The aging and color development of writing in paper archives

doi:10.21203/rs.3.rs-3751068/v1

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The aging and color development of writing in paper archives

https://doi.org/10.21203/rs.3.rs-3751068/v1

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published 13 Jun, 2024

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The most important value of archives lies in the handwritten information they record. However, during the storage process, handwriting may change color or fade due to the influence of the storage environment, especially when writing in blue-black ink. Therefore, in response to the phenomenon that files with blue-black ink handwriting are prone to fading, α,α’bipyridine with the ethanol solution was selected as a developer to perform color-developing treatment on naturally aged ink samples to assess the color-developing effect of faded blue-black ink handwriting. The results showed that the artificially aged ink sample had significantly faded. After being treated with a chromogenic agent, the color of the faded ink sample deepened and the outline of the handwriting became clear. The yellow blue color b value changed to -3.15, the residual ink amount increased by 41.74%, and the difference in absorbance curve area decreased by 296.75. Moreover, the chromogenic agent did not cause significant damage to the paper. This result indicates that the chromogenic agent has a good effect on the recovery of naturally aged blue-black ink blots, and this study has certain practical application value for the protection of ancient literature.

Protection of ancient books

Blue-black ink

Chromogenic agent

Paper literature archives are precious historical relics that can reflect the development of politics, an economy, and culture in different eras, and have extremely high preservation and research values ^[1–2]. However, in the process of preserving paper documents, the environment and its own physical and chemical changes affect their condition, which can cause certain damage to the paper and graphic information, affecting the ability to conduct future research using such paper archives. Therefore, it is crucial to study the protection of paper archives ^[3–4].

The most important value of paper archives lies in their recording of textual information, and protecting this textual information is a key issue in the protection of paper archives. Archives containing various handwriting materials are preserved in various archival institutions in China, among which archives written in blue-black ink are the most representative. Blue-black ink handwriting is widely present in paper archive literature, with its main components being tannic acid, gallic acid, and ferrous sulfate ^[5–6]. Due to a series of reactions that occur during storage when encountering acid, light, heat, and humidity, the handwriting on the paper may discolor or fade, thus affecting the historical value of the archives ^[7–8]. Performing color development on faded ink for restoration to a recognizable state ^[9] is an important part of ancient book protection and also of great significance for advancing research into historical information.

Currently, three main types of methods are available for blue-black ink handwriting, namely chemical, physical, and computer methods. Among which, the chemical methods are tannin (ellagic acid) handwriting recovery method, sulfide reaction method, yellow blood salt handwriting recovery method ^[10] and LC color-fixing agent ^[11]. Utilizing water and ethanol solution of phosphoric acid and methanol solution of barium hydroxide as reinforcement, experiments of Luo Yujia ^[12–13] and others have proved that the reinforced handwriting is less prone to scattering and discoloration under light. However, the use of these chemical reagents may cause irreversible damage to the paper and handwriting, which is not conducive to its subsequent protection.

Physical and computer methods are mostly applicable to handwriting that is not badly faded, and cannot be employed to identify badly faded handwriting ^[14–15]. Zeng Fei et al. ^[16] first classified damaged strokes, and then processed the two types of images using grey-scale histogram impact point detection and the classification method based on the connective component, respectively, to extract the damaged characters and improve the efficiency of handwriting recognition. Wu T. et al. ^[17] investigated the changes in the spectral curves of the wood and the ink through use of the sequential maximum angle convex cone (SMACC)method, used the density splitting method for the extraction, and finally succeeded in the exhumation of the tomb referred to therein. More than 60 Chinese characters were found on the wood pieces. In 2017, Hu Rui et al. ^[18] utilized multi-resolution super-pixels combined with other computer technologies to design a system that can automatically extract the strokes of hieroglyphic characters, which can better divide the width of the strokes, and can preserve the details of the strokes of hieroglyphic characters in a more perfect manner. These physical methods, as well as science and technology applied to textual information extraction, can obtain good results, but their techniques are complicated, and there are some limitations to their use.

Therefore, in view of the fading of blue-black ink, suitable reagents were selected according to the fading principle of blue-black ink. The practical application effect of the color developer was verified by testing the changes in the appearance and morphology of the ink before and after the treatment using the color developer, the color ^[19], the residual ink amount, the absorbance, and the metal ions in the ink. In this way, the practical application effect of the color developer was verified.

Preparation of ink samples

To prepare the ink samples, a 45 mm × 35 mm rectangle was divided on A4 paper by using a mold. The blue-black ink was evenly applied to the rectangular area and written underneath, marked with the date, and hung in a ventilated place to dry. Then, the samples were pressed with a heavy object until they were flat.

Aging of ink samples

The prepared ink samples will be placed in the sun can be irradiated indoors for a period of six months of natural aging, indoor temperature, humidity changes with the weather, not to the sample as well as any artificial interference with the environment, the aging ink samples placed in a chamber with constant temperature and humidity to balance the moisture and facilitate subsequent testing.

Preparation of color-developing reagents

Some 1.5 g of α, α' bipyridine was weighed on an analytical balance and dissolved in 70 g of ethanol solution to make a homogeneous mixture and stored in a cool, ventilated place for later use.

Color development of ink samples

An appropriately sized piece of skimmed cotton wool was cut and dipped in the prepared color developer and applied to the faded ink blot. Afterwards, the coated paper was placed in a fume cupboard to dry naturally.

Testing and characterization

Ink sample color testing

The color measurement function of the whiteness tester was employed to measure the values of L*, a*, and b* of the ink blot at the same position before and after treatment using the developer. Then, the color difference △E ^[20] of the ink blot before and after color development was calculated. The greater the △E, the more pronounced the change in color is.

Residual ink-level test

The residual ink amount on paper before and after color development was tested adopting a residual ink tester.

Ultraviolet-visible spectroscopy (UV-vis) testing

The ink layers on the paper surface before and after color development were tested utilizing a UV-vis spectrophotometer, which tested and recorded the UV-vis absorption spectra of the samples over a scanning range of 200 nm to 800 nm.

Fourier transform infrared spectroscopy (FTIR)

The chemical structures of the ink before and after color development were tested using a Fourier transform infrared spectrometer with air as the background, 32 background scans and 32 sample scans, which were scanned to acquire the absorption spectra of specimens from 400 cm^− 1 to 4000 cm^− 1.

X-ray photoelectron spectroscopy (XPS)

The ink samples before and after color development were tested leveraging a photoelectron spectrometer, in which the samples were equilibrated in a constant temperature and humidity room for 72 h after natural aging and color development, and the samples were cut into 5 mm × 5 mm pieces for sample preparation and testing.

Measuring the tensile strength of paper

The transverse tensile strengths of paper before and after color development were tested adopting the L&W Tensile Strength Meter. The test standard was in accordance with GB/T 12914 − 2008.

Tear test

The transverse tensile strengths of paper before and after color development were tested adopting an L&W Tensile Strength Meter. The test standard was in accordance with GB/T 455–2002.

X-ray diffraction (XRD)

The ink blots before and after color development were tested using a multi-bit auto-sampling X-ray diffractometer, where the test samples were cut into circles with a diameter of about 10 mm and pasted onto the XRD sample stage.

Macroscopic changes in the ink surface before and after color development

The blue-black ink blots without any treatment were placed indoors under natural conditions for 180 days, and the appearance and morphology of the aged ink blots are shown in Figs 1(b, d). Comparing with Figs 1(a, b, d), the original blue-black color on the surface of the blue-black ink blots faded away completely after 180 days of aging in the natural environment, and the surface of the aged ink blot became yellowish.

The naturally aged ink blot was then subjected to color-developing treatment, and the appearance of the ink blot after treatment is shown in Figs 1(c, e). Comparing Figs. 1(b) and 1(c), it can be seen that the color of the ink blot after the color-developing treatment has recovered more obviously, the color becomes darker, and the uniformity of the ink blot after color-developing treatment is good. Comparing Figs. 1(d) and (e), the color of the text after development treatment has been deepened, and the specific text can be identified more clearly. Through observation of the macroscopic changes on the surface of the ink blot, it can be seen that the color of the ink blot after development is darker than that of the ink blot after natural aging, with a certain restorative effect.

Changes in ink color before and after color development

The CIE L*a*b* value and the color difference value ∆E of the ink blot at the same position before and after aging and before and after color development were measured, and the results are shown in Fig. 2.

As illustrated in Fig. 2, after 180 days of natural aging of the paper coated with blue-black ink, the color of the ink blot produced a great change, with a color difference value ∆E of 47.3, and this degree of color change can be identified by the human eye. After the naturally aged ink was treated with the color-developing reagent, the color difference value ∆E was 13.29 compared with the ink before color-developing treatment, in which the yellow and blue color change was -3.15, indicating that the color changes from yellow to blue. That is to say, the blue-black handwriting was restored to a certain extent, and after the action of the color-developing reagent, the value of ∆a* was +11.87. This indicates that the color was on the reddish side. This is because this color-developing reagent achieves ink reduction due to combination of the nitrogen atom with Fe²⁺ and the resulting Fe²⁺ precipitate being reddish in color, so the ∆a* value is positive. Meanwhile, the ∆L* value is -5.08, which indicates that the brightness of the ink blot decreases and the color is deepened. From the changes in the color values of the ink blots, it can be seen that after 180 days of natural aging, the original color of the blue-black ink blots faded almost completely, while after the action of the color-developing reagent, both the color and the brightness of the ink blots were restored. This indicates that the color-developing reagent has a more obvious effect on color restoration.

Changes in ink residue before and after color development

Changes in the residual ink amount can be a more objective reflection of variation of the ink amount, before and after aging and before and after color development at the same position of the residual ink amount of the test. They can be used to calculate the percentage of change, and the results are shown in Fig. 3.

As shown in Fig. 3, after 180 days of natural aging, most of the blue-black ink on the paper is lost, and the residual ink amount decreases significantly. This indicates that after 180 days of natural aging, the blue-black ink blots that will produce a greater fading of the ink components are decomposed and faded, and the ink amount carried on the paper surface decreased. The ink after the color-developing treatment and the residual ink amount have increased, indicating that the color developer plays a role in the recovery of blue-black ink, which can deepen the color of faded ink after natural aging.

Change in absorbance of ink before and after color development

The absorbance of the ink blot at the same position before and after aging and before and after color development was measured (Fig. 4).

As can be seen from Fig. 4, the absorbance curves before and after natural aging are quite different: the absorbance value is much smaller compared with that before aging, and the area gap between the two curves is large, indicating that after 180 days of natural aging, the absorbance of the ink blot surface decreases, macroscopically expressed as significant fading of the color of the ink blot. After 180 days of natural aging, the ink blot is processed, and the absorbance curve of the ink blot is significantly higher than that before color development, which indicates that after color development, the absorption of light on the surface of the ink blot is increased and the macroscopic performance of the ink blot is deepened in color. It shows that the color-developing agent has a certain restoration effect on the color of the ink on the faded paper. Comprehensive comparison of the color-developing effect of the color-developing agent on the artificially aged ink blot and the naturally aged ink blot shows that the reagent has a better effect on the recovery of the naturally aged ink blot.

Changes in the chemical structure of the ink before and after color development

Fig. 5 shows the infrared characteristic absorption spectra of the ink blots before and after color development. From Fig. 5, it can be seen that the ink blot after color development shows more obvious new absorption peaks near 1260 cm^-1 and 800 cm^-1. Combining with the absorption analysis of the infrared spectrum of blue-black ink, the peak near 1260 cm^-1 is a typical absorption peak of the water-soluble gum Arabic in the ink blot. The absence of gum Arabic in the infrared spectrum before color development is due to the fact that the gum Arabic in the ink blot is decomposed by leaving the paper in the room for a long period of time for natural aging. The second new absorption peak appears near 800 cm^-1, which may be a partial absorption peak of ethanol ^[21].

Changes in iron ion content of ink before and after color development

Fig. 6 shows the percentage change of different valence states of iron ions contained in the ink blot before and after color development. The energy peaks of Fe2p_3/2 and Fe2p_1/2 orbitals were fitted by peak splitting method ^[22], and the fitted XPS spectra are shown in Figs 6(a, b). By calculating the peak areas pertaining to Fe²⁺ and Fe³⁺, the valence transition of iron ions in the blue-black ink blot was studied before and after color development. As shown in Fig. 6(c), the percentage of Fe²⁺ in the ink blot was 68.41% before color development, and the high content of divalent iron on the ink blot surface was mainly due to the lignin in the paper that inhibited the oxidation of part of the divalent iron, in addition to the reducing property of gallic acid contained in the ink blot itself. The percentage of Fe²⁺ after treatment was 64.74%, and the relative percentage content of divalent iron ions decreased, which is due to the principle of restoration of blue-black ink blot by the color-developing reagent entailing certain components in the reagent bind with Fe²⁺ to produce color precipitation, resulting in a decrease in the content of Fe²⁺ on the surface of the ink blot. In addition, the relative increase in the content of trivalent iron ions is because the reaction of divalent iron ions caused by the total amount of iron ions on the surface of the paper is reduced, so the content of trivalent iron ions is increased (in relative terms).

Effect of color developers on the strength of the paper

The mechanical properties of the samples before and after color-developing treatment were measured (Fig. 7).

After 180 days of aging under natural conditions, paper degrades in all aspects of its own performance and becomes more sensitive to chemical reagents. Whether or not the color developer used will cause severe secondary damage to the paper is an important consideration in the study of ink recovery reagents.

As can be seen from the Fig. 7, after the chromogenic reagent treatment of the sample, both the tensile index and tear index show a slight downward trend, indicating that the role of the chromogenic reagent on the surface of the ink in deepening the color of the faded ink at the same time, but also the internal role of the paper on the fibers, thus affecting the strength of the paper. Comprehensive paper tensile index and tear index of the specific changes in the value of the paper can be seen, the mechanical strength of the paper decreases to a small extent, and the color-developing reagent does not cause any significant decrease in the strength of the paper.

The effect of color developers on the fiber structure of paper

To examine whether the treatment of ink blots by the color-developing reagent affects the paper loaded with ink blots, the XRD spectra of the paper before and after color-developing treatment were acquired (Fig. 8). The red curve in the figure is the XRD spectrum before color development, and the black curve is that after color development. The crystallinity data of the internal fibers of the paper before and after color development were also calculated based on the spectra.

The X-ray diffractometer of all the samples was used to sweep over the paper samples before and after color development at a diffraction angle 2θ in the range of 20°-70°, and the experimental data were processed using Origin software to calculate and analyze the crystallinity of the fibers. To determine whether the color-developing reagent causes major damage to the paper, not only the changes in tensile strength and tear of the paper before and after color development need to be observed, but also more importantly the impact of the chemical reagent on the paper fibers should be studied, by exploring the crystallinity of the fibers. As shown in Fig. 9, the number and position of XRD absorption peaks of the paper do not change significantly before and after the treatment of ink blot development, indicating that the treatment of ink blots by the color developer does not affect the intrinsic structure of the internal fibers of the paper. According to the XRD spectra used to calculate the crystallinity of the paper fiber before and after color development, it is known that the crystallinity of the fiber of the paper after aging for 180 days under natural conditions is about 33.95%; after treatment with the color-developing reagent, the crystallinity is 32.02%. The change in crystallinity is small, suggesting that the chemical reagent does not damage the paper and is local in its action.

(1) After 180 days of aging in a natural environment, the blue-black ink faded badly, the ink color changed from the original blue-black to yellowish, and the value of the color difference before and after aging reached 47.3. The color recovery of this ink was assessed, and the value of the color difference of the ink before and after color development was 13.29, and the value of ∆b* was − 3.15, implying that the color of the faded ink was recovered to blue after color development;

(2) After 180 days of natural aging, the residual ink amount decreases from the original 120.81 mm²/m² to 23.91 mm²/m², then increases to 33.89 mm²/m² after color development, with an overall increase of 41.74% before and after color development. Natural aging causes a difference of 413.65 in the area under the ink absorbance curve, indicating that the color-developing agent helps color recovery of faded ink;

(3) After natural aging of the ink blot taken for color-developing treatment, the tensile index of the paper decreases from 17.67 Nm/g to 16.84 Nm/g, the tear index reduces from 2.61 mN·m²/g to 2.34 mN·m²/g, and the crystallinity of the fibers decreases from 33.95–32.02%; the overall changes are small, indicating that the color developer has a little destructive effect on paper strength and fiber crystalline area, and does not cause significant damage to the structure of the paper;

(4) Comparing the changes in color, absorbance, and residual ink amount before and after color development of naturally aged ink, it can be seen that the color developer has a better effect on the recovery of naturally aged blue-black ink, and handwriting can be clearly identified after color development.

Funding

We received no financial support for the research and/or authorship of this paper.

Author Contribution

The first author (Wq) designed the experiments and provides comments on the revision of this article. The second author (Gx) is responsible for data analysis and article writing, was a major contributor in writing the manuscript. The third author (Xz) performed the experiments. All authors read and approved the final manuscript.

Availability of data and materials

All date are incorporate into the article and its online supplementary materials.

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No competing interests reported.

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Journal Publication

published 13 Jun, 2024

Read the published version in Chemical Papers →

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The aging and color development of writing in paper archives

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Abstract

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Introduction

Experimental work

Testing and characterization

Results and discussion

Conclusion

Declarations

Funding

Author Contribution

Availability of data and materials

References

Additional Declarations

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Journal Publication

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