Image credit: AI-generated illustrative image created by OpenAI based on the study by Petronic S., Milovanovic D., Sibalija T., et al., Study of ink laser removal processing effects on the surface and mechanical properties of aged paper, npj Heritage Science (2026), https://www.nature.com/articles/s40494-026-02654-w
In a recent research article published in the journal npj Heritage Science, researchers systematically investigated the effects of Nd:YAG laser cleaning on the surface morphology, chemical composition, and mechanical properties of aged, ink-contaminated paper to optimize preservation of its material integrity under controlled experimental conditions.
Materials and Heritage Context
Paper artifacts primarily consist of cellulose fibers, often modified through aging processes involving oxidation and hydrolysis, which affect mechanical strength and color. During aging, cellulose molecules form chromophores that absorb light, contributing to discoloration and weakening of the paper.
Inks historically used for stamping and writing can further complicate conservation due to their chemical complexity and deep absorption into paper fibers. Removing ink typically risks damaging the underlying cellulose matrix or leaving residues that accelerate degradation.
Previous research on laser cleaning of paper materials has explored various wavelengths and pulse durations, showing that certain laser regimes can remove surface contaminants with limited material damage.
Laser Cleaning: Experimental Design
The research involved artificially contaminating aged paper samples, taken from a privately owned remaining copy of a book printed in 1900, with commercial stamp ink to simulate realistic conditions. The protected original collection was not used, and the work was framed as methodological research rather than an active conservation treatment. The inked paper was then subjected to laser cleaning using a pulsed nanosecond Nd:YAG laser operating at 532 nm with a pulse duration of about 6.2 nanoseconds.
Experimental laser parameters included varying pulse energies (10, 30, and 50 mJ), pulse frequencies (2-10 Hz), scanning speeds, and the number of passes to control the cumulative energy delivered.
Surface morphology was examined using optical microscopy and scanning electron microscopy (SEM), with both secondary and backscattered electron imaging, to reveal changes in fiber exposure and ink removal.
Chemical composition was analyzed by energy-dispersive X-ray spectroscopy (EDS) to assess semi-quantitative elemental shifts in carbon, oxygen, calcium, silicon, and aluminum before and after laser treatment. Additionally, colorimetric measurements assessed changes in lightness and chromatic properties indicative of ink removal.
The mechanical integrity of the paper after laser treatment was evaluated by tensile testing to measure the maximum stress and compare it with that of untreated samples. The study also used a Taguchi experimental design, combined with principal component analysis (PCA) and grey relational analysis (GRA), to systematically optimize laser parameters affecting the cleaning process.
Laser-Paper Interaction Analysis
Microscopic inspection revealed that laser cleaning progressively removed the ink layer covering the paper surface, with more effective cleaning under optimized irradiation conditions. Optical and SEM images showed distinct re-exposure of cellulose fibers and inter-fiber pores previously obscured by the ink, without observable fiber melting or surface damage.
The roughness parameters measured after cleaning approached those of untreated aged paper, suggesting restoration of surface topography rather than mere pigment discoloration. This suggests the laser predominantly removed the ink layer physically, preserving the cellulose matrix below.
Colorimetric data indicated that lightness (ΔL*) changes correlated strongly with ink removal efficiency, gradually approaching the original paper’s optical properties as the number of laser pulses increased.
Changes in the blue-yellow chromatic coordinate (Δb*) signaled progressive removal of the blue ink component, while red-green shifts (Δa*) remained minimal, supporting the selective ink removal hypothesis. Notably, excessively high cumulative irradiation doses led to larger chromatic shifts but did not improve cleaning outcomes, underscoring the importance of dose optimization.
Chemical analyses supported the interpretation that after laser cleaning, the elemental composition of paper surfaces shifted closer to that of untreated paper. The dominant elements, carbon and oxygen, moved toward untreated-paper values, while some underlying inorganic signals became more detectable after ink removal.
The decrease in carbon and increase in oxygen following laser treatment were consistent with ink ablation without significant chemical alteration of cellulose structure. The presence of silicon and aluminum in deeper layers became more pronounced following ink removal, further supporting layer-selective cleaning.
Mechanical testing showed that laser-cleaned inked paper had higher maximum stress than untreated old paper, although the inked paper itself showed the highest maximum stress. The reported increase may arise from microstructural changes associated with ink deposition and laser interaction, such as fiber overlapping or compaction, but should not be interpreted to imply that laser cleaning outperforms mechanical cleaning across all sample types.
Parametric optimization through Taguchi design and associated statistical analyses identified an optimal regime at 30 mJ pulse energy, 0.45 mm/s scan speed, 10 passes, and 3 Hz frequency. This parameter set maximized cleaning efficiency and preserved paper integrity by balancing fluence and cumulative pulse count (pulses per spot).
Optimized Laser Cleaning Outcomes
Using a nanosecond Nd: YAG laser at 532 nm, the study confirms that parameters can be optimized to efficiently remove ink from delicate paper surfaces without compromising the fiber network or mechanical strength within the tested experimental window. The carefully controlled laser processing preserved the measured chemical and structural indicators under the tested conditions, although accelerated-aging studies are still needed to assess long-term artifact stability.
Future research directions include extended chemical characterization and aging studies to verify the long-term stability of laser-treated paper. Expanding dataset size and developing neural network models aim to enhance optimization and predictive capabilities for diverse heritage materials.
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