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Study on the restoration of glass slides dating back to the 1940s

Heritage Science volume 12, Article number: 335 (2024) Cite this article

Abstract

The Museum of Sun Yat-sen University houses a collection of antique glass slides dating back to the 1940s. These historical artifacts not only serve as a record of the past but also bring history to life. During extended storage, the emulsion layer on glass slides may harden and become brittle, leading to cracking and buckling. This study suggests a method to enhance the physical property of the emulsion layer by using a combination of nonionic surfactant isomeric alcohol ethoxylates eight (TO-8) and waterborne epoxy resin (WER). We investigated the microscopic action mechanism of the two on the emulsion layer of glass slides using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), water contact angles, and other techniques. Our study revealed that TO-8 can significantly eliminate the spherulitic crystal structure of the emulsion layer, as well as improve its softness and hydrophilicity. The addition of the WER ensures that the cracking and warping of the emulsion layer film is fully corrected, resulting in a flat surface. Additionally, the size of the emulsion layer film remains stable even after wetting. The WER has minimal impact on the image information of glass slides. The emulsion layer of the glass slides, restored using the softening protection solutions developed in this study, showed almost full recovery of image information. This research holds significant theoretical and practical value for repairing cracked and warped emulsion layers on glass slides.

Introduction

Dry gelatin photography, also known as glass slides, is a historic photographic technique that originated in the nineteenth century. In the early days of dry gelatin photography, glass sheets were coated with a light-sensitive film. The production process involved mixing dissolved gel with potassium bromide and potassium iodide, preparing a silver nitrate solution in complete darkness, stirring the silver nitrate solution with the gel mixture. The temperature of the whole process is controlled between 50 and 60 °C. This meticulous process resulted in the creation of the light-sensitive film. The glass slides coated with the light-sensitive film were then cooled, exposed to light, developed, fixed, and finally produced. Glass slides have been in use since the mid-nineteenth century, serving as a valuable tool for recording historical figures and scenes. Over the past century, they have formed a unique and precious archive [1,2,3]. Sun Yat-sen University is located in Guangdong Province, in the vibrant city of Guangzhou on the subtropical coast. The city enjoys an oceanic subtropical monsoon climate, with mild winters and hot summers. Thanks to oceanic currents, the area receives between 800 and 1000 mm of precipitation annually, creating a humid environment. The university has a valuable collection of photographic images that have been carefully preserved for decades. However, the storage conditions lack proper temperature and humidity control, leading to fluctuations in overall temperature and humidity based on the seasons. Glass slides are affected by environmental temperature and humidity during long-term storage. This can cause the emulsion layer to crack, warp, and peel off (Fig. 1).

Fig. 1
figure 1

a Photographs of wrinkled and cracked emulsion layers on glass slides; b photograph of cracked and warped emulsion layer on glass slides

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The emulsion layer on glass slides primarily consists of gelatin, a protein derived from collagen denaturation [4, 5]. Gelatin, as a protein macromolecule, shares similar properties with other proteins [6]. The gelatinization process of gelatin solutions involves the formation of a three-dimensional network, with physical cross-linking of gelatin gels achieved through weak intermolecular bonding such as van der Waals forces, hydrogen bonding, or electrostatic interactions. The gelation process of gelatin can be analyzed through kinetic modeling, with variations in concentrations, extracts, and temperatures impacting the process [7,8,9,10]. The unique physicochemical properties of gelatin are determined by its molecular structure, which is composed of peptide bonds linking amino acids’ carboxyI and amino groups [11]. These peptide bonds are susceptible to hydrolysis in the presence of water molecules, making glass slides vulnerable to high temperatures and humidity during storage [12,13,14,15]. When exposed to these conditions, the ordered triple helix structure of the emulsion layer transitions to a disordered single helix structure, leading to a loss of colloidal adhesion. This can result in the emulsion layer becoming hard, brittle, and eventually cracking, warping, or peeling off (Fig. 2).

Fig. 2
figure 2

Causes of cracking and warping of the emulsion layer on glass slides

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When repairing cracked and warped glass slides, the first step is to use a mixture of water and surfactant to slightly dissolve the hard and brittle emulsion layer. This will soften and flatten the wrinkled areas, making it easier to repair the slides. Surprisingly, there is a lack of information on the protection and repair of glass slides both domestically and internationally.

Surfactants play a crucial role in reducing the surface stress of glass slides and softening the emulsion layer [16,17,18]. There are two main types of surfactants: ionic and nonionic [17, 19]. Ionic surfactants typically cause the substrate surface to arrange hydrophilic groups inward and hydrophobic groups outward. However, this arrangement renders the substrate surface hydrophobic, reducing the likelihood of infiltration. Nonionic surfactants, which have excellent solubility in water and are electrically neutral or micellar, are known for their stability in acidic, alkaline, and neutral electrolyte solutions [20]. The nonionic surfactant TO-8 utilized in this study exhibits exceptional wetting properties for the emulsion layer of glass slides. The neutral molecules in TO-8 possess high surface activity, maximizing the original system’s surface activity without compromising the emulsion layer of the glass slides [21,22,23,24]. The TO-8 aqueous solution effectively softens the emulsion layer of the glass slides.

During the softening process, it is crucial to prevent significant deformation or dissolution of the emulsion layer caused by excessive water absorption, as this can negatively impact the repair process. The WER contains numerous epoxy groups and boasts exceptional properties [25, 26]. Compared to non-aqueous epoxy resins, the WER has been found to be more stable [27,28,29]. It has the ability to fully penetrate the emulsion layer of glass slide films, forming functional group bonds with gelatin within the layer. This results in an improvement in the physical properties of the gelatin within the emulsion layer, preventing excessive water absorption, swelling, or dissolution during the impregnation process. Additionally, the WER can work synergistically with TO-8 to enhance the softening and leveling repair effects of the emulsion layer [30,31,32].

This paper utilized TO-8 and WER as restorative agents for the restoration of glass slides. Various analytical techniques including SEM, FTIR, Confocal laser scanning microscope (CLSM), softness meter, colorimeter, and optical contact angle measuring instrument were employed to analyze the emulsion layer before and after restoration. The results of these tests were used to comprehensively evaluate the effectiveness of the restorative agents on the emulsion layer. The practical application of the restorative agents on glass slides was demonstrated in this study, providing valuable insights and methods for the restoration and conservation of glass slides.

Experimental section

Experimental material

Gelatin, Ethanol absolute, Shanghai McLean Biochemical Technology Co., Ltd; Ammonium Bromide, Chengdu Jiaye Biotechnology Co., Ltd; Potassium iodide, Shanghai Zhan Yun Chemical Co., Ltd; Silver nitrate, Xilong Science Co., Ltd; Waterborne epoxy resin; Ultra-pure water; Ammonium hydroxide, Wuhan Jiye Sheng Chemical Co., Ltd; Isomeric alcohol ethoxylates eight (TO-8), Shandong Yusuo Chemical Technology Co.

Preparation of simulated samples

To begin, a 1 mol/L silver nitrate solution was prepared using dilute ammonia, while separate solutions of 1 mol/L ammonium chloride, 1 mol/L potassium iodide, and gelatin with a mass concentration of 15% were each configured using distilled water. The solutions of ammonium chloride, potassium iodide, and gelatin were then thoroughly mixed before slowly adding the silver nitrate solution under protection from light. The mass ratio of the solutions was 10:5:46:2, respectively.

After cooling, refrigerate the slices overnight before rinsing them with distilled water to remove any remaining halides. Heat the emulsion again to complete the configuration. Apply the emulsion evenly to warm steel or glass slides within 25 h, allowing the unexposed slides to dry before exposing them. Develop the exposed slides to produce simulated samples [33].

Place the glass slides in a high and low-temperature alternating hygrothermal aging chamber, following a program of 48 h at 50 °C and 20% RH, then 48 h at 20 °C and 50% RH for one cycle, repeating for 2–3 cycles. Finally, allow the emulsion layer to wrinkle overnight in a desiccator with pure water before drying.

Preparation of protection softening solutions

Reagents selected for the preparation of softening protection solutions included TO-8, anhydrous ethanol absolute, and WER. The following solutions were prepared:

  1. 1.

    TOOE: A 6% TO-8 solution was configured with ultrapure water.

  2. 2.

    TOEA: TO-8 and ethanol were mixed uniformly ultrapure water with mass percentages of 0.6% and 80%, respectively.

  3. 3.

    TEERA: TO-8, ethanol, and water were mixed uniformly with mass percentages of 0.6%, 75% and 22.5%, respectively. After that, WER was added with a mass percentage of 2%–2.5%.

Simulated samples of wrinkle emulsion layer were treated with TOOE, TOEA, and TEERA to evaluate their softening and protection effects.

Softening protection process for simulated samples

Place the simulated samples on a clean tabletop. Using a rubber-tipped dropper, slowly apply TOOE, TOEA, and TEERA onto the surface of the samples, ensuring they are completely saturated. Allow the samples to rest for 2 h before removing them from the table. The sample without any softening protection solution will be referred to as CT, while the samples treated with TOOE, TOEA, and TEERA will be labeled as C-TOOE, C-TOEA, and C-TEERA, respectively.

Characterization methods

Measurement of elongation and contraction lengths

Initially, 50 mm × 50 mm simulated samples are carefully chosen and subjected to treatment with TOOE, TOEA, and TEERA. Subsequently, the samples are allowed to rest for approximately 30 s before their horizontal length is measured using a vernier caliper. Following the initial measurement, the samples are placed in the ultra-clean bench (VD-650; Tianjin Xinbode Instrument Co., Ltd.) for drying. Finally, the horizontal length is measured once more using the vernier caliper.

Water contact angle test

Three points are sequentially taken at different positions of CT, C-TOOE, C-TOEA, and C-TEERA. The water contact angle is measured using an optical contact angle meter (Model OCA20; Dataphysics, Germany), and the contact angle values of the samples are recorded at 0 s, 5 s, 10 s, and 15 s.

Softness test

The CT, C-TOOE, C-TOEA, and C-TEERA specimens are cut into 100 mm × 100 mm pieces. The softness test, following GB/T 8942-2002 standards, is conducted using a feeler-type softness instrument (DCP-RRY1000; Sichuan Changjiang Paper Instrument Co., Ltd.). The instrument has a slit width of 10 mm, and the depth of specimen indentation is 8 ± 0.5 mm.

Surface roughness test

The simulated samples, CT, C-TOOE, C-TOEA, and C-TEERA, are positioned directly on the CLSM (Model VK-X250K; Keens Corporation, Japan). The surface morphology is then observed and the roughness is measured in sequence. Following data processing, the Sa, Sz, Str, Spc, and Sdr values of the simulated samples are determined.

Color difference value test of glass slides after artificial accelerated aging

CT, C-TOOE, C-TOEA, and C-TEERA samples are subjected to hygrothermal aging in a chamber (HCP type; Mermelte Trading Co., Ltd.). The temperature is set to 50 °C, relative humidity to 80% RH, and aging time to 12 days. (Equivalent to 1.5 to 2 years of natural storage at 20 ± 2 °C and 65 ± 5% RH [34]). Every 3 days, a batch of specimens is removed and immediately analyzed for color difference using a colorimeter (X-RiteVS-450 model; AISI Instruments Co., Ltd., USA).

Microcosmographic analysis

CT, C-TOOE, C-TOEA, and C-TEERA are placed in a vacuum freeze-dryer (model FD8-4a, GOLD-SIM, USA) for 24 h. Following freeze-drying, the samples were cut into 10 mm × 10 mm pieces and fixed onto the stage of a scanning electron microscope using conductive adhesive. The samples were then coated with gold using an ion sputtering apparatus (SCD005, BAL-TEC, Switzerland) for 120 s. Subsequently, the surface morphology of the samples was examined using a tungsten filament scanning electron microscope (SU3500, Hi-Tech, Japan) at an accelerating voltage of 3 kV and a magnification of 1000 times.

Infrared spectroscopy analysis

Infrared spectroscopic testing of CT, C-TOOE, C-TOEA, and C-TEERA films was conducted using a Fourier infrared spectrometer (PE-Frontier model, Platinum Elmer, USA) in the wavelength range of 4000 to 500 cm−1. The WER is coated inside the PTFE molds and allowed to form a film. The infrared spectra of the gelatin film and WER were then tested using the same PE-Frontier Fourier infrared spectrometer in the wavelength range of 4000 to 500 cm−1.

Evaluation of restoration effect

A digital camera is utilized to capture images of the glass slides both before and after restoration. Additionally, a scanner (V800; Seiko Epson) is employed to scan the glass slides in professional mode at 16-bit grayscale and 600 DPI resolution.

Explanation of abbreviations in articles

Abbreviations for reagents

The nonionic surfactant isomeric alcohol ethoxylates eight (TO-8); waterborne epoxy resin (WER); the abbreviations of the prepared reagents TOOE, TOEA and TEERA are given in the “Preparation of protection and softening solutions” section.

Abbreviations for simulated samples

CT, C-TOOE, C-TOEA and C-TEERA correspond to the untreated simulated samples and the simulated samples treated with TOOE, TOEA and TEERA, respectively.

Abbreviations for instruments

The scanning electron microscopy (SEM); Fourier transform infrared spectroscopy (FTIR); Confocal laser scanning microscope (CLSM).

Results

Expansion of simulated samples through absorption

In this study, three different softening agents-TOOE, TOEA, and TEERA-were utilized to soften samples, and their dimensional changes were measured post-softening. Each group of samples was tested six times to compare the effectiveness of the three agents. Results depicted in Fig. 3a show that samples softened with TOOE had lengths ranging from 5.60 to 5.85 cm, while samples treated with TOEA ranged from 5.45 to 5.65 cm, and TEERA-treated samples ranged from 5.10 to 5.15 cm. Notably, TEERA-treated samples exhibited the least elongation. Similarly, Fig. 3b illustrates that samples treated with TOOE had lengths ranging from 4.50 to 4.65 cm post-drying, TOEA-treated samples ranged from 4.65 to 4.80 cm, and TEERA-treated samples ranged from 4.75 to 4.85 cm. The shrinkage length of TEERA-treated samples was also minimal. Overall, it can be inferred that the interaction of the WER with the samples during wetting helps maintain their dimensions. Furthermore, the wetting performance was assessed by measuring the water contact angle to understand the interaction mechanism between the WER and the samples.

Fig. 3
figure 3

a Elongation length of simulated samples after softening; b shrinkage length of simulated samples after drying

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Wetting properties of simulated samples

Figure 4 displays the results of the water contact angle values for CT, C-TOOE, C-TOEA and C-TEERA at different times. All four groups exhibited contact angles of less than 90°, indicating that both the untreated and treated samples are hydrophilic [35, 36]. It is evident from the figure that CT and C-TOOE have relatively poor hydrophobicity, with the contact angle decreasing rapidly upon contact with water droplets. This rapid penetration of liquid into the samples leads to over-absorption, causing dissolution and deformation. Among the samples, C-TOOE shows the most significant changes in appearance and size, which could potentially result in overlapping of image portions on glass slides during the restoration process, leading to damage to image information.

Fig. 4
figure 4

Contact angle values for CT, C-TOOE, C-TOEA and C-TEERA at different times

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On the other hand, C-TOEA and C-TEERA exhibit larger contact angles, indicating improved hydrophobic properties. This enhancement may be attributed to the hydrogen bonding between the hydroxyl group of absolute ethanol and gelatin, as well as between the hydroxyl group of WER and the amino group of gelatin. The intermolecular force of hydrogen bonding between the hydroxyl and amino groups is stronger than that between hydroxyl groups.

The softness of simulated samples

Flexibility is determined by the combination of the bending force of the specimen and the friction force between the specimen and the slit when pressed with a plate-like probe. A lower flexibility value indicates a softer specimen, which is crucial for repositioning the emulsion layer on glass slides [37]. If the emulsion layer is too hard, it may break during repositioning, while if it is too soft, it may stick together and not flatten properly. In the softness test of simulated samples (as shown in Fig. 5), the softness values were measured as follows: CT—740.33 mN, C-TOOE—12.67 mN, C-TOEA—423.33 mN, and C-TEERA—331 mN. The C-TEERA sample exhibited the most suitable softness after treatment, preventing bonding during spreading and ensuring a better fit to the glass during repositioning, reducing the risk of air bubbles and preserving the glass slides long-term.

Fig. 5
figure 5

Softness values of CT, C-TOOE, C-TOEA, C-TEERA

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Figure 5 demonstrates that the WER in TEERA optimizes the softness of the simulated samples. Microscopic changes in the samples were observed using CLSM to confirm the flattening effect of the treatment.

Three-dimensional flattening effect

Figure 6 displays three-dimensional schematic diagrams of roughness parameters Sa, Sz, Str, Spc, and Sdr, while Fig. 7 illustrates the roughness values of CT, C-TOOE, C-TOEA, C-TEERA. Sa represents the arithmetic mean height, reflecting the surface traits’ height parameter. The CT simulated sample exhibits the highest arithmetic mean height, while C-TEERA has the lowest, suggesting that TEERA can reduce wrinkles and flatten the samples. Sz indicates the maximum height difference on the surface. The histogram of Sz shows that the CT sample has significant bumps and depressions, while C-TEERA appears relatively flat, demonstrating improvement in surface irregularities after TEERA treatment. Str represents the aspect ratio of surface traits, with values near 0 indicating a stripe-like pattern and values near 1 suggesting orientation independence. Spc denotes the arithmetic mean curvature of peaks, with smaller values indicating rounded points in contact with objects, and larger values indicating sharper points. Compared to other samples, C-TEERA effectively reduces Spc, rounding sharp parts of the film. Sdr, the interfacial extended area ratio, is 0 when the surface is flat. Figure 7 shows that Sdr for C-TEERA is close to 0, indicating a very flat surface on the simulated sample.

Fig. 6
figure 6

3D schematic representation of roughness parameters Sa, Sz, Str, Spc, Sdr

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Fig. 7
figure 7

The surface roughness values of CT, C-TOOE, C-TOEA, C-TEERA

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Color difference test

Table 1 displays the color difference values of CT, C-TOOE, C-TOEA, C-TEERA after hygrothermal aging. The color difference values of CT simulated samples increased gradually after hygrothermal aging at different time intervals. In high-humidity conditions, CT simulated samples underwent a hydrolysis reaction, resulting in the production of ethylene glycol [38, 39]. A schematic diagram of the hydrolysis reaction of gelatin to produce ethylene glycol is depicted in Fig. 8. Ethylene glycol, containing more hydroxyl groups, acts as a color enhancer by deepening the color of the simulated samples through interaction with color-generating groups such as carbonyl groups [40]. Among the softening protection solutions, C-TOEA exhibited the smallest color difference compared to the original simulated samples without moist heat treatment. This can be attributed to the presence of ethanol in TOEA, which aids in the volatilization of water, thereby delaying the hydrolysis reaction. While the color difference value of C-TEERA was slightly higher than that of C-TOEA, it showed the least variation in color difference values during long-term hygrothermal aging. This stability may be attributed to the chemical interaction of WER in TEERA with the simulated samples, leading to the formation of a more stable structure. This stability results in a minimal color change trend in the simulated samples in high humidity environments, indicating excellent aging resistance post-TEERA treatment.

Table 1 Color difference values of CT, C-TOOE, C-TOEA, C-TEERA after hygrothermal aging
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Fig. 8
figure 8

Schematic diagram of the hydrolysis reaction of gelatin to produce ethylene glycol

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Morphological analysis

Figure 9 displays scanning electron micrographs of CT, C-TOOE, C-TOEA, and C-TEERA. The surface of CT appears rough with large particle agglomerations, likely due to gelatin shrinkage and embrittlement during aging, leading to the formation of large spherulitic crystal structures [41]. C-TOOE has a relatively smooth surface with some large particles and undulations, attributed to the wetting properties of TO-8 reducing surface tension and gelatin film crystal size [42]. C-TOEA shows a smoother surface without large particle agglomerates, possibly due to the incorporation of absolute ethanol interacting with gelatin film molecules through hydrogen bonding. In contrast, the surface of C-TEERA is smooth, flat, and free of particle agglomeration, likely because WER within TEERA disperses and eliminates gelatin crystal structures. Further analysis of functional groups by FTIR helps determine the bonding mechanism between WER and the emulsion layer’s functional groups.

Fig. 9
figure 9

SEM of a CT, b C-TOOE, c C-TOEA, d C-TEERA

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Infrared spectral analysis

Figure 10 illustrates the infrared spectra of C-TOOE, C-TOEA and C-TEERA. The characteristic peaks of the samples remain mostly unchanged after exposure to the softening protection solutions, except for a noticeable change in intensity at 1700–1600 cm−1, corresponding to the amide I band of the protein. The stretching vibration of the C-TEERA characteristic peak at this location appears smoother compared to C-TOOE and C-TOEA.

Fig. 10
figure 10

Infrared spectra of C-TOOE, C-TOEA and C-TEERA

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Using XPSPEAK analysis software, we analyzed the infrared spectra of the samples at 1700–1600 cm−1, identifying changes in the secondary structure of the proteins. Figure 11 displays the fitted curves of the secondary structure characteristic peaks after treatment with different protection softening solutions. Interaction with waterborne epoxy resin led to a decrease in the proportion of α-helices and an increase in β-turns and random coils in the samples [43]. This resin disrupts hydrogen bonds within the samples, weakening intermolecular forces and increasing free volume between molecules. This enhances flexibility in the simulated samples [44].

Fig. 11
figure 11

Fitted curves of secondary structure characteristic peaks of C-TOOE, C-TOEA and C-TEERA

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Mechanism of action of WER with emulsion layer on glass slides

Figure 12 illustrates the infrared spectra of pure gelatin film with a 15% mass concentration (Fig. 12a), as well as the infrared spectra of WER (Fig. 12b). The absorption peak of -OH is observed at 3500 cm−1, the C=O stretching vibration of the amide I band is seen at 1700–1600 cm−1, and the N–H deformation vibration of the amide II band is observed at 1600–1490 cm−1 [45]. In the infrared spectrum of WER, the absorption peak of –OH is also at 3500 cm−1, the characteristic absorption peak of alkyl C–H is around 2900 cm−1, the symmetric telescopic vibrational absorption peak of –CH2 is at 1465 cm−1, and the absorption peaks of the –C–O bond are found in the range of 1200 to 1100 cm−1 [46].

Fig. 12
figure 12

a Infrared spectra of pure gelatin film with 15% mass concentration, b WER

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Figure 13 depicts the mechanism of TEERA interacting with the emulsion layer film on glass slides. In this study, TEERA containing WER was applied to the surface of the emulsion layer film on the glass slides. The main component of the emulsion layer film is gelatin, and the amino groups within the gelatin and the methylene groups of the WER form a more stable network structure by linking with each other. This linking process affects the carbonyl stretching vibration of the gelatin amide I band, resulting in noticeable changes in the carbonyl stretching vibration peaks of the simulated samples after TEERA treatment.

Fig. 13
figure 13

Schematic diagram of the mechanism of TEERA interaction with the emulsion layer film on glass slides

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Practical restoration applications of softening protection solutions in cultural relics

Figure 14 displays the transformation of glass slides before and after restoration. In Fig. 14a1, b1, the emulsion layer film on the glass slides from the Archives’ collection is severely cracked and warped, resulting in poor light penetration and visible black lines in scanned photographs. Following restoration, depicted in Fig. 14a2, b2, the wrinkled emulsion layer is flattened and reattached to the glass base, nearly eliminating the black lines and restoring clarity to the images. Figure 14c1, d1 shows digital photographs of the damaged emulsion layer before repair, with wrinkles and peeling making it difficult to match the original image. After restoration, the emulsion layer is fully restored, resulting in clearer scans of the glass slides. The TEERA-infiltrated emulsion layer film on the glass slides appears smoother and less prone to diffuse reflections, contributing to the improved image quality.

Fig. 14
figure 14

a1–d1 Digital photographs of glass slides before restoration; a2–d2 digital photographs after restoration

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Conclusions

  1. (1)

    Treatment with TO-8 alone can lead to excessive water absorption and swelling of the negative emulsion layer membrane, resulting in strong hydrophilic properties. However, when used in combination with waterborne epoxy resin, the repair reagent can have a wetting effect on the emulsion layer membrane of glass slides without altering its original size. Additionally, the hydrophobicity of the emulsion layer film can also be improved with the use of waterborne epoxy resin, making it easier to reposition the emulsion layer membrane for film restoration.

  2. (2)

    The emulsion layer film exhibits adhesive properties, becoming soft when exposed to liquid, making it easily bonding. By treating the emulsion layer membrane with waterborne epoxy resin, the softness can be adjusted to a moderate level. This ensures that the emulsion layer membrane remains flat and prevents unintended bonding during the spreading process.

  3. (3)

    In harsh temperature and humidity conditions, the waterborne epoxy resin-treated samples exhibit stability and resist significant changes. This indicates that the aging resistance of the emulsion layer film has been effectively enhanced.

  4. (4)

    Using TO-8 alone can help eliminate the spherical crystal structure within the emulsion layer film, but some agglomerated particles may still remain on the surface. By incorporating waterborne epoxy resin, the two components can work together to disperse and eliminate any remaining spherical crystal structures, resulting in a smooth and flat surface for the emulsion layer film.

  5. (5)

    The waterborne epoxy resin plays a crucial role in forming a stable network structure with the amino group of gelatin in the emulsion layer of the substrate. This chemical bond linking affects the carbonyl stretching vibration within the gelatin membrane, leading to strong intermolecular interactions that alter the protein secondary structure. This alteration makes the molecules within the sample more mobile, allowing the emulsion layer membrane to be softened and flattened, ultimately facilitating complete recovery of the image information it carries.

Availability of data and materials

No datasets were generated or analysed during the current study.

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Acknowledgements

The authors are grateful to the Sun Yat-sen University Archives for the support with the samples for this research.

Funding

This work is supported by the National Natural Science Foundation of China (Grant No. 51043002) and the Science and Technology Major Project of the National Archives Administration of China (Nos. 2014-B-07 and 2017-B-01).

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Authors and Affiliations

  1. School of Materials Science and Engineering, Engineering Research Center of Historical Cultural Heritage Conservation, Ministry of Education, Shaanxi Normal University, Xi’an, 710119, China

    Yuyao Ruan, Yanli Li, Fei Yang, Yanru Chen, Panpan Liu, Mantang Ge, Kezhu Han & Yuhu Li

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  1. Yuyao Ruan
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Contributions

Conceptualization, Y.R., Y.L (Yanli Li).; data curation, Y.R., F.Y.; investigation, Y.R., P.L., M.G., and K.H.; writing-original draft, Y.R., Y.C.; writing-review and editing, Y.L (Yanli Li).; supervision, Y.L (Yuhu Li).; funding acquisition, Y.L (Yuhu Li). All authors have read and agreed to the published version of the manuscript.

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Correspondence to Yuhu Li.

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Ruan, Y., Li, Y., Yang, F. et al. Study on the restoration of glass slides dating back to the 1940s. Herit Sci 12, 335 (2024). https://doi.org/10.1186/s40494-024-01454-4

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