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Paper des Monats
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Paper is one of the most important materials representing and witnessing human culture particularly as a carrier medium for text and image. As soiling hampers the reception of information, paper cleaning techniques are needed. Traditional mechanical and chemical cleaning methods are used by conservator-restorers. In some cases, a classical cleaning procedure of paper objects yields unsatisfactory results or a conventional treatment is even impossible. Especially, fragile paper objects cause problems due to mechanical instabilities. Laser cleaning as a non-contact method might be a way to overcome some of the limitations of classical cleaning techniques. Laser parameters have to be chosen to achieve removal of the soiling without influencing the artwork. Any immediate as well as long-term effects causing an irreversible change of the artwork have to be avoided. At present, most laser applications are found in stone and metal conservation, while laser treatment of complex organic materials like paper is still not fully developed for application in conservators' workshops. This contribution describes recent work of pulsed laser cleaning of soiled model samples. Pure cellulose, rag paper and wood-pulp paper were mechanically soiled with pulverized charcoal in a standardized procedure to make model samples representing essential characteristics of contaminated real-world artworks. Afterwards, model samples were cleaned using short and ultrashort laser pulses in the nanosecond and femtosecond time domain, respectively. An extensive analysis of the model samples after laser treatment using an optical microscope and a multi-spectral imaging system allows a comparison of the cleaning results obtained with both laser sources.
One of the most important materials presenting and witnessing human culture is paper. The cleaning of paper is often necessary because contamination must be removed so that the fragile organic substrate can be preserved. The conventional cleaning methods are mechanical or involve the application of chemicals. These methods can damage drawings or print layers to some extent or make the original paper substrate brittle. More specifically, the use of a scalpel blade can cause damage to fibers. Chemical cleaning is difficult to perform locally, can dissolve foreign matter that then migrates into the paper substrate, or involves volatile organic compounds that can be harmful to the conservator. There is, therefore, a need for new conservation technologies aimed at the safe cleaning of paper. Lasers have proved to be an appropriate tool for cleaning as the energy dose and penetration depth at the specific point of contamination can be controlled. Additionally, if used properly, laser cleaning is not destructive to the paper.
Cleaning of paper is a challenging task due to the fact that a contamination should be removed and a fragile organic original material has to be preserved. Pulsed laser cleaning of artificially soiled Whatman© filter paper samples serving as models for historical paper was performed. Different cleaning strategies employing 8-ns laser pulses at 532 nm wavelength were applied to clean paper avoiding undesired effects like discoloration (yellowing) and mechanical deterioration of the substrate. Multi shot experiments with low-energy pulses were compared with single pulse investigations utilizing high pulse energies achieving a constant energy load incident on the samples in both cases. The cleaning efficiency and possible yellowing effects were evaluated by means of a multi spectral imaging system. An extensive microscopic analysis of the cleaned parts of the samples provided insight into the remaining soiling on the surface and in the bulk of the paper material after laser treatment. As a reference, a hard and a soft eraser were used to clean the samples.
Cleaning of artificially soiled paper using nanosecond, picosecond and femtosecond laser pulses
(2010)
Cleaning of cultural assets, especially fragile
organic materials like paper, is a part of the conservation
process. Laser radiation as a non-contact tool offers
prospects for that purpose. For the studies presented here,
paper model samples were prepared using three different paper
types (pure cellulose, rag paper, and wood-pulp paper).
Pure cellulose serves as reference material. Rag and woodpulp
paper represent essential characteristics of the basic
materials of real-world artworks. The papers were mechanically
soiled employing pulverized charcoal. Pure and artificially soiled paper samples were treated with laser pulses of
28 fs (800 nm wavelength) and 8–12 ns (532 nm) duration in
a multi pulse approach. Additionally, the cellulose reference
material was processed with 30 ps (532 nm) laser pulses.
Damage and cleaning thresholds of pure and soiled paper
were determined for the different laser regimes. Laser working
ranges allowing for removal of contamination and avoiding
permanent modification to the substrate were found.
The specimens prior and after laser illumination were characterized
by light-optical microscopy (OM) and scanning
electron microscopy (SEM) as well as multi spectral imaging
analysis. The work extends previous nanosecond laser
cleaning investigations on paper into the ultra-short pulse
duration domain.
Cleaning of artifically soled paper using nanosecond, picosecond and femtosecond laser pulses
(2009)
In this contribution, chemical, structural, and mechanical alterations in various types of femtosecond laser-generated surface structures, i.e., laser-induced periodic surface structures (LIPSS, ripples), Grooves, and Spikes on titanium alloy, are characterized by various surface analytical techniques, including X-ray diffraction and glow-discharge optical emission spectroscopy. The formation of oxide layers of the different laser-based structures inherently influences the friction and wear performance as demonstrated in oil-lubricated reciprocating sliding tribological tests (RSTTs) along with subsequent elemental mapping by energy-dispersive X-ray analysis. It is revealed that the fs-laser scan processing (790 nm, 30 fs, 1 kHz) of near-wavelength-sized LIPSS leads to the formation of a graded oxide layer extending a few hundreds of nanometers into depth, consisting mainly of amorphous oxides. Other superficial fs-laser-generated structures such as periodic Grooves and irregular Spikes produced at higher fluences and effective number of pulses per unit area present even thicker graded oxide layers that are also suitable for friction reduction and wear resistance. Ultimately, these femtosecond laser-induced nanostructured surface layers efficiently prevent a direct metal-to-metal contact in the RSTT and may act as an anchor layer for specific wear-reducing additives contained in the used engine oil.
Titanium and its alloys are known to allow the straightforward laser-based manufacturing of ordered surface nanostructures, so-called high spatial frequency laser-induced periodic surface structures (HSFL). These structures exhibit sub-100 nm spatial periods – far below the optical diffraction limit. The resulting surface functionalities are usually enabled by both, topographic and chemical alterations of the nanostructured surfaces. For exploring these effects, multi-method characterizations were performed here for HSFL processed on Ti–6Al–4V alloy upon irradiation with near-infrared ps-laser pulses (1030 nm, ≈1 ps pulse duration, 1–400 kHz) under different laser scan processing conditions, i.e., by systematically varying the pulse repetition frequency and the number of laser irradiation passes. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), tactile stylus profilometry, as well as near-surface chemical analyses hard X-ray photoelectron spectroscopy (HAXPES) and depth-profiling time-of-flight secondary ion mass spectrometry (ToF-SIMS). This provides a quantification of the laser ablation depth, the geometrical HSFL characteristics and enables new insights into the depth extent and the nature of the non-ablative laser-induced near-surface oxidation accompanying these nanostructures. This allows to answer the questions how the processing of HSFL can be industrially scaled up, and whether the latter is limited by heat-accumulation effects.