Abstract
As part of a project to develop adequate measures for the conservation of cultural heritage, an evaluation of the air quality was done in some Brazilian museums through the use of metallic coupons. To evaluate and control museums climate in countries where environmental control, specially concerning cultural heritage, is neither traditional nor a priority is a challenging task. The goal of this work is to initiate a series of actions aiming at gathering information about museums internal atmosphere through simple and easy-to-evaluate methods. A method is proposed in which silver coupons are exposed in a room or showcase in a museum for a defined period of time. A qualitative and quantitative evaluation of the interactions between the silver coupons and their environment can be done by electrochemical stripping. Series of silver coupons (pure silver and alloyed with copper- 80/20) were exposed in four different museums in southwest Brazil, in the cities of São Paulo (urban) and Rio de Janeiro (maritime and urban). Results of these first evaluations show that most of the samples present a sulfide containing surface layer, whereas only few coupons present a chloride containing layer on their surface. Indoor pollution source was detected in some rooms and is probably related with construction materials used in it. Furthermore, waxed and varnished samples show a very good appearance after 4 months exposure and tests should continue for a couple of months.
Keywords
silver, indoor, sulphide, electrochemistry, copper, varnish, wax, tarnishing
Introduction
In a tropical climate the majority of museums are "open door" all day. Obviously there are some disadvantages: no air-conditioning, high level of pollution in the big cities and large temperature and relative humidity variations. When metallic artefacts are exposed, the consequence is a rapid tarnishing, which leads to frequent cleaning and results in accelerated wear.
To minimize this problem it would be necessary to control the environment, or to apply a protective coating to the objects surface. Before deciding the most appropriate solution, it is necessary to evaluate the real indoor conditions. For that purpose, silver coupons were exposed in different museums environments. Although it is considered as noble metal, silver presents a strong tendency to tarnish when in contact with the atmosphere, specially if oxidising or sulphur compounds and high relative humidity are present.
Experimental
Pure silver and, in one case, silver-copper alloy coupons (1x5cm2) were exposed to the museum's environments for a given time period, after what they were removed and the surface alterations evaluated by visual inspection and by electrochemical technique. For the later, the experimental set up consisted in a three-electrode arrangement with a potentiostat, as described elsewhere [1]. The experiment was carried out in a 0,1 M NaNO3 solution by slowly sweeping (5mV/s) the potential of the silver coupon from the rest bias toward negative values (reduction), and simultaneously recording the current flowing through the electrolytic cell.
From such polarisation plots (Figure 1) two important parameters can be obtained: (a) the potential at which a current increase is observed: in a given solution it is characteristic of the reduction of a specific compound and, in this way, allows its identification; (b) the area inside the peak, which corresponds to the charge needed for the reduction. It is the product of current by time, expressed in Coulombs. After Faraday's law it is possible to calculate the corresponding mass, in grams and also its equivalent in thickness of the tarnished layer. For silver, 10 mC/cm2 would correspond approximately to 5.2 nm, what means as low as few atomic layers.
Four museums in two Brazilian cities were chosen to expose the silver coupons. In São Paulo the coupons were exposed in a new museum with air-conditioning and another older museum, without this facility. The same work was done in Rio de Janeiro in the herbarium section of a big and old fashioned museum and also in a museum of astronomy and scientific instruments.
Figure 1: Typical polarisation plot of a tarnished silver coupon. The potential (E) was scanned and the resulting current (i) recorded. (a) potential characteristic of a given compound; (b) charge involved in the reduction of the compound, related to its amount.
Results
(a) Museum with air-conditioning (SP)
The museum is situated in a new 6 floor building, where temperature and relative humidity are maintained nearly constant (T=22°C; RH=60%). The coupons were randomly distributed and evaluated after exposure periods of 1 and 3 months. The appearance of some of them after three months can be observed in Figure 2a. A general tendency was observed: the coupons in the lower levels were considerably less tarnished than those on the 5th and 6th floor.
Curves for the "best" case (basement level) and for the "worst" case (upper levels) are presented in Figures 2b and c, respectively. In all curves the current peak situated at ca E= -1,0 Vsse indicates the presence of a silver sulphide layer. After 1 month in the basement a very low contamination level (Fig 2b) can be noticed. For longer exposure (3 months), the calculated reduced charge is about 10 mC/cm2, which means ca 5 nm thickness. Figure 2c shows the results for the "worst" case, for which a 10 times larger equivalent thickness can be estimated (note the different scale of current).
These results could be related to the construction materials used in the renovated exhibition rooms at the upper levels (rubber flooring, wood showcases, etc). Further experiments are in progress.
Figure 2: (a) An overview of the coupons after three months exposure in different rooms of the museum with air-conditioning; (b) polarisation plots for the "best" condition (slightly tarnished coupon) after 1 and 3 months exposure; (c) polarisation plots for the "worst" condition (heavy tarnished coupon) after 1 and 3 months exposure (notice the current scale).
(b) Museum without air-conditioning (SP)
This is an old colonial house, which remained in its original form but is now situated in a very busy and polluted area. Because it is not possible to change building conditions, people are evaluating the possibility of using protective coatings to reduce the frequency of tarnishing removal. In this way, four types of silver coupons were exposed in 10 different places in the museum: 1- coupons protected with varnish (Paraloide B-72); 2- coupons protected with wax (Renaissance); 3- coupons protected with wax (Lopes Ferreira); 4- unprotected coupons.
The appearance of the different coupons after 4 months exposure in one site can be seen in Figure 3a. The effectiveness of the protection is confirmed from this test period and exposure shall continue to evaluate the performance of the different coatings. For the unprotected sample the corresponding polarisation plot indicates two reduction peaks. The bigger one corresponds to reduction of Ag2S and the smaller one is assigned to AgCl reduction (Figure 3b).
Figure 3: (a) aspect of the coupons after four months exposure in the museum without air-conditioning: 1-coupon protected with varnish (Paraloide B-72); 2- coupon protected with wax (Renaissance); 3- coupon protected with wax (Lopes Ferreira); 4- unprotected coupon; (b) polarisation plot for the unprotected sample, showing two peaks, corresponding to Ag2S and AgCl.
(c) Herbarium (RJ)
In this museum there is a project to optimise storage of the botanical collection. They intend to substitute the old metallic boxes, because they are not sealed enough to avoid penetration of insects and deterioration of the specimens. They will use polypropylene boxes and are presently comparing climate conditions in both cases. Silver coupons were exposed together with data loggers in 3 different places: inside a metallic box, inside a polypropylene box and outside, simply on the shelve. The recorded values of temperature (T=25-35°C) were the same inside and outside boxes. The relative humidity data were however quite different: RH= 55-65% inside boxes and RH=40-80% on the shelve.
Although the coupons were only exposed for 1 month, a period too short to observe remarkable visual differences, it was enough to evidence, through the electrochemical technique, that the tarnishing rate was slower for the coupons stored in boxes. The worst case here - on the shelve - corresponds to a tarnish layer of about 5 nm thickness (Figure 4). This result could be attributed to the higher and variable relative humidity outside the boxes.
Figure 4: (a) polarisation plots for coupons stored in a polypropylene box, in a metallic box and on the shelve in the herbarium. The last sample shows slightly higher amount of formed products.
(d) Scientific Instruments Museum (RJ)
In this case not only silver, but also silver-copper alloys were exposed in a unique room. The environmental conditions during 2 months were T=29-32°C and RH=57-78%. The appearance of the coupons can be seen in the Figure 5a. Concerning the pure silver samples the corresponding electrochemical curves confirm the presence of Ag2S and the thickening of the tarnished layer with increasing exposure duration.
In regard to the Ag-Cu alloys, the polarisation curves look quite different (Figure 5b). After 1 month the peak is no longer characteristic of Ag2S, but it is shifted to more negative potentials, corresponding certainly to the contribution of Cu2S reduction. When comparing both curves one notice an unexpected decrease of this peak intensity with increasing exposure duration, possibly compensated by the 2 other peaks appeared in the low potential range. These last might be attributed to copper oxides and will be investigated in more detail.
Figure 5: (a) aspect of the silver and silver-copper alloy coupons after 1 and 3 months exposure in the scientific instruments museum; (b) polarisation plots for the pure silver samples showing a unique peak, characteristic of Ag2S, which increases remarkably with the exposure duration; (c) polarisation plots for the silver-copper alloy samples showing two distinct peaks, probably corresponding to copper sulphide and oxide reduction.
Conclusion
Concerning the electrochemical method, it was shown that it is sensible enough to evidence the presence and even evaluate the amount of very small changes in the surface chemistry. It can be also considered accessible, because no complicated or expensive equipment is needed and it allows to get a better understanding about the nature of the reactions taking place.
Dealing with museum's need, this is the first step to evaluate the actual environment, of primordial importance to further progress in the optimisation of the conservation conditions, always accordingly museum's reality and limitations.
References
[1] Costa, V., "Electrochemistry as a conservation tool", Conservation Science 2002 - Edinburgh, Scotland, Townsend, J.H., Eremin, K., Adriaens, A., eds., Archetype Publications 2003.
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Author to whom correspondence may be addressed:
Virginia Costa
LACOR / Universidade Federal do Rio Grande do Sul
Av. Osvaldo Aranha, 99/706
90035-190 Porto Alegre
RS Brasil
E-mail: virginiaco@aol.com
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