Assessment of the efficiency and safety of ND:YAG laser cleaning of foxing stains on rag paper in historical printed editions. Printing and Publishing. Ukrainian Academy of Printing

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Якимець М. В., Myklushka I. Z.	№ 1 (89)	129-137

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References

This article investigates the applicability of solid-state Nd:YAG laser technology for cleaning “foxing” stains from historical printed books made from rag paper. Foxing, characterized by yellow-brown discolorations, remains among the most common and persistent forms of degradation observed on archival and library paper materials. Despite decades of research, the exact origin of foxing is still debated; however, it is widely associated with oxidative reactions involving metal ions, high humidity, and microbiological contamination. Traditional restoration techniques—including mechanical, chemical, and enzymatic methods—often fail to fully eliminate these stains without risking damage to the cellulose-based paper matrix.

In this research, we utilized the MED-810 Nd:YAG laser system, typically used in dermatological procedures, and adapted it for heritage conservation purposes. The laser operates at wavelengths of 1064 nm (infrared) and 532 nm (visible green) with pulse energies ranging from 200 to 1200 mJ. Experimental samples consisted of pages from a rare 1834 publication printed on ragbased paper. All specimens underwent preliminary mechanical and chemical cleaning using conventional restoration protocols, yet residual foxing stains persisted, making them suitable candidates for laser-based treatment.

Colorimetric analysis was conducted to assess visual changes, using the ΔE parameter in the CIE Lab color space. In addition, microscopic evaluation via a SIGETA Expert digital microscope was employed to determine whether any structural damage occurred at the fiber level. The results showed that despite the use of varying energy regimes and both laser wavelengths, there was no significant reduction in foxing stain intensity. However, no microstructural damage to the paper fibers was observed under any tested laser regime, confirming the technique’s safety for historical rag paper.

The study concludes that while Nd:YAG laser irradiation under the tested conditions is safe for use on rag-based paper, it is ineffective for removing foxing stains. The findings suggest that the foxing phenomenon may originate from deep, chemically bonded inclusions within the fiber matrix or from metal-based oxidation that does not absorb the applied laser energy effectively. Future research should focus on exploring alternative laser wavelengths with higher specificity, as well as combined cleaning techniques involving photothermal or plasma-based methods.

Keywords: Nd:YAG laser, rag paper, foxing stains, early printed books, laser cleaning, cultural heritage preservation, colorimetry, microscopic analysis, paper restoration.

doi: 10.32403/0554-4866-2025-1-89-129-137

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