_ / 1647

THE FRESCOES OF THE ORATORY OF S.STEFANO (LENTATE SUL - ): COMPARISON BETWEEN THE INDOOR AIR QUALITY AND DECAY.

NEGROTII ,R. , REALINI, M. and TONIOLO L. Centro CNR "Gino Bozza" - Politecnico di Milano, Milan,

SUMMARY The mural paintings of the Oratory of $.Stefano ( - Milan) have been subjected to a complete characterisation both of the painted mortars and of the decay products, in order to plan correctly the conservation work of the whole church. Different analytical techniques allowed to determine the soluble salts content of the painted mortars (XRD coupled with IC) and the decay products over the painted mortars (FTIR). A monitoring program of the indoor suspended particulate matter during one year was carried out to determine the air quality of the Oratory. The total amount of suspended particulate matter has been compared with some literature data relating to similar situations. The qualitative and quantitative determination of the soluble fraction of the particulate matter was compared with the nature of the decay products pointed out on the painted surfaces. The particulate matter doesn't play an important role in the decay of the frescoes.

1. INTRODUCTION

The effect of polluted atmosphere on works of art has been widely studied by many Authors, and the important role of gaseous acid pollutant in the decay of carbonatic substrates (stone, mortar, plaster) has been stressed. The suspended particulate matter is also responsible of stone material decay [1-4]. owing to the typical alteration named "surface deposit" [5). Particulate matter can also chemically react with a carbonatic surface, owing to the activation of its intrinsic acidity. The frescoes of the Oratory of S.Stefano in Lentate sul Seveso (Milan), dated from the end of the XIV century, have been studied to investigate their state of conservation. This paper points out the relationship between surface decay and some pollution parameters; the role of the suspended particulate matter in the formation of efflorescences and salt patinas is discussed.

2. EXPERIMENTAL

2.1. State of conservation 2.1.1. FTIR spectrophotometry The FTIR analyses have been carried out by a Perkin Elmer 1725X spectrophotometer equipped with DTGS detector on powdered samples from frescoed surface dispersed in KBr micropellets.

2.1.2. Ion chromatography The soluble salt content (Na+. NH/, K+. Mg++ · Ca++. F , er. N02-. N03-, PO/, SO/) was determined by a Dionex DX100 ion chromatography system equipped with Dionex AS4A/CS12 columns using a NaHC03 I Na2C03 or methanesulfonic acid solutions as eluent.

2.1 .3. X-ray diffractometry The XRD analyses have been carried out by a Phillips PW1130/90 diffractometer equipped with a CuKa tube on powdered samples between 3° to 60°. 1648

2.1.3. X-ray diffractometry The XRD analyses have been carried out by a Phillips PW1130/90 diffractometer equipped with a CuKa tube on powdered samples between 3° to 60°.

2.2. Indoor pollution The suspended particulate matter (SPM) has been collected on cellulose nitrate filters (diameter 47 mm, pore size 0.45 µm) using a Tecora Bravo pump with a Sentinel 3 control unit. Every filter has been used for 24 hours, and during this period the air flow was constant (20 litres/minute). Before and after their employ, the filters have been conditioned at 60 °C and weighted with a Mettler analytical balance (precision 0.00001 g), so that the amount of collected particulate matter has been calculated for difference of weight, and related to the volume of sucked air normalised to 25°C and 1013 mBar using the equation (1)

V · P · (273 + Tnl Vn = P0 • (273 +Tl where: Vn = air normalised volume V = air measured volume P = measured atmospheric pressure Pn = 1013 mBar T= measured temperature in °C Tn = 25 °C

Two periods during the cold season (from 7 November to 4 December 1994 and from 20 February to 19 March 1995) and one period during the hot season (from 8 May to 2 July) have been chosen for the particulate sampling. This choice is aimed in order to evaluate the influence on the indoor air composition of the increase of outdoor pollution due to the heating systems. The hydrosoluble fraction (HsF) of the collected particulate has been characterised by ionic chromatography (see § 2.1.2.) on solution obtained by shaking every filter for 4 hours in ultrapure water. During the last cold period (from 17 November 1995 to 20 February 1996) a test to quantify the environmental corrosion potential has been carried out, using a Purafil copper/silver coupon. The test consists in measuring the thickness of the corrosion film formed on the copper coupon placed in the Oratory; this parameter allows to assign a particular air quality class [6] to the considered indoor environment. Additional information about the presence of chlorides and sulfates are provided from the silver coupon corrosion.

3. RESULTS

3.1. State of conservation The fresco painted surface of the Oratory have been investigated to characterise the plasters and the decay products. The south wall of the nave is covered by a white veiling that is present also in the lower part of the apse; the north wall of the nave shows only some isolated efflorescences near the floor. On the whole, the painted surfaces are in a quite good state of conservation. except for some small repainted areas. The plasters are composed of a lime and a quartz-silicatic sand, coming from the Ticino river. The weight compositional ratio binder/aggregate is almost constant (1 :2). that is the painted plasters have been manufactured by the same raw material mixture. The sol4ble salts content of the mortar collected samples is reported in table 1. While the sulfate ion content is quite low, ranging between 0.2 and 1.2 o/ow, the nitrate and chloride ions amount is considerably high (No3- up to 2.7 o/ow - c1- up to 0.4 %w). The FTIR analysis of the white surface veiling (Figure 1) of the nave south wall pointed out the presence of calcite (1430, 875 cm-1>, nitrate (1385 cm-1), gypsum (1622, 1140, 1120,671,603 cm-1) and - 7 1649 weddellite (1646, 1326 cm-1)_ Gypsum and nitrate prevail over the other compounds, explaining the appearance of the white patina that covers the painted surface of the nave wall. The efflorescences, collected from different zones of the nave and the apse, are composed mainly of gypsum and, only in a few samples, of weddellite and nitrate. XRD analysis identified the nitrate crystals as nitronatrite (NaN03) . Evidently, the prevalence of gypsum on the painted surface is determined by the indoor microclimatic conditions of the Oratory, while the lower amount of nitronatrite is due to the permanence of nitrate ion inside the plaster owing to its high solubility. This occurs, particularly, on the south wall of the nave and on some zones just under the roof, being the masonry and the plasters of these areas critically damp.

Table 1 - Soluble salts content (o/ow) of the painted plasters.

SamQle K"' NH/ Na"' Mgn Can F er N02- N03- PO/' $04- 101 0.25 0.01 0.31 0.94 - 0.08 1.58 0.42 102A 0.29 0.01 0.07 0.79 - 0.09 0.04 0.29 0.19 1028 0.23 tr 0.03 0.84 - 0.11 0.43 0.16 102C 0.30 0.02 tr 0.04 0.78 - 0.07 tr 0.31 0.19 103A 0.44 0.02 0.03 0.16 1.33 - 0.42 2.72 0.29 1038 0.41 0.02 0.16 1.18 tr 0.34 tr 2.33 0.17 103C 0.52 0.04 0.17 1.20 tr 0.37 0.02 2.49 0.20 104A 0.23 0.16 0.65 tr 0.03 tr tr 0.19 1048 0.26 0.15 0.62 tr 0.03 0.18 105A 0.16 tr 0.20 0.53 tr 0.05 0.27 0.42 106A 0.20 0.14 0.49 tr 0.02 0.05 0.33 1068 0.20 tr 0.08 0.43 tr 0.04 tr 0.01 0.18 106C 0.23 0.01 0.05 0.48 tr 0.04 tr 0.09 0.24 107A 0.26 tr 0.17 0.87 tr 0.11 0.68 0.72 1078 0.28 0.01 0.15 0.73 tr 0.11 0.69 0.38 108 0.65 0.02 tr 0.08 1.07 tr 0.04 tr 0.07 1.25 109 0.25 0.16 0.56 tr 0.07 0.21 0.32 110A 0.29 0.16 0.65 tr 0.12 0.38 0.47 1108 0.25 0.16 0.55 tr 0.05 0.10 0.23 111A 0.27 tr 0.08 0.53 - 0.04 0.01 0.01 0.36 1118 0.25 0.04 0.55 tr 0.04 0.03 0.22

100.0

-AT 470 1647 671 603 1621 1384

3405 20 · ~ooot-----.3000____ _2 ooor---cm-:-.,;----:, -::500::---~~,~ooo~....,_--~500~400

Figure 1 - FTIR spectrum of the white veiling of the south wall of the nave. Powdered sample dispersed in KBr micropellet. 1650

3.2. Indoor pollution The indoor concentration values of suspended particulate matter (SPM) and of its hydrosoluble fraction (HyS) are reported in table 2. The mean SPM value referred to the cold period is higher than the hot period one; the same observation is true considering the concentration values of the hydrosoluble fraction. In particular, the ratio between these values (HyS/SPM) is constant (0.43) during the considered periods, pointing out a quite good homogeneity of the indoor air composition during the year. In figure 2 the frequency (%) of the SPM concentration values is shown. From the figure it is evident that the concentration values of SPM are asymmetrically distributed around the mean value and the frequency trend doesn't fit a normal distribution. In figure 3 the outdoor SPM yearly values monitored by the Local Health Dept. of Milan [7] and the Oratory values are shown: the Oratory indoor mean concentration (51 µg/m~ is higher than eight outdoor values of the Province of Milan; moreover, the outdoor value of Lentate is the highest. This situation could be ascribed to the location of the Oratory, placed in an heavy vehicular traffic zone, along a downhill road with a S-shaped bend.

Table 2 - Indoor concentration of suspended particulate matter and of hydrosoluble fraction (µg/m~.

Total particulate Hydrosoluble fraction

Cold Hot Year Cold Hot Year ~riod ~riod ~eriod ~riod Mean 62 39 51 26 17 22

Count 56 50 106 56 50 106

Standard error 3.2 2.0 2.2 1.2 1.1 0.9

Median 61 38 45 27 16 21

Standard deviation 24.1 14.1 23.1 9.3 7.9 9.8

Relative st dev. 38.8 36.2 45.0 35.1 45.8 44.1

Variance 581 .9 199.2 531 .6 86.4 63.1 95.6

Curtosi -0.6 2.5 0.1 -0.8 1.5 -0.7

Asymmetry 0.4 1.0 0.8 0.0 1.1 0.4

Minimum 17 14 14 9 5 5

Maximum 119 88 119 45 43 45

The whole reported data seem to outline a very dangerous situation for the building and frescoes conservation. Nevertheless, comparing these data with those available in literature [8-11), the Oratory indoor SPM concentration should be considered quite low (mean total SPM of Sistine Chapel, Scrovegni Chapel, S.Francesco in Arezzo, Borghese Gallery is around 120 µg!m\ However, some very important differences should be considered: the different date, period and duration of sampling, the different geographic and climatic conditions, the different indoor situations (presence/absence of visitors, restoration, air filter system). Therefore, after these considerations the SPM concentration in the Oratory seems to be less worrying. The indoor quiet situation of the air (no heating system, no visitors) could help in keeping low the SPM amount. It cannot be excluded that a different destination of the building might produce the same situation monitored in the Sistine Chapel and in the Cenacolo Vinciano [11) where a very large increase of powder is detected during the visitors incoming. At the moment the Oratory situation is in accordance with the typical indoor SPM concentrations (39-66 µg/m~ reported for residences and public buildings [11). 1651

Different consideration can be argued about the chemical composition of the SPM. 25,------.

~ 15 +------> (.) c Cl> :l CT ~ 10------u..

0 0 0 0 0 0 0 0 0 0 0 0 0 ';I "? "r u;> <-? "':" a;> a;> ..- ..- 6 ..- ..- ..- ..- ..- ' ' N <") '

Figure 2 - Concentration class distribution of total suspended particulate matter.

150 -

Lentate outdoor

100 - -

SPM (µg/m3) Provi ne e of Milan

50 Lentate indoor -

0 -

Figure 3 - Outdoor mean concentration of suspended particulate matter (1994/95) in some stations of the Province of Milan and in the Oratory of Lentate. The hydrosoluble ion concentrations are reported in table 3. The considered ions have been chosen owing to their well known interaction with the plaster surfaces.

Table 3 - Indoor mean concentration (µg!m3) of hydrosoluble ions in the suspended particulate matter N03. 804- Na• NH/ K; Can F er N02· 8.18 8.33 Cold period 2.00 5.40 0.38 0.92 0.15 0.71 0.03 5.06 6.84 Hot period 0.95 3.65 0.00 0.91 0.12 0.57 0.00 6.62 7.59 Year 1.48 4.53 0.19 0.92 0.13 0.64 0.02

Note: Magnesium, phosphate and oxalate are always absent. 1652

It is always present a significant difference between the considered periods, being all the ion concentrations larger in the cold period. The total concentrations of sulfate and nitrate fit well the typical concentration range of the chemical species which contribute to dry deposition of particulate (0.2-40 µg/m3 for the sulfate and 0.1-10 µg/m3 for the nitrate) and the nitrate amount is very close to the top value of the range. In particular, the nitrate concentration is higher than others reported for indoor environments with very high total SPM concentration [8, 1 OJ. In figure 4 the frequencies (%) of the sulfate and nitrate concentration values is shown. Their trend is very similar to the total SPM one (Fig. 2), being the values very dispersed towards the highest classes. The ammonium concentration is very high too, being this ion very diffused in the Milan atmosphere. No significant data are available for indoor sites, besides our experience. The environmental potential corrosion test identify the indoor air as belonging to the ISA class G1 (Mild). The pollutants concentration level of this class are reported in table 4. The copper corrosion film was 202 A I 30 days thick, while the silver corrosion thickness was 312 A I 30 days thick. No further considerations can be carried out because of the lack of similar data. At the end of the monitoring campaign in 28 indoor Italian environments, actually in course, more specific conclusions will be drawn about the relation between the coupons behaviour and the materials decay .

•Nitrate DSulfate

30 -+------

>

gQ.) 20 ------::J a ~ u..

10-+------

0 CJ) CJ) .q- CJ) ~ ~ ~ O> 0 N .q- ,.... O> N .q- ,.... O> N 0 LO LO LO " N r.: 0 LO LO Lt? N ,... Class (µg/m3) Figure 4 - Class distribution of daily concentrations of nitrate (black) and sulfate (white) in suspended particulate matter.

Table 4 - Concentration values of pollutants for the ISA G1 class (Mild). [6)

Pollutants Concentration (ppb) H2S < 3 S02, S03 < 10 Cl2 < 1 NOx < 50 HF < 1 NH3 < 500 03 < 2 Copper reactivity level < 300 A/30 days _ / 1653

4. CONCLUSION

The obtained results allow to affirm that particulate doesn't play an important role in the decay of the frescoes of the Oratory of S.Stefano. This assumption is based on the following considerations: • the colder wall (the north side of nave) [13) should be the more interested by particulate deposition [14), nevertheless it is in a better state of conservation, not covered by the salt white patina . • the indoor climatic conditions don't produce dew events upon the wall surfaces [13). Hence the climatic conditions do not favour neither the particulate deposition nor its chemical activation . . the loss of painted surface in some limited areas is surely due to accidental water penetration in the masonry. The painted surface under the salt patina doesn't show chemical corrosion phenomena and it is in a good state of conservation. The patina arises from a crystallisation process of the salts solution present in the plaster coming from different sources. - no oxalate was detected in the particulate, while it is present as weddellite in the salt patina. This study demonstrates that the presence of suspended particulate matter inside the Oratory is not directly correlated to the observed decay, and so the particulate filtration should not be considered necessary. On the contrary, it is necessary a temperature and relative humidity control, owing to their direct influence on salt crystallisation, the most important alteration detected on the frescoes.

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