Environ. Sci. Technol. 2000, 34, 2859-2865
Tempera-Painted Dosimeters for Environmental Indoor Monitoring: A Spectroscopic and Chemometric Approach MAURO BACCI,* MARCELLO PICOLLO, SIMONE PORCINAI, AND BRUNO RADICATI Istituto di Ricerca sulle Onde Elettromagnetiche “Nello Carrara” - CNR, Via Panciatichi 64, 50127 Firenze, Italy
The importance of minimizing the deterioration of works of art has stimulated the development of new methods for the monitoring and control of environmental parameters. The effect caused to paintings by the “global” indoor environment can be evaluated by using dosimeters that reproduce the pictorial structure of paintings as closely as possible. After having prepared these dosimeters with specific pigments and dyes, they were aged in aging chambers under different conditions. Subsequently, mock paintings, prepared by using a reduced set of the more sensitive dosimeters, were placed in several museums and historical sites. Information on the alteration process induced by the environmental factors was obtained from a comparison between the natural and artificial aging dosimeters.
Introduction As time progresses, paintings on display in galleries, museums, and churches are inevitably subject to changes caused by environmental factors. Ultraviolet (UV) and visible light, temperature, relative humidity (RH), particulate, atmospheric gases, and pollutants are the environmental parameters responsible for the aging of paintings and, particularly, in affecting their color (1-10). A milestone in the study of the effect induced on artworks by light, for instance, was the Russel and Abney Report in 1888 (1). Since then, numerous studies have been performed in order to monitor the damage caused by light to paintings, textiles, photographs, etc. Alone, or in combination with other environmental parameters (temperature, RH, particulate, atmospheric gases, and pollutants), light has been shown to initiate fading, yellowing, and darkening in paintings. Indeed, the “Predella della Trinita`” panel by Luca Signorelli (1445-1523), on display in the Leonardo room at the Uffizi Gallery, has shown that a noticeable change in color occurred after 66 months (May 1990-November 1995) of regular exhibition to the public, despite the environmental control in the gallery (11). Studies of these environmental parameters mainly focus on monitoring a number of preselected factors. However, since the internal environment is always subject to fluctuations, the monitoring of environmental parameters by separately measuring the individual factors at a particular point in time does not necessarily yield an accurate assessment of the potential damage to the art on display. In fact, only an analysis of the synergistic effects of the various * Corresponding author phone: +39 055 4235217; fax: +39 055 410893; e-mail:
[email protected]. 10.1021/es991437d CCC: $19.00 Published on Web 06/06/2000
2000 American Chemical Society
environmental factors on paintings can provide data relative to the global impact of the environment on the exposed artworks. Thus, the conditions or the change in conditions, which may alter the stability of the objects and affect their chances of longevity, must be evaluated by using a different approach. For this purpose and following previous experiences (11), paintings themselves can be considered as dosimeters that integrate all the effects of the environment. However, because of their uniqueness, actual paintings cannot be considered as “test panels”, to be sampled without any restrictions. Therefore, mock paintings, constructed from relevant materials, could act as dosimeters and serve for monitoring the quality of the museum environment. As color is one of the most important factors constituting paintings, recording visible spectra before and after aging makes it possible not only to obtain information about the chemical changes induced but also to have a direct measurement of the potential aesthetic damage to the work of art. Moreover, the data obtained could be better related to the actual damage produced on paintings, starting from the fact that color changes are a macroscopic and global index of alteration. In the present work, noninvasive reflectance spectroscopic analysis in the visible region was performed on mock paintings. The samples simulating the actual paintings were artificially aged under different exposure conditions. It is quite obvious that in the natural environment a combination of various factors may also produce a synergistic effect. The number and the amount of such factors are variable and dependent on the climatic situation, which also includes the number of visitors. Therefore, there are several difficulties in simulating this complicated setup in the laboratory. As a consequence, the most important factors (light, thermal, and pollutant exposure) were singled out in order to isolate the effects induced by each factor separately. These (mock) paintings were prepared by using whole egg tempera as a binding medium. This is because it dries in a shorter time than other media (i.e. drying oils such as linseed oil, poppy oil, nut oil, etc.), and because it was mostly used in Medieval and Renaissance Italian paintings. The selected set of pigments and dyes used as dosimeters had to meet the following requirements: (a) different stability to light and pollutants; (b) different electronic structure of the lightabsorbing system; (c) natural and synthetic origins covering a wide range of colors; (d) presence or absence of metal ions; and (e) compatibility with the egg-tempera technique. Accordingly, the following choice was made (Table 1): indigo, azurite, smalt (blue pigments); turmeric (curcumin), lead(II) chromate, lead antimoniate yellow (yellow pigments); madder (alizarin), cinnabar (red pigments); white lead (white pigment); and Sienna earth (brown pigment). The alteration was evaluated according to the modification of the reflectance spectra of the selected pigments/dyes acquired before and after the aging procedures. Last, a comparison was made between the data obtained in the laboratory and the results from measurements on mock paintings that had been placed in various field sites.
Experimental Section All the pigments and dyes were purchased from various firms, and their quality was checked by means of several analytical techniques (XRD, FTIR, PIXE, micro-RAMAN). The tempera medium was prepared at the Opificio delle Pietre Dure (OPD, Florence) mostly following the recipe reported in Il libro dell′arte by Cennino Cennini (12): (a) one egg yolk; (b) one egg white, beaten (which was left to settle for one night in VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Pigments Chosen for the Preparation of the Mock Dosimeters pigment
source
source code chemical formula
extra fine azurite Kremer, Germany 1026 extra fine smalt Kremer, Germany 1001 synthetic indigo Janssen, Belgium 21.213.67 mountain Kremer, Germany 1060 cinnabar lead white Aldrich, U.S.A. 24,358-2 lead antimoniate Zecchi, Italy 0772 a,b yellow raw Sienna Zecchi, Italy 0813 eartha lead(II) chromate Aldrich, U.S.A. 31,044-1 curcumin Acros, U.S.A. 21.858.33 alizarin Acros, U.S.A. 15.369.43
2CuCO3 Cu(OH)2 SiO2-K2O-CoO C16H10N2O2 HgS 2PbCO3 Pb(OH)2 Pb2Sb2O6(O,OH) RFeOOH, RFe2O3 PbCrO4 C21H20O6 C14H8O4
a Lead antimoniate yellow (also known as Naples yellow) and raw Sienna earth pigments were not 100% pure; however, the amount of impurities was negligible, so that the two pigments were used as standards. b Lead antimoniate yellow (Naples yellow) is an artificial pigment which has been manufactured in various periods. The formula most often given for Naples yellow in the pigment literature is Pb3(SbO4)2 in which the mass ratio of antimony to lead is 0.39. The second most frequently cited formula is Pb(SbO3)2, having a high Sb/Pb ratio of 1.18. Other formulas which might be encountered are Pb8(SbO4)2, Pb(SbO4)2, and PbSb2O4. Here, the formula and the ASTM reference of the mineral Bindheimite, Pb2Sb2O6(O,OH), which is the analogue of lead antimoniate yellow, are reported.
order to separate the liquid from the foam; the foam was not used); (c) mastic varnish (one spoon ∼ 15 cm3); and (d) a few drops of apple cider vinegar. After all the compounds were mixed, they were shaken in order to emulsify the obtained solution. When the tempera was ready for painting, water was added in order to keep the mixture medium together with the pigment ready to be easily brush-spread. The tempera dosimeters were spread over 125 µm-thick poly(ethylene terephthalate) sheets (Melinex) using a film applicator (Braive Instruments, mod. Bird Film Applicator), to obtain an opaque painted layer with a constant thickness (200 µm wet layer thickness). The Melinex sheets provided an inert, light, and stable substrate for the tempera-pigment system. The sheets of Melinex thus painted (one for each pigment or dye) were cut into small strips; two sets of strips were prepared for each pigment and dye. One set was artificially aged, while the second was used to prepare the mock paintings (dosimeters) that were to be placed in natural environments (i.e. galleries and museums). Both sets had their own reference strip stored under exclusion of oxygen and light: L00 for the artificially and CON for the naturally aged strips. The artificial aging tests consisted of three separate exposures under different environmental parameters: light (visible component only), temperature, and pollutants (SO2, NOx). The tests were carried out at the Tate Gallery (London, England), for light and temperature exposures, and at TNO (Delft, The Netherlands), for exposure to air pollutants. The samples aged under light were exposed in a light box for 4 (L04), 8 (L08), 16 (L16), 32 (L32), and 64 (L64) days, with the use of Philips TLD94 58 W daylight rendering fluorescent tubes. These tubes were filtered with Perspex VE ultraviolet filters: these had a cutoff for wavelengths less than 400 nm and maintained a constant sample illuminance of 18 klux. During this aging process, the temperature and relative humidity (RH) in the light-box were around 30 °C and 30%, respectively. For the thermal aging exposure, the samples were placed in an oven without any light at 60 °C and at 55% RH for 7 (T07), 14 (T14), and 21 (T21) days. Exposure to air pollutants was carried out in the dark for 4 (NOX) days under a continuous flow of SO2, NOx, and air. The consequent overall concentrations of SO2 and NOx in the gas chamber were 2860
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approximately 10 and 20 ppm, respectively (T ) 23 °C; RH ) 55%). The identical mock paintings prepared for the natural aging were placed in six different field sites. These locations were selected on the basis of known differences in the indoor environments: (a) sites with controlled local environment, as the Nightwatch room at the Rijksmuseum, Amsterdam (RNW), the Clore Gallery at the Tate Gallery, London (TAT), and the Leonardo room at the Uffizi Gallery, Florence (UFF), and (b) uncontrolled local environments, as the Cord room at the Alca´zar in Segovia (ALC) and the Sandham Memorial Chapel in Burghclere, U.K. (SAC). A dosimeter panel was placed in the storage facility (Depot Oost) of the Rijksmuseum (RDO), which did not have a controlled environment but had very low light levels. Subsequently, the choice regarded the importance of the sites or rooms of the galleries or museums as expressed by the number of visitors. The Uffizi Gallery was chosen for this reason as it is generally visited by more than a million visitors per year, and it is assumed that most of them pass through the Leonardo room. Moreover, additional environmental data and spectroscopic measurements from previous studies were available for this room (11, 13). Furthermore, from studies carried out in the Pollaiolo room at the Uffizi Gallery (two rooms from Leonardo room) during June 1997, it was found that the NOx and SO2 levels were in the 16-27 ppb and 0.42-2.09 ppb ranges, respectively (14). Each naturally aged strip was exposed for 9 months (from mid-December 1996 to end-September 1997). All strips were analyzed using a spectrophotometer (Perkin-Elmer mod. λ19) equipped with a 60 mm integrating sphere. The reflectance spectra were acquired from 360 to 2500 nm step 1 nm. The resolution of the spectrophotometer was ( 0.2 nm and ( 0.8 nm for the vis and near-IR ranges, respectively. The spectra obtained were recorded without any damage to the samples, and each sample was measured three times (three different cycles of measurements) in order to reduce possible errors which might be induced by external sources. Calibration was performed by means of plates of pressed BaSO4 powder (purity > 99.99%). These spectrophotometric measurements gave access to three different approaches for evaluating changes in the dosimeters that were induced by artificial or natural aging processes: (i) display of the difference between the reflectance spectra of the aged sample and the control (∆R%), which made it possible to monitor the spectral variations related to the chromophore and/or to the scattering properties of the surface [An almost constant value of ∆R% over the visible spectral range indicated that no change in the chromophore had occurred, and the reflectance variation was mainly due to a different scattering of the investigated surface.]; (ii) calculation of the color change (∆E), together with its three components ∆a*, ∆b*, and ∆L*, according to the CIELAB 1976 recommendation for 2° standard observer and standard illuminant D65 (15, 16); and (iii) principal component analysis (PCA) was performed in order to stress the variability within the spectra of the artificially aged samples. To explain the spectral variation that occurred as a consequence of the natural aging processes, the reflectance spectra collected from the exposed strips at the field site were projected onto the PCA model (17, 18), which was built using the artificially aged data. The PCA algorithm was implemented in the MATLAB environment and run on a PC. Method (i) was more suitable for identifying possible chemical changes in the chromophores that could be induced by the aging process, while method (ii) was directly correlated to the visual perception. Clearly, the two alternatives were related to each other, because colorimetric parameters were evaluated on the basis of the reflectance spectrum, but, in addition, reflectance data were weighted, to take into account the eye response of a standard observer (15, 16).
TABLE 2. Colorimetric Values Calculated on the Artificially and Naturally Aged Lead White Dosimeters artificial aging
∆E
∆a*
∆ b*
∆ L*
light 4 days - L04 light 8 days - L08 light 16 days - L16 light 32 days - L32 light 64 days - L64 temperature 7 days - T07 temperature 14 days - T14 temperature 21 days - T21 pollutant gases - NOX
1.0 1.1 1.9 2.0 2.4 1.5 2.2 2.6 5.5
-0.2 -0.2 -0.3 -0.3 -0.2 0.4 0.0 0.0 -0.9
-0.9 -0.9 -1.6 -1.8 -2.1 1.0 2.1 2.3 5.4
0.4 0.5 0.9 0.8 1.0 -1.0 -0.8 -1.2 -0.9
natural aging
FIGURE 1. The reflectance spectra in the visible region (360-830 nm range) of control (solid line), light-aged 64 days (dashed line), thermal-aged 21 days (dotted line), and pollutant gas aged 4 days (dash-dotted line) lead white dosimeters.
Cord room; Alca` zar, Segovia - ALC Depot Oost; Rijksmuseum, Amsterdam - RDO Nightwatch room; Rijksmuseum, Amsterdam - RNW Sandham Memorial Chapel, Burghclere - SAC Clore gallery; Tate Gallery, London - TAT Leonardo room; Uffizi, Florence - UFF
∆ E ∆a *
∆b* ∆L*
3.2 0.1 -3.0 1.3 0.8 -0.2 -0.6 0.5 2.6 -0.1 -2.3 1.2
2.7 0.0 -2.5 1.0 2.7 -0.1 -2.4 1.2
FIGURE 3. DSI values for the artificially and naturally aged lead white dosimeters (for legend see text). FIGURE 2. The reflectance spectra (360-830 nm range) reported as difference spectra (∆R ) Raged - Rcontrol) of the artificially aged lead white dosimeters. For legend see text.
Results and Discussion The evaluation of the reactivity of the 10 pigments/dyes to the artificial exposure conditions was based on their color variations (∆E) between aged and nonaged strips. Starting from these ∆E values on the artificial aging tests, the pigments/dyes were grouped into fugitive, durable, and permanent categories. In setting the ∆E threshold for these three categories, the error of the measurement, due to incorrect repositioning operations, nonhomogeneous layers, etc., was evaluated as about (0.7 ∆E units, and, correspondingly, the uncertainties associated with ∆a*, ∆b*, and ∆L* were around 0.3, 0.4, and 0.3, respectively. On the basis of the above data, lead white, alizarin, and lead chromate dosimeters were investigated in detail, and their artificial exposure data were compared with the natural exposure ones. Lead White Pigmented Tempera. The reflectance spectra in the visible region of lead white pigmented tempera control, light aged 64 days, thermal aged 21 days, and pollutant gas aged 4 days were reported in Figure 1. A small difference in the four spectra was clearly evident, and this difference could be better highlighted when the control spectrum was subtracted from the others (Figure 2). From these data, it could be seen that the light exposure affected the lead white tempera spectrum in a different way with respect to tem-
perature or pollutant gases. Moreover, from the reflectance spectra it was evident that the light-aged strip reflected more light than the control, whereas the contrary occurred for the gas and temperature aged strips. In colorimetric terms (Table 2), this is indicated by a positive ∆L* for light-aged samples and a negative one for temperature- and gas-aged ones. However, the main contribution to the color variation (∆E) was made by ∆b*, instead of ∆L*. The behavior of the temperature and gas aged samples was different from that of the light-aged samples: in fact, in the former case the samples were more yellow after aging, whereas they were bluer in the latter. These aging effects were also determined by evaluating the integrals of difference spectra (DSI). The integrals of difference spectra were the differences between the areas subtended by the reflectance spectrum of a sample after aging and of the related control. Measuring the differences of these areas with time corresponds to evaluation of the global changes induced in the chromophore. The bar diagram with the values of DSI for the artificially aged lead white strips is reported in Figure 3. DSI values were well correlated with aging time exposures for light- and thermalaged dosimeters. Information concerning correlation with pollutant gas exposure was not available, because only one aging trial was performed. However, the adopted exposure for pollutant gases (4 days) affected this dosimeter to a large extent. The results obtained by applying PCA on the reflectance spectra (360-830 nm range) of lead white artificially aged dosimeters have been summarized in the PC1-PC2 scores VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. DSI values for the artificially and naturally aged alizarin dosimeters. For legend see text. FIGURE 4. PC1-PC2 scores plot of the reflectance spectra (360830 nm range) of artificially aged lead white dosimeters. The reflectance spectra of the naturally aged lead white dosimeters were projected on the same diagram. For legend see text.
FIGURE 5. PC1 and PC2 eigenvectors of artificially aged lead white dosimeters. plot, which gave 99.5% of the total variance (Figure 4). From these data, three groups of objects were distinguished, according to the different artificial aging processes. In particular, a good correlation between aging time and PC1 scores was found for light-aged samples. The spectral variability accounted by PC1 (97.5% of total variance) was related to the 360-600 nm range, in which the PC1 eigenvector had the highest absolute values and corresponded to the overall reflectance of the samples. The PC2 eigenvector (2% of total variance), on the other hand, indicated that a large contribution to the variability was due to the spectral region around 550 nm (Figure 5). According to Figures 3 and 4, light provided the main contribution to the natural aging of the dosimeters. In fact, the DSI positive values for the naturally aged strips (Figure 3) were in good accordance with the data obtained from the artificial light exposure strips (DSI > 0). This result was also confirmed by projecting the naturally aged samples into the PC1-PC2 score plot (Figure 4). The field exposure dosimeters were positioned close to the light-aged strips even if most of them were more changed than the 64-days light-aged strips. The close position of the two references is a good validation for the experimental procedure followed. All the dosimeters placed in natural sites, except the Rijksmuseum Deposit Oost, showed a noticeable and quite similar color variation (Table 2). ∆b* and ∆L* data indicated that light seemed to be the main factor inducing alteration on these dosimeters. The data regarding the Sandham Memorial Chapel were not available for this set of dosimeters. 2862
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FIGURE 7. PC1-PC2 scores plot of the reflectance spectra (360830 nm range) of artificially aged alizarin dosimeters. The reflectance spectra of the naturally aged alizarin dosimeters were projected on the same diagram. For legend see text. Alizarin Pigmented Tempera. The integral bar graph (Figure 6) showed that the alizarin dosimeter was extremely sensitive to pollutants. The effect of light and pollutants aging factors produced an increase of reflectance. On the other hand, the spectra of thermal-aged alizarin dosimeters had a reflectance lower than that of the reference spectrum, and the changes due to this aging factor seemed to be saturated after 7 days of exposure. It follows that positive values of DSI for naturally aged dosimeters should exclude any contribution from thermal aging. From the PC1-PC2 scores plot (99.9% of total variance), three groups corresponding to the three different aging conditions were clearly separated (Figure 7). The same conclusions were obtained from the colorimetric analysis (Table 3): the alizarin pigmented tempera was very sensitive to the pollutant aging (∆E ) 36.6) and sensitive to the light aging (∆E ) 8.9). The main contribution to ∆E was given by ∆b*, which was linked to a relevant increase in reflectance in the 500-700 nm range. This increase produced a shift in the color evaluation toward reddish-yellow hue, compared with the reference strip. This dosimeter was not very sensitive to thermal aging, since the ∆E value for the 21day-aged sample was slightly above the proposed threshold ∆E value for the permanent tempera-pigments and dyes. The absorption peak observed around 620 nm in the control strips was strongly reduced in light and pollutant aged alizarin dosimeters (Figure 8a). Moreover, it is known that the absorption spectrum of alizarin is strongly dependent on pH (19), and, in particular, the band around 600 nm tends to disappear when the pH decreases. Accordingly, the presence of free carboxylic acids, which was previously found by direct
TABLE 3. Colorimetric Values Calculated on the Artificially and Naturally Aged Alizarin Dosimeters artificial aging
∆E
∆a *
∆ b*
∆L*
light 4 days - L04 light 8 days - L08 light 16 days - L16 light 32 days - L32 light 64 days - L64 temperature 7 days - T07 temperature 14 days - T14 temperature 21 days - T21 pollutant gases - NOX
3.2 3.4 4.4 5.8 8.9 1.8 1.9 2.3 36.6
1.0 0.7 1.3 2.1 3.6 -1.2 -1.0 -1.6 17.8
2.9 3.0 3.6 4.6 6.9 -1.0 -1.1 -1.6 27.7
1.0 1.3 2.1 2.9 4.4 -1.1 -1.2 -0.6 16.0
natural aging
∆E ∆a* ∆b* ∆L*
Cord room; Alca` zar, Segovia - ALC Depot Oost; Rijksmuseum, Amsterdam - RDO Nightwatch room; Rijksmuseum, Amsterdam RNW Sandham Memorial Chapel, Burghclere - SAC Clore gallery; Tate Gallery, London - TAT Leonardo room; Uffizi, Florence - UFF
3.6 1.4 1.9 2.7 4.5 2.3 3.1 2.2 4.2 2.3 2.8 2.2 3.8 2.0 2.0 2.5 4.3 1.6 2.7 3.1 6.1 3.1 4.3 3.0
FIGURE 9. DSI values for the artificially and naturally aged lead chromate dosimeters. For legend see text. responsible for the color change in the natural aging dosimeters. The strong variation in color observed in the artificially aged dosimeters due to pollutants could suggest that the pollutants play a major role in the exposure sites. However, the interpretation of the DSI and PCA data for the alizarin dosimeters (Figures 6 and 7, respectively) pointed out that, once again, light provided the main contribution to their natural aging, as previously reported for the lead white dosimeters. Finally, it has to be remarked that both the decrease and the blue shift of the absorption around 620 nm, which were observed in the naturally aged dosimeters (Figure 8b), seem to indicate again a possible role of the pH in affecting the spectral response of the alizarin dosimeters.
FIGURE 8. (a) Reflectance spectra (reported as A′ ) log(1/R)) of the naturally aged dosimeters reference (solid line), Uffizi Gallery dosimeter (dashed line), 64 days aged dosimeter (dash-dotted line) ,and pollutant gas aged dosimeter (dotted line) and (b) second derivative of the reflectance spectra, reported in Figure 8a, of naturally aged dosimeters reference (solid line) and Uffizi Gallery dosimeter (dashed line).
Lead Chromate Pigmented Tempera. The DSI values (Figure 9) indicated that light and thermal aging played a different role from pollutants in the alteration process of lead chromate tempera dosimeter. In fact, the spectral modification induced by light and thermal aging was a decrease in reflectance (in the 360-830 nm range) with respect to reference spectrum, while pollutants produced an increase in reflectance. Visible absorption in lead chromate pure pigment corresponds to a ligand-to-metal chargetransfer transition, where light induces the transfer of electrons from the oxygen ions to the Cr(VI) ion with a consequent photoreduction of the latter and a variation in the absorption spectrum. However, it could not be excluded that the binding medium could also act as a reducing agent, with respect to the Cr(VI) ion. The fact that light-aged DSI values were not in perfect accordance with the time exposure (32 days light-aged and 21 days thermal-aged dosimeters showed a change smaller than 16 days light-aged and 14 days thermal-aged dosimeters, respectively) might be caused by the lack of homogeneity in the painted dosimeters. PC1 (71.3% of total variance) divided thermal-aged dosimeters from pollutants and light ones, and PC2 (28.2% of total variance) split the latter into two groups (Figure 10). In particular, light-aged dosimeters were spread along the PC2 axis following the trend of integral values. Color analysis indicated a decrease in lightness ∆L* < 0 (darkening) for all the lead chromate dosimeters investigated, and color change (∆E) that was well correlated to the aging time (Table 4).
temperature-resolved mass spectrometry (DMTS) in aged egg tempera (20), suggests that in the present case the hydrolysis of the medium could have affected the spectral features of alizarin. The color change of the naturally aged alizarin dosimeters (Table 3) showed that a slightly higher ∆E value than the others was recorded in the Uffizi Gallery and that all the other field sites were characterized by similar color variations (within the experimental error). The three components, ∆a*, ∆b*, ∆L*, indicated that both light and pollutants might be
The only colorimetric parameter useful for the comparison between artificially and naturally aged dosimeters was ∆b* (Table 4). Indeed, its behavior suggested that, in natural conditions, light and temperature could be effective on lead chromate dosimeters. This consideration was also confirmed by DSI data (Figure 9). However, the projection of the naturally exposed set of dosimeters into the PC1-PC2 plot (Figure 10) clearly showed their similarity with light-aged dosimeters and excluded any contribution due to thermal aging. The two references (L00 and CON) lie practically in the same position. VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 10. PC1-PC2 scores plot of the reflectance spectra (360830 nm range) of artificially aged lead chromate dosimeters. The reflectance spectra of the naturally aged lead chromate dosimeters were projected on the same diagram. For legend see text.
TABLE 4. Colorimetric Values Calculated on the Artificially and Naturally Aged Lead Chromate Dosimeters artificial aging
∆E
∆a *
∆ b*
∆L*
light 4 days - L04 light 8 days - L08 light 16 days - L16 light 32 days - L32 light 64 days - L64 temperature 7 days - T07 temperature 14 days - T14 temperature 21 days - T21 pollutant gases - NOX
2.7 4.6 6.2 6.8 7.5 4.6 7.1 10.5 2.4
-1.9 -1.2 -4.7 -2.8 -3.1 -2.2 -3.8 -5.2 -1.6
-1.2 -3.9 -2.5 -5.2 -5.7 -3.4 -4.9 -8.7 1.7
-1.5 -2.0 -3.3 -3.4 -3.8 -2.3 -3.4 -2.8 -0.6
natural aging Cord room; Alca` zar, Segovia - ALC Depot Oost; Rijksmuseum, Amsterdam - RDO Nightwatch room; Rijksmuseum, Amsterdam -RNW Sandham Memorial Chapel, Burghclere - SAC Clore gallery; Tate Gallery, London - TAT Leonardo room; Uffizi, Florence - UFF
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Acknowledgments
6.2 -3.3 -4.4 -2.7
This work was supported by the EC Project Environmental Research for Art Conservation (ERA, Contract No. EV5V CT 94 0548) and, partially, by the Progetto Finalizzato “Beni Culturali” of the National Research Council of Italy. The authors wish to thank Dr. Marianne Odlyha (Birkbeck College, London), Prof. Jaap Boon, and Dr. Oscar van den Brink (FOM Institute, Amsterdam) for providing them with the artificially aged samples. They are also grateful to the staffs of the Tate Gallery (London) and the TNO (Delf) for performing the aging tests and to Mr. Bellucci of the Opificio delle Pietre Dure (Florence) for preparing the tempera-samples. Thanks are expressed to Dr. Ornella Casazza and Dr. Anna Maria Petrioli (Soprintendenza dei Beni Artistici e Storici of Florence), who allowed the authors to put the dosimeters in the Uffizi Gallery.
7.7 -5.4 -4.1 -3.5 9.5 -6.2 -5.5 -4.6
Literature Cited
∆ E ∆a *
∆ b*
∆L*
9.9 -6.0 -6.3 -4.8 1.9 -1.9 -0.3 -0.2 6.9 -4.6 -3.8 -3.5
As expected, ∆E data and DSI values for the naturally aged dosimeters indicated that the Deposit Oost of the Rijksmuseum could be considered the safest place (Table 4), and the most affecting environments were both at the Alcaza´r in Segovia and the Uffizi Gallery. Comparison of Dosimeter Response. The results obtained for the three different dosimeters were quite consistent. However, owing to the experimental error, which affected the entire measurement procedure, a simple list of the sites ordered by increasing (or decreasing) environmental stress could be misleading. In fact, in some cases the data obtained from the same dosimeter (i.e. lead white) but exposed in different sites (i.e. Uffizi Gallery, Tate Gallery, Rijksmuseum, etc.) showed differences in the values of DSI (or ∆E) which fell within the variability due to the error in the measurement procedure. As a consequence, a well-defined order of the natural sites investigated was questionable regarding the environmental risk. According to this premise, the Rijksmuseum Deposit Oost was the safest site for every dosimeter, as expected. Then, from the lead white and lead chromate dosimeters, which were mainly affected by light in the field exposure, it was seen that the worst situation was found at the Alcaza´r in Segovia and the Uffizi Gallery, while the Rijksmuseum Night 2864
Watch, Tate Gallery, and Sandham Memorial Chapel were safer. As regards the alizarin dosimeter, good stability was found at the Rijksmuseum Night Watch (quite comparable to the results from Rijksmuseum Deposit Oost), whereas the Tate Gallery and Sandham Memorial Chapel, which are quite safe as regards light, are situated in a position comparable to or even worse than that of the Uffizi Gallery or the Alcaza´r in Segovia. As the alizarin dosimeter can be affected also by pollutants, pH and, consequently, CO2, mainly due to the number of visitors, a strict control of these environmental factors is advisable. Based on this experiment, it can be concluded that mock paintings could well act as an early warning system for indicating the potential risk of a local environmental situation. As these dosimeters were prepared with materials that have been encountered in actual works of art, the damage detected by them, though amplified, can be considered a more reasonable approximation than that obtained with other materials not related to the painting practice. The reported results showed that, in general, light exposure is one of the main causes for aging alterations that occur in galleries and historical sites. Moreover, a suitable assembly of different pigments/dyes could provide information about the most affecting environmental factors in the alteration processes.
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Received for review December 28, 1999. Revised manuscript received April 17, 2000. Accepted April 24, 2000. ES991437D
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