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Abstract
In museum lighting, traditional Chinese paintings are the artworks with the highest light sensitivity. They are vulnerable to the color damage to pigments and the mechanical damage to substrates after the irradiation from light sources. As the basis of effective preventive protection, the research on the two lighting quantity indexes of illuminance and annual exposure (illuminance × time) is currently missing. In this study, the halogen lamp was used as the experimental light source to conduct a 1440 h irradiation experiment on the samples of paper and silk substrates under 4 illuminance levels, respectively, and the test of infrared spectrum was carried out on the samples every 240 h. The oxidation index of paper and crystallinity degree of silk were calculated then. Three-dimensional visual curved surface plots of mechanical damage to samples with the change of illuminance and time were established and then fitted into damage evaluation equations, which revealed and described mathematically the mechanical damage law of samples. Through the equations, the recommended values of illuminance and annual exposure for samples could be calculated. Combining the previous researches on pigments, the lighting quantity indexes of traditional Chinese paintings with different combinations of substrates and pigments were proposed.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Traditional Chinese paintings are an important part of the world art, with high historical and artistic value. At the same time, there are more than 640,000 pieces in existence, with a huge inventory. In China, there are 3993 history and art museums that preserve such exhibits. However, due to the characteristics of materials and craftsmanship, traditional Chinese paintings are the exhibits with the highest light-sensitivity level stipulated by the International Lighting Commission (CIE) and are extremely vulnerable to light. At the same time, the exhibition halls of traditional Chinese paintings are all fully artificial lighting environments isolated from natural light, so artificial lighting is the most important cause of paintings damage [1]. The lighting requirements of general buildings are to reduce energy consumption on the basis of meeting visual requirements, while traditional Chinese paintings exhibition halls in museums first need to ensure the light safety of cultural relics. Therefore, it involves cultural relics protection, visual effects and energy efficiency [2], among which protection is the key. In order to avoid and minimize “future” losses [3], preventive protection has attracted attention and the establishment of scientific lighting quantity indexes is the basis for effective preventive protection.
At present, there are different standards for illuminance and annual exposure (the product of illuminance and irradiation time in a year) in different countries and organizations [4–9], as shown in Table 1. However, the inconsistent standards without enough accuracy cannot meet the requirements of both cultural relic protection and visual needs comprehensively. For example, CIE 157-2004, GB/T 23863-2009 [4,5] and other standards stipulate that the upper limit of illuminance value of painting cultural relics is 50 lx. The illuminance value is the lowest illuminance that can meet the color discrimination of human eyes [10], but it cannot guarantee that the visual effects of different cultural relics can meet the viewing needs. The difference of materials will affect the absorption and reflection characteristics of cultural relics, resulting in different visual effects of cultural relics under the same light source. In addition, with the policy orientation of “let cultural relics lively”, the lighting standards of Chinese museums are constantly being revised in an attempt to balance preventive protection with visual effects by increasing illuminance. According to our previous researches, it is found that increasing the illuminance within 200 l× doesn’t cause sudden and large increase in the damage to cultural relics [11]. Therefore, it is of great social significance for us to meet the visual comfort of human eyes as far as possible while minimizing the damage degree of cultural relics, and it is an urgent problem to propose a scientific and reasonable recommended value of lighting quantity indexes.
Table 1. Limit Values of Illuminance and Annual Exposure of Different Standards for the Artwork With the Highest Light Sensitivity
Through the lighting aging experiments, the influence law of different illuminance and irradiation time on painting materials can be obtained, which is an internationally accepted method to obtain the lighting quantity indexes [12]. In the previous research of our studio, the research group used halogen lamps as experimental light sources to conduct a 1440 h aging experiment on 25 kinds of pigment samples under four illuminance levels [11]. The experimental specimens were all common painting pigment types, including 18 inorganic pigments and 7 organic pigments. The color parameters were tested every 240 h and the color differences were calculated. Based on the data analysis, the damage evaluation model of 25 pigments was established, which calculated the recommended values of illuminance and annual exposure for different types of paintings, including organic pigment paintings, inorganic pigment paintings, organic and inorganic pigment paintings. However, the above research only took the degree of color damage to pigments as the evaluation index, which is suitable for some painting types such as oil painting whose pigments are fully spread on the substrates. As the substrates are completely covered with pigments and are not exposed to light, these types of paintings only need to consider the color damage to the pigments rather than the mechanical damage to the substrates. One of the biggest characteristics of traditional Chinese paintings is that there is a large amount of white space in the painting, that is, pigments only occupy a part of the painting and a large number of substrates are still not covered by pigments, as shown in Fig. 1. Therefore, in addition to color damage to pigments, the mechanical damage to substrates must also be considered.
Traditional Chinese paintings usually use Xuan paper or silk as substrates. The main component of Xuan paper is plant fiber, and the main component of silk is silk protein, both of which are organic materials with unstable properties. The photo-induced damage in paintings is caused by photochemical reaction after the irradiated material absorbing the spectral energy of the light source, which results in denaturation of functional groups of the material molecules [13,14]. The mechanical properties will decline [15–17], leading to some photoaging phenomena such as cracking, embrittlement, etc. The damage will greatly reduce the historical and artistic value of cultural relics. The damage to paper is caused by the hydrolysis and oxidation reactions in the aging process of cellulose, which gradually reduce the physical and chemical properties of paper, and eventually lead to the loss of material quality [16]. J. Lojewska et al. used the oxidation index of cellulose as an index to evaluate the mechanical damage degree of paper [18]. The damage to silk is caused by the degradation of silk fibroin during the aging process. These reactions lead to yellowing and embrittlement, and reduce the mechanical properties of materials. L. Li et al. used the crystallinity of silk fibroin as an index to evaluate the mechanical damage degree of silk [19].
Using a microscope to observe the microscopic changes of paper and silk samples before and after irradiation is a traditional method for mechanical damage evaluation [20–22]. However, this method can only find the tiny cracks which cannot be recognized by naked eyes, and cannot quantitatively evaluate the oxidation index of paper and the crystallinity of silk at the micro scale. In the field of analytical chemistry, Fourier transform infrared (FTIR) spectroscopy is an effective method to study the micro-structure of materials. Like the fingerprints, each material has its characteristic infrared peaks, which correspond to different molecular functional groups of the materials. When the molecular structure of materials change due to external stimuli, the corresponding infrared peaks will change accordingly. Therefore, the quantitative characterization of micro-structure changes can be realized through the analysis of infrared peaks [23]. FTIR spectroscopy has been widely used in the field of material micro-structure analysis. For example, M. Ali et al. [24] characterized the aging of cellulose paper by using FTIR spectroscopy to evaluate the insulation status of transformer paper. J.Z. Shao et al. [25] used ATR - FTIR to study the effects of different environments on the aging and deterioration properties of silk. A. Munajad et al. [26] used FTIR to detect the change of light transmittance of functional groups representing cellulose aging characteristics in cellulose paper. In addition, this technology has the advantages of no sample pretreatment, non-contact and non-destructive analysis, and fast analysis speed. It is especially suitable for analyzing the influence of lighting on the mechanical damage to traditional Chinese painting substrates.
The mechanical damage degree of substrates is related to the photochemical material properties, irradiation intensity and irradiation time. Although FTIR spectroscopy provides a method to evaluate the mechanical damage to substrates at the micro level, there is no report on the coupling study of the damage with the irradiation intensity and irradiation time. The establishment of mathematical model is the basis of obtaining the lighting quantity indexes, however, there is no mathematical model that can describe the above law uniformly. To solve the problem that the lighting quantity indexes of traditional Chinese paintings are missing, this study revealed the mechanical damage law of paper and silk caused by illuminance and time at the micro level, and established mathematical models to describe the above law. The recommended values of illuminance and annual exposure for paper and silk were calculated. Combining the previous research results on pigments, the lighting quantity indexes of traditional Chinese paintings with different combinations of substrates and pigments were proposed. The research results can provide suggestions for the exhibition and protection of traditional Chinese paintings, improving the quality of light environment in museums.
2. Experiment
2.1 Experimental light source
Although LED lighting is now becoming the standard of museums, and there are many adjustable spectra that can meet the requirements of paintings lighting, it is difficult to extract and study all the LED spectra that meet the requirements in the experiment. It can be found that the common points of these spectra are that they have low short-wave content, high long-wave content and relatively continuous visible spectra. So we chose the halogen lamps as the experimental light sources to represent the commonly used light sources with low correlated color temperature, high color rendering and continuous spectra in museum lighting. In this experiment, PHILIPS 13289 halogen lamps with Andover 700FL07-25 low-pass infrared filter (short wave pass) were selected as experimental light sources [5]. The relative spectral power distributions of the halogen lamp are shown in Fig. 2. The light sources don’t contain ultraviolet and infrared spectra, whose color temperature Tc is 2700 K ± 30 K and average color rendering index Ra is 99. In addition, in order to avoid attenuation of the light source after a long time of irradiation, the light flux of the light source was measured before the experiment. HJ S-480-0-12 voltage regulator was used to ensure voltage stability. The light source would be replaced by a backup light as soon as it was decayed and caused an abnormality.
2.2 Experimental samples
The paper and silk samples used in the experiment were ancient originals at the end of the 19th century. Traditional Chinese mounting technology was used to make samples level. Then the paper and silk were cut into 1 cm × 1 cm squares and combined as a standard experimental sample. Three test points were marked on each sample and detected in the later infrared spectrum test. Then the average value was taken to reduce the measurement error, as shown in Fig. 3. Five groups of the same standard samples were made by the above method, which were used in the irradiation experiment under 4 different illuminance values and the control experiment under 1 dark condition.
2.3 Experimental device
In museums, paper and silk relics are usually kept in constant temperature and humidity display cabinets. In order to restore the real museum environment, a closed experimental cabinet was set up in the all-dark optical laboratory to eliminate the interference of temperature and humidity. The temperature and humidity parameters of the experimental cabinet were set according to the environmental parameters specified in JGJ 66-2015 standard [27], in which the internal temperature was kept constantly at 23 ± 0.5°C and the relative humidity was kept constantly at 50 ± 5%. According to the parameters of the display cabinet specified in GB/T 36110-2018 standard, the internal air exchange rate of the display cabinet was set as 0.5 d−1, which met the requirements of the sealing performance of the display cabinets [28]. In order to prevent the light sources of different irradiation groups in the cabinet from interfering with each other, the movable partition boards were used to divide the cabinet into 5 independent spaces. Among them, the top of 4 spaces were equipped with experimental light sources that could be adjusted by power, and the inner wall of every space was placed with black velvet to prevent the influence of reflected light on the wall. The last space was a dark control group without light source. A rotating platform was installed under each experimental light source to place the experimental samples. During the irradiation process, the platform rotated uniformly at a speed of 0.5 rpm to ensure the uniformity of illuminance on the specimen surface, as shown in Fig. 4.
2.4 Experimental scheme
CIE 157-2004, GB/T 23863-2009 and other standards stipulate that the illuminance of highly sensitive cultural relics such as calligraphy and painting in the museum should not exceed 50 lx [4,5]. However, the illuminance at 50 lx only meets the minimum requirement for human color discrimination, and provides unsatisfactory exhibition results for many calligraphy and painting. A research shows that the illuminance at 200 lx is sufficient to provide visibility and achieve exhibition design goals based on the visual comfort of gallery visitors when appreciating artworks [29]. Previous studies of the studio found that increasing the illuminance within 200 lx would not cause sudden and large changes in the damage to cultural relics [11]. Therefore, the illuminance parameters of the irradiation groups were set in the range of 50–200 lx and divided into four illuminance gradients: 50 lx, 100 lx, 150 lx, and 200 lx. At the same time, a control group of 0 lx was set to exclude the effect of natural aging. The same samples were irradiated for 6 days a week under five illuminance values of 0 lx, 50 lx, 100 lx, 150 lx, and 200 lx. And they were irradiated periodically for 8 h per day for 180 days with a total of 1440 h. Every 30 days was a measurement cycle. Nicolet 6700 infrared spectrometer was used to measure the infrared spectra of paper and silk samples in the wavelength range of 4000–400 cm−1. The spectral resolution was better than 0.1 cm−1 and the scanning times was 32.
2.5 Evaluation indexes
The oxidation index OIFTIR is used to evaluate the carbonyl conversion of cellulose in Xuan paper, and then to characterize the mechanical damage degree caused by different illuminance levels. The larger the oxidation index value, the higher the mechanical damage degree of Xuan paper [18]. The oxidation index is defined as the ratio of the integral area of the characteristic infrared spectral peak at 1900–1500 cm−1 to the integral area of the characteristic infrared spectral peak at 3000 - 2800 cm−1 [30], as shown in Eq. (1).
The crystallinity CFTIR is used to evaluate the peptide bond breakage of proteins in silk, and the mechanical damage degree caused by different illuminance levels is characterized. The smaller the crystallinity value, the higher the mechanical damage degree of silk [25,31]. The crystallinity is defined as the ratio of the area of the characteristic infrared spectral peak at 1263 cm−1 to the sum of the areas of the characteristic infrared spectral peaks at 1230 cm−1 and 1263 cm−1 [19], as shown in Eq. (2).
3. Analysis
3.1 Paper
According to the infrared spectra measured in seven test cycles, the integral areas of 1900–1500 cm−1 and 3000–2800 cm−1 in the spectra were calculated to obtain A1900–1500 and A3000–2800. Equation (1) is used to calculate the oxidation index ΔDpi of Xuan paper relative to the initial state OFTIR0 in each cycle:
After calculation, the change values of oxidation index of paper samples corresponding to different illuminance levels and irradiation time are shown in Table 2.
According to Table 2, some individual abnormal values were excluded after necessary data processing. We used MATLAB software to establish a 3D visualization surface of paper with illuminance E as X axis, time t as Y axis, and change values of oxidation index ΔDp as Z axis, as shown in Fig. 5. It can be seen that the damage law of paper is caused by the changes of time and illuminance, and oxidation indexes of paper increase with the increase of illuminance and time. This shows that the lighting damage degree of paper is positively correlated with illuminance and time.
Fig. 5. Three-dimensional surface plot of the oxidation index changes with the illuminance and time.
Table 2. Change Values of Oxidation Index of Paper ΔDp
The 3D surface is fitted by binary polynomial, and its function expression is obtained (R2 = 0.8905), as shown in Eq. (4):
As the lighting damage degree is positively correlated with the illuminance E, when E reaches a certain value, the lighting damage degree of paper will increase significantly, that is, the change rate of damage will increase significantly. The value of E is called illuminance threshold, and the change rate can be expressed by derivative. At present, the average exhibition time of paintings in Chinese museums is 156 days, with 8 h per day, for a total of 1248 h. Therefore, t = 1248 h/y is brought into the Eq. (4), and the derivative of E value is calculated. The change rate of lighting damage degree with illuminance is obtained when the paper has been irradiated for 1248 h, as shown in Eq. (5):
All illuminance values of 0 lx ≤ E ≤ 200 lx are brought into Eq. (5), and the change rate of lighting damage degree ΔDp′ with different illuminance values can be obtained, as shown in Fig. 6.
CIE 157-2004, GB/T 23863-2009 and other standards stipulate that the illuminance of highly sensitive cultural relics such as calligraphy and painting in the museum should not exceed 50 lx [4,5]. Therefore, the change rate of lighting damage at E = 50 lx is taken as the threshold, that is, |fp′(E, 1248)| ≤ |fp′(50, 1248)|. It can be seen from Fig. 6 that the above requirements are met when 50 lx ≤ Ep ≤ 146.3 lx, so the recommended value of illuminance is Ep ≤ 146.3 lx. Since the equation of annual exposure is Q = E · t, the recommended value of illuminance is multiplied by the average annual exhibition time 1248 h, and the recommended value of annual exposure is Qp ≤ 182582.4 lx·h/y.
3.2 Silk
According to the infrared spectra measured in seven test cycles, the two peak areas, A1263 and A1230, near 1263 cm−1 and 1230 cm−1 were calculated. Equation (2) is used to calculate the crystallinity value ΔDsi of silk relative to the initial state CFTIR0 in each cycle:
After calculation, the change values of crystallinity of silk samples corresponding to different illuminance levels and irradiation time are shown in Table 3.
According to Table 3, some abnormal values were excluded after necessary data processing. We used MATLAB software to establish a 3D visualization surface of silk with illuminance E as X axis, time t as Y axis, and change values of crystallinity ΔDs as Z axis, as shown in Fig. 7. It can be seen that the quantitative damage law of silk is caused by the changes of illuminance and time. From the overall trend, the crystallinity of silk decreases after long-term light aging, which is consistent with the research results of Shao et al. [25]. The crystallinity of silk first slightly increases and then decreases with the increase of illuminance and the extension of irradiation time. We speculate that in the early stage of aging test, the aging speed of amorphous region on the surface of silk fiber is faster than that of crystalline region, so the proportion of crystalline region rises [32]. After a period of time, the aging rate of the crystalline region is higher than that of amorphous region, the proportion of crystalline region decreases, and finally the crystallinity of the silk decreases slowly.
Table 3. Change Values of Crystallinity of Silk ΔDs
The 3D surface is fitted by binary polynomial, and its function expression is obtained (R2 = 0.8324), as shown in Eq. (7):
All illuminance values of 0 lx ≤ E ≤ 200 lx are brought into Eq. (8), and the change rate of lighting damage degree ΔDs′ with different illuminance values can be obtained, as shown in Fig. 8.
It can be seen from Fig. 8 that the above requirements are met when 7.2 lx ≤ Es ≤ 116.2 lx, so the recommended value of illuminance is Es ≤ 116.2 lx. The recommended value of illuminance is multiplied by the average annual exhibition time 1248 h, and the recommended value of annual exposure is Qs ≤ 182582.4 lx·h/y.
4. Conclusions
According to the above analysis, the recommended values of illuminance and annual exposure for paper and silk are shown in Table 4.
Table 4. Recommended Lighting Quantity of Substrates
The recommended illuminance values of pigments obtained in the previous research of the studio are shown in Table 5 [11].
Table 5. Recommended Lighting Quantity of Pigments
Traditional Chinese paintings can be divided into six types according to the types of substrates and pigments used: paper with organic pigments, paper with inorganic pigments, and paper with organic and inorganic pigments, silk with organic pigments, silk with inorganic pigments, silk with organic and inorganic pigments. Based on Tables 4 and 5, in order to achieve the best protection effect, the recommended lighting quantity indexes of traditional Chinese paintings are obtained by taking the low value of single material in painting combination materials as the standard, as shown in Table 6.
Table 6. Recommended Lighting Quantity of Traditional Chinese Paintings
The above results mainly put forward the lighting quantity indexes of traditional Chinese paintings from the perspective of cultural relic’s protection. The recommended values of illuminance range from 41.6 lx to 146.3 lx. From the perspective of exhibition, the minimum value of 41.6 lx can basically meet the exhibition requirements, which can be confirmed according to the existing standards and relevant literature [4–6,10]. From the perspective of lighting energy consumption, the maximum value of 146.3 lx is a low illuminance value in the building lighting, and there is no problem of excessive energy consumption. Therefore, in general, the recommended lighting quantity indexes of Chinese traditional paintings can well balance the three aspects of protection, exhibition and energy efficiency. In addition, if there are some professional researches on exhibition and energy efficiency, more accurate recommended values will be got. This study didn’t give special consideration to them, but our conclusion can provide a reference for the follow-up research work in the protection of cultural relics.
Funding
National Natural Science Foundation of China (52078331); Science Fund for Distinguished Young Scholars of Tianjin (20JCJQJC00200); Peiyang Scholars Fund (1801).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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