolved by the mechanical impact of the drops. We have found an experimental way to achieve this but our laboratory tests are still to be concluded. In particular, we have to take into account that in our previous experiments, the results of which are shown in Fig. 3, we assumed as AR = 0 the dropping on a surface of 15 cm 2 so that all this wet area is hit by the drops (and so we are only referring to the irregular runoff which corresponds to the impact of the water and its detachment from the tile). This is because the "additional" runoff was intended as something in addition to the normal impact. If we plot the same recession rate values in Fig. 3 as a function of the wet area (so from

15-50 cm -2 and not from 0-233%) our diagram can be extrapolated to lower values. The intersection with 0 cm 2 will give, in our opinion, the additional corrosion due to the mechanical effect of dropping only. In Figs 4a and 4b the variation of rainfall frequency, surprisingly, shows very little influence on the recession rate. This result is very useful as we can consider annual rainfall as a single precipitation thus simplifying the evaluation of corrosivity. Figure 5 reveals another unexpected result that differs from some theoretical assumptions by other scholars (Livingston and Baer, 1983): the recession rate of the marble is practical!y .unaffected by the surface .roughness in the large range of variations

Corrosion of ancient monuments by rain

345

0.20.

0.15.

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•

0.05.

Q n,

PH

Fig. 8. Recession rate of marble surfaces as a function of rain acidity (pH).

proposed for this parameter. This is another simplification of our modelling of the rain-marble interaction. This does not affect, of course, the possibility of relating an increase of roughness to the increase in surface weathering so using the roughness as an index of weathering (Grimm, 1983). In Fig. 6 the effect of temperature is shown. The recession rate increases notably with the increase of the temperature both with weakly acid and acidsimulated rain. This could be expected, as the influence of temperature on almost all the other parameters regulating corrosion is notable. Anyway, our simulation was not extended to a large range of temperatures as this would not have reflected the real thermal excursion of rain. In Figs 7a and 7b the recession rate vs concentrations of various salts in simulated rain is shown. Even though an increase of corrosion can be seen, the effect is lower than expected. In particular, the presence of sodium nitrate causes a very modest additional corrosive effect (Fig. 7a) as does the presence of sodium chloride while a more evident increase of the recession rate occurs in the presence of sodium sulphate (Fig. 7b). We must take into account, however, that the majority of the salt concentrations used in the simulations are very high in comparison to real rain. The limited effects we have observed are, however, related to direct and primary reactions and, therefore, do not take into account the secondary effects of surface or internal salt deposits which are generally considered responsible for the mechanisms of deterioration and which are extremely destructive for marble and stone. In Fig. 8 we can see the effects of simulated rain acidity on the recession rate. The acidity, together with

the salt concentration, is the parameter that specifically introduces the effect of air pollution. For this reason it was inserted as an additional variable in most of the other simulations (see Figs 1-4, 6 and 7b) even though only two pH values 5.6 (or 5.5) and 4 were used. The same data, expressed as calcium concentration in the runoff solution, have already been published (Guidobaldi, 1981) but have only rarely been assimilated in subsequent publications (Charola, 1988). Frequently, the same kind of considerations, supported or not by experimental evidence, have been reported as new results (Livingston and Baer, 1983; Gibbs, 1986; Sherwood and Reddy, 1988; Bawden, 1988) but in most of the publications our results have not been applied and acid rain, even if very weak, is still retained more corrosive than it really is (Winkler, 1982, 1988; Amoroso and Fassina, 1983; Torraca, 1988; Brocco et al., 1988). The graph in Fig. 8 shows, in fact, that the dissolution by acid equates the solubilization by water only at a pH of about 4. The dissolution increases in proportion to the acidity, only at a pH lower than 4 and so at pH 3 the dissolution is about 10 times higher than it is at pH 4. The calcium carbonate solubilization by water, being relatively constant in constant dynamic conditions, predominates in weakly acid conditions (when the acid corrosion contribution is practically negligible) but becomes negligible with the increase of acidity. The corrosive effect of acid rain, then, is extremely worrying ira pH lower than 3.5 is maintained for long periods of time; is moderately worrying if the pH is between 3.5 and 4.5 and is practically negligible, in our accuracy levels, if the pH is higher than 4.5. If we then also take into account that, during one year, the

346

F. GUIDOBALD]and A. M. MFCCH]

highest levels of acidity are reached only occasionally and, however, these are maintained only for short periods (Hicks, 1982), the responsibility of acid rain in the deterioration of marble monuments can be completely reconsidered.

the range of our laboratory variations (continuous line) and in the range of real variations (broken line). It is immediately evident that the variations of the surface roughness, rainfall frequency and salts in solution (especially in the very low concentrations in the real rain) have a negligible effect on the recession rate. Relatively modest variations of corrosive effects are also shown when the temperature and the additional runoff are varied; furthermore, the fact that this last parameter is conditioned by the geometry of the surface and its relative position (and is thus a constant for a specific monument) allows us to minimize its significance in dynamic conditions. The quantity, intensity and acidity of the rainfall appear to be the main factors responsible for variations in the recession rate in a polluted atmosphere, that is to say, when the acidity levels can be very high. We can therefore assume that the recession rate of marble exposed to pre-industrial rain (and thus unpolluted) depends almost exclusively on two parameters: rainfall quantity and intensity. A semi-quantitative evaluation of this recession rate can easily be obtained on the basis of our simulation data. If we refer to the standard conditions, we can find a recession rate of approximately 0.01 per century but, as stated before, the rainfall intensity value used here is much higher than real rain. To compensate for this

Semiquantitative recession rate evaluations

As shown in Figs 1-8 our simulations were carried out, not only with mono-parametric variations but also with the contemporaneous variation of more than one parameter. It was possible to establish that, in general, the results obtained in synergic conditions were equal to the sum of those corresponding to mono-parametric variations. It would, therefore, be possible to express the recession rate with a mathematical formula. This, in fact, has already been proposed by other scholars on the basis of the data obtained from new marble samples exposed to real atmospheric conditions monitored continuously (Haynie, 1983; Reddy and Sherwood, 1985) or, more specifically, for the tombstone studies (Livingston and Baer, 1985). We have not done the same because, until now, we had not found a satisfactory mathematical expression for the semi-quantitative approach of our research. This will be the subject of further research. Our basic evaluation can be extracted, in fact, from an analysis of the synoptic graph in Fig. 9, where our parameters are reported for the recession rate, both in

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500 0.05 __ 400

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A

pH

:

Corrosion of ancient monuments by rain

347

Fig. 10. Rome, pyramid-shaped tomb of Caius Cestius (lst century B.C.) next to the ancient city walls at Porta San Paolo. Two arrows indicate the position of the details shown in Figs 1la (larger arrow) and 1lb (smaller arrow).

(which was conditioned by experimental needs) we have to introduce a factor from 3 to 5 which approximately corresponds to the increase of corrosion from a minimum to a maximum of average intensities closer to reality. So we obtain for a vertical marble surface partially sheltered by a roof or projecting architectural element, a recession rate of 0.03-0.05 mm, where the lower values refer to climatic areas with higher rain intensities and the higher values refer to those with lower rain intensities: R o m e and London can exemplify, respectively, the first and the second values. For a marble surface totally exposed to rain (that is, very inclined and not sheltered at all) we have to multiply our data by a factor of 5 or more and so we obtain a notable increase of the recession rate up to 0.154).25 m m per century. This agrees with the actual situation of the Caius Cestius pyramid, a marble tomb built as an Egyptian imitation between 18 and 14 B.C. (Fig. 10), now over 2000 years old. The surface corrosion does not, in fact, seem to have caused a surface recession of more than a few millimetres because the original inscriptions still to be seen on two sides are very well preserved and still deeply carved (Fig. 11). We must take into account, however, that monuments in these conditions are relatively rare so the lower values previously mentioned are more generally referred to.

Up to now we have considered a standard rainfall quantity of 500-800 mm (which is that of Rome). For other climatic conditions we can foresee an increased recession rate. This increase cannot be very high because higher rain intensities (and so a decrease in the recession rate) must correspond to higher rainfalls so there will be a certain compensation between the effects of the two parameters and the eventual increase of the recession rate will not exceed, in the most favourable conditions, a factor of 2. In any case, it is not necessary to consider the higher values of rainfall quantity as the majority of marble monuments are not found in climatic conditions where these occur.

Observations on other research on rate recession

We have reported in Table 2 a list of recession rates, most of which have been published in the last decade. The values are all expressed in millimetres per century in order to make a comparison with our evaluations easier. We have chosen a time interval of a century so that the corresponding recession value is high enough to allow us to visualize the induced damage. We have often had to change the dimensions of the original data that were expressed in micrometres per year or in other units. Sometimes the recession rates, not specified in the original publications, were calcu-

348

F. GUIDOBALDIand A. M. MECCHI

b Fig. 11. Rome, particular of the inscriptions on the tomb of Caius Cestius: (a) the larger inscription; (b) the smaller inscription.

Corrosion of ancient monuments by rain

349

Table 2. Recession rate (mm per century) of marble and calizareous stones as obtained in different studies Object

Marble Tombstones Tombstones Tombstones Tombstones Samples (tiles) Samples (tilesJ Samples (tiles) Various monuments Calcareous stones Various buildings Cologne Cathedral London Cathedral Washington Cathedral Mechelen Cathedral Theoretical Rocks Theoretical Monuments or simulations Samples (cubes)

Country

Minimum (min) or rural (r) environment

U.S.A. U.S.A. U.S.A. U.K. U.S.A. U.S.A. U.S.A. U.S.A.

0.2* (r) 0.75* (min) 0.5 (r) 0.2~0.65 (r) 0.5 (min) 0.3* (rain) 0.3

U.S.A. Germany U.K.

3.2 (average)

Maximum (max) References or polluted (u) environment 0.5* (u) 1.25" (max) 1.2 3.5 (u) 0.65-1.1 (u) 1 (max) 0.5* (max) 3.25

3.5 (u) 13.5 (max)

7.8 (min)

U.S.A. Belgium U.S.A. U.S.A.

2.3 0.7 7.8 (for the past)

U.K. U.K.

5 (min)

lated by us from other similar data or integrated from incomplete series of data. We apologise to the authors for these arbitrary manipulations that are, however, always indicated by an asterisk. The data are presented in Table 2. The list is not to be considered complete because some data have not been inserted due to the difficulty of interpretation and transformation or because it has been difficult to find the original publications. As can be seen, the recession rates of marble, compared to those of other calcareous stones, show an average difference of about an order of magnitude. Obviously, the experimental conditions are often different, except perhaps, for the marble tombstones. In this report we will not examine in detail the results obtained for the calcareous stone but we have observed that, probably, very fragile types of stone were examined in the reported studies or the experimental methods applied were not comparable with real alteration. F r o m the marble data we have to extract, as inconsistent, the recession rate obtained by Winkler, that is, unfortunately, the only one usually included in the general text books on marble and stone deterioration (Winkler, 1973; Amoroso and Fassina, 1983). All the other data are included in a limited range between 0.2 and 1.4 mm per century and can be further sub-divided into other groups from 0.2 to 0.65 and from 0.5 to 1.4, respectively, for rural and urban-industrial environments that only partially overlap. If we compare the data obtained for a rural environment (that can be roughly assimilated with a pre-

Husar et al., 1985 Livingston and Baer, 1985 Feddema and Meierding, 1987 Attewell and Taylor, 1988 Haynie, 1983 Reddy and Sherwood, 1985 U.S. Bureau of Standards, 1928 Winkler, 1966 Loughlin, 1931 Luckat, 1976 Sharp et al., 1982

0.95 (u) 1.9 (u)

Livingston and Baer, 1985 Roekens et al., 1987 Miller, 1952 Carrol, 1970 Sharp et aL, 1982

27 9 (max)

Butlin et al., 1985 Martin and Barber, 1987

industrial one) with our evaluations for the preindustrial environment we can easily observe that the first differs by an order of magnitude to our lower data referring to a vertical sheltered wall. If we take into account that the tombstones, however, are totally exposed, not sheltered and often also inclined, we realise that the case of the tombstones cannot be compared to the runoff situation of a monument. In fact, the recession rates obtained for those tombstones are better compared to the data we calculated for a totally exposed surface, even though our values are lower, ranging from 0.2 to 0.4 but another factor has not, up to now, been considered by other scholars. A tombstone normally has two faces exposed to weathering and each of them has a recession rate that has to be considered separately from the other, so it is completely false to calculate a recession rate from the total difference in thickness of the marble slabs because in this way the recession of both surfaces are accumulated and the result is almost double or, at least, greater than the true one. This fact is not generally taken into consideration and even in the only publication where it has been noted (Livingston and Baer, 1985), the results have not, however, been halved or reduced. So we can conclude that our data agree well with the lower values which are those more frequently found in the literature. It is possible that the higher values found up to now for marble have to be regarded with some suspicion: the maintenance and cleaning with strong acids and abrasives or with other mechanical actions (especially to remove lichen encrustations) has been, in

350

F. GUIOOBALDIand A. M. MECCHI

fact, extremely c o m m o n even recently a n d as this practice has not usually been recorded, it is possible to confuse a mechanical or chemical a b r a s i o n with a recession rate caused by weathering.

R E F E R E N C E S

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