Cleaning of pages of rare printed editions from organic contaminants using ND:YAG laser

This paper presents an innovative approach to cleaning rare printed editions from organic contaminants using an Nd:YAG laser. The key drawbacks of traditional cleaning methods (mechanical, chemical, and thermal) are analyzed, and the feasibility of using laser technologies is justified. The core principles of laser beam interaction with organic compounds and the potential effects on paper structure are outlined.

The proposed research methodology involves determining the optimal operating parameters of the Nd:YAG laser, including pulse frequency, pulse duration, and laser power. The study was conducted on pages of printed editions dated before 1918. The experiments included surface analysis using microscopy and colorimetry to assess the cleaning efficiency and its impact on the paper’s structure. The use of modern equipment, such as the MED-810 professional Nd:YAG laser, enabled the application of precise and controlled cleaning processes. This device, originally designed for medical and cosmetic applications, was adapted for use in the restoration of rare printed materials.

The traditional cleaning methods used in restoration — mechanical, chemical, and thermal — have significant limitations. Mechanical cleaning can cause scratches, tears, and deformation of paper fibers. Chemical methods often involve solvents that may alter the paper’s color or damage its structure. Thermal methods, on the other hand, can lead to yellowing and deformation of the sheets due to heat exposure. Given these limitations, the introduction of laser technology is seen as a safer and more effective alternative.

Laser cleaning relies on selective interaction between the laser beam and organic contaminants. The laser beam targets and destroys contaminants without causing mechanical damage to the paper surface. The key mechanism involves the ablation and destruction of unwanted particles. The experiments demonstrated that the Nd:YAG laser, with wavelengths of 1064 nm and 532 nm, is effective for the removal of a wide range of contaminants, from dark spots to light-colored stains. For this purpose, laser parameters were carefully adjusted to avoid damage to the paper fibers.

The cleaning process consisted of several stages. First, test samples were prepared by selecting paper pages with visible organic stains. The second stage involved determining the optimal operating parameters of the laser, such as energy (400–1800 mJ), pulse frequency (1–6 Hz), and pulse duration (5–20 ms). Experimental cleaning was conducted under various laser settings, and the results were recorded using optical microscopy and colorimetric analysis. The comparison of paper conditions before and after cleaning showed a significant reduction in contamination while maintaining the integrity of the paper’s structure.

One of the primary evaluation methods for cleaning efficiency was colorimetric analysis, where the color difference (ΔE) before and after cleaning was measured. ΔE indicates the distance between two color points in the CIE Lab color space. For this purpose, pages were scanned and converted into the Lab color space using Adobe Photoshop, and average values of L, A, and B were obtained for each contaminated area. Measurements were taken both before and after laser treatment, and the ΔE values were calculated for each power level. The results confirmed that optimal cleaning efficiency was achieved at power levels of 800–1200 mJ for a wavelength of 1064 nm and 400–800 mJ for a wavelength of 532 nm.

The impact of laser cleaning on the structural integrity of the paper was studied using optical microscopy with the SIGETA Expert digital microscope. The analysis revealed no significant changes in the morphology of the paper fibers. No cracks, fiber separation, or other defects were observed after treatment with the laser, which confirmed the safety of the selected cleaning parameters. This is a crucial result, as preservation of the original structure of historical documents is essential for restoration.

The research findings highlight the potential of Nd:YAG laser technology in the conservation of cultural heritage. Laser cleaning offers several key advantages over traditional methods:

1. Non-contact cleaning — The process avoids mechanical friction and physical contact with the surface, thereby reducing the risk of surface damage.

2. Controlled selective action — The laser can be adjusted to target specific types of stains while preserving uncontaminated areas of the paper.

3. Chemical-free approach — Unlike chemical cleaning methods, laser treatment does not introduce foreign substances or solvents that could alter the chemical structure of the paper.

As a result, the use of laser cleaning enhances the ability to create high-quality digital copies of rare prints. This is especially important for the digitalization and preservation of library, archive, and museum collections. Through the use of the Nd:YAG laser, archivists and restorers can significantly improve the preservation of printed heritage while ensuring minimal impact on the authenticity and original structure of the documents.

Keywords: Nd:YAG laser, laser cleaning, rare books, paper cleaning, organic contamination, cultural heritage preservation, paper restoration, restoration technology, digitalization of rare prints.

doi: 10.32403/0554-4866-2024-2-88-123-132