Laser Cleaning Studies Research Articles

Effects of laser cleaning on the condition of different silk model samples using varying wavelengths and pulse durations

The cleaning of aged silk fibers poses a common challenge in the conservation of textiles, since traditional cleaning techniques often yield unsatisfactory results or even harm objects. In this regard, cleaning objects with laser radiation is a promising addition to the range of available methods. Due to it being contactless, even brittle and touch-sensitive objects with disfiguring or harmful soiling could potentially be cleaned and therefore made accessible for research and presentation. Examples of treatment have sometimes shown spectacular results. Still there is some skepticism concerning the safety of this treatment for textile materials, which has been strengthened through previous 532 nm wavelength nanosecond laser cleaning studies on silk fibers. Taking these published results into account, the range of examined laser parameters has been extended in this study, from 532 nm nanosecond laser to 1064 nm nanosecond and even 800 nm femtosecond laser, reevaluating the effect of this treatment on the fibers. The physicochemical processes taking place on the silk fibers when cleaning with lasers are complex and still not fully understood. The aim of this project was therefore to bring more clarification about potential effects of those processes on the condition of silk samples treated with a set of different parameters for wavelength, pulse duration, energy density and number of pulses per spot. It also looks at the influence of the presence of soiling on the results. The analysis of potential effects was then carried out using statistical methods and advanced analytics. Scanning electron microscopy, Fourier-transform infrared spectroscopy and colorimetry technology provided the required insights to better assess the effects. Results show that laser cleaning of silk fibers, like most other conventional cleaning techniques, is not completely without risk, but knowing what the possible effects are helps making decisions on whether the benefits of the technique used justify these risks.

Laser Cleaning as Novel Approach to Preservation of Historical Books and Documents on a Paper Basis

The purpose of this work is the study of laser cleaning of historical paper. The effect of laser exposure of the paper reflectance, fracture resistance and acidity was investigated. The paper surface roughness before and after laser treatment was analyzed by optical and scanning electron microscopy. It was shown that use of multi-pulse micromachining in combination with high-speed scanning of laser beams provides high safety for paper cleaning. The optimal parameters of laser radiation for effective and safe cleaning are a power density of about 2 × 105 W/cm2 at a wavelength of 1.06 μm, pulse repetition rate is 20 kHz; and a beam scanning speed of 200 mm/s–500 mm/s. The selective laser cleaning method for books and documents was proposed. Selective cleaning is achieved by means of high-precision control of the trajectory of movement of laser beams.

A study of laser cleaning to remove by-products occurring after arc welding

Recently, demand for the welding process largely increases in the manufacturing fields. The welding process is an essential process but the formation of by-products including slag, oxide, and heat tint after welding adversely affect the corrosion resistance and painting performance of the products. The conventional cleaning method generates environmental pollution because of secondary waste. Besides, a worker is exposed to danger. The mechanical method is limited due to the difficulty to access curved geometry of the workpiece. However, laser cleaning can overcome these problems. Especially, this technique can remove the welding by-products with no critical damage on the surface and secondary waste is barely formed during processing. Thus, it can be considered as an eco-friendly processing method. To analyze the interaction characteristics between laser and material, different laser parameters such as pulse duration, laser power, and scanning speed are set. The microstructure and chemical composition are observed using a scanning electron microscope (SEM) and energy dispersive x-ray (EDX), respectively. Consequently, when the laser power increases, the oxygen component on the surface decrease because the heat tint and oxide layer are removed. Also, when the scanning speed decreases, the over-cleaning is generated on the surface and the surface of the substrate is damaged.

High efficiencies for laser cleaning of glassware irradiated from the back: application to glassware historical objects

We present a systematic study of laser cleaning of black paint deposited on both standard and frosted glasses. We performed laser cleaning of black paint layers of different thicknesses in both front- and backside laser irradiation geometries. Using laser ablation induced photoacoustics (LAIP), we determined the ablation threshold of the paint that turns out to be independent of the paint thickness and substrate’s properties. To characterize the efficiency of the cleaning process as a function of the number of laser shots, we measured the transmission of the glass in the ablated region and simultaneously the amplitude of the acoustic signal generated by the ablation process. We show that laser cleaning is much more effective when the glass sample is irradiated from the back. To explain this effect, we propose a phenomenological model. This model also predicts the existence of a critical thickness, above which backside cleaning is no longer efficient. The method of back incidence laser cleaning was successfully applied to two real objects, namely a piece of advertising glass covered with black paint and an antique glass bottle with black dirt inside, both archeological objects founded in excavations made in the city of Buenos Aires.

Femtosecond laser cleaning of historical paper with sizing

Lasers have served as cleaning tools for historical objects and artworks for about 40 years. In many cases, superior results of laser cleaning were achieved with respect to traditional methods. In this technique, contaminations on the surface of the object are ablated by laser irradiation. In order to apply laser cleaning method to fragile materials such as paper made of cellulose or parchment, heat deposition to the bulk should be minimal, to prevent damage. In this work, it is demonstrated that laser pulses with femtosecond (fs) duration can exhibit non-thermal ablation of contaminants on paper samples. In particular, laser cleaning studies are concentrated on paper samples with sizing. Fs laser cleaning is performed on artificially soiled and aged samples, as well as on historical ones. The laser used in the experiments has pulse duration of 550 fs and 1030 nm center wavelength. The fluence of the laser is varied and the post-cleaning statuses of samples are investigated. The analyses are color changes, fiber integrity, chemical composition changes and mechanical strengths. These results show that fs lasers can be very efficient in cleaning paper samples, yielding minimal discoloration and no damage to fibers distinguishable on microscopic examination. The presence of sizing also provides further protection against possible side effects.

Laser Cleaning and Spectroscopy: A Synergistic Approach in the Conservation of a Modern Painting

We present results from preliminary laser cleaning studies performed on a 20th century modern painting, in which laser-induced breakdown spectroscopy (LIBS) was employed for monitoring the progress of material removal. This synergistic approach, that combines laser ablation cleaning with spectroscopic control, is of obvious importance as it offers a reliable means of ensuring proper conservation and could be the basis of a standard protocol for laser-based restoration procedures.

A Comprehensive Study for the Laser Cleaning of Corrosion Layers due to Environmental Pollution for Metal Objects of Cultural Value: Preliminary Studies on Artificially Corroded Coupons

This paper is focused on the systematic investigation of the layer-by-layer removal of corrosion products on artificially corroded metal coupons aiming to introduce a methodology for the optimum laser cleaning approach of historical metal objects. Thus, it is very important to determine the chemical composition of the studied surfaces before and after irradiation. A series of laser cleaning studies has been performed on test coupons (reference and artificially corroded). Wavelength and pulse duration effects are investigated. Initial studies were focused on the use of infrared (1064 nm) and ultraviolet (355 nm and 248 nm) radiations of nanosecond (ns) pulse duration. Damage and removal threshold values were determined for the substrates and the corrosion layers, respectively. The irradiated surfaces are evaluated microscopically under the optical and the scanning electron microscope, while the mineralogical and chemical composition of the various layers is determined with X-ray diffraction and SEM-EDAX analyses, respectively. The results obtained are providing a comprehensive approach for understanding the main mechanisms that are significant in the different laser cleaning regimes, while the optimum cleaning methodologies for the studied materials are being established.

Preliminary Laser Cleaning Studies of a Consolidated Prehistoric Basketry Coming from the Pile Building of Fiavè-Carera in the North-East of Italy

In the archaeological field, some specific advantages are recognized to laser cleaning, like, for example, the absence of mechanical contacts with the sample. The cleaning procedures generally involve a multilayer structure (dust, dirty, organic deposits, and, in some cases, consolidant substances). In this work, prehistoric wood samples (found and consolidated in 1970s) have been laser irradiated (KrF excimer laser) in order to restore their original surface aspect. A certain amount of burned matter was also present. Samples came from a fragment of a prehistoric basketry found in the lake dwelling site of Fiavè-Carera, Trento, Italy (1500–1400 BC). It was observed that the laser cleaning effects are strictly dependent on the irradiation parameters (power density and number of pulses). Efficient material removal was possible by using appropriate energy density. Moreover, for lower laser energy density, special structures appeared on the surface of the consolidating substance that we attributed to heating-induced stresses on the consolidant surface.

Studies towards a thorough understanding of the laser-induced discoloration mechanisms of medieval pigments

A significant issue in the current research on laser cleaning studies is the discoloration effect that the laser radiation may cause to many medieval pigments. Towards a thorough study of this effect, specially prepared limestone plates coated with lead, ferrous and copper pigments in linseed oil or the powder pigment itself, have been irradiated with Q-switched Nd:YAG laser radiation at 1064 nm and the discoloured material has been identified using spectroscopic and surface analysis techniques. The observation that the darkening effect in the irradiated lead based pigments decays, and the original colour is recovered after some time, stimulated further research on the possible conditions that may influence this recovery process. A model to describe the phenomenon is suggested where oxidation is not favourable but instead the pigments decompose to their constituent elements.

Reduced threshold ultraviolet laser ablation of glass substrates with surface particle coverage: A mechanism for systematic surface laser damage

A recent study of ultraviolet laser cleaning of silica glass surfaces contaminated with medium density alumina particles has shown a systematic type of laser-induced surface damage. This is characterized as a pit which increases in diameter and depth with increasing irradiating fluence. The damage pit occurs at lower fluence levels when smaller contaminant particles are used. The pits are due to laser ablation of the particle coated glass surface. The threshold for this laser ablation is well below that for a sample of the glass in its uncontaminated condition and the efficiency of ablation is estimated as being about 1000 times greater than that for silica. This laser damage is difficult to visualize by standard optical microscopy and could easily go undetected in laser cleaning studies. It may have general implications in the application of the new laser cleaning technologies to a variety of surfaces, including dielectrics and semiconductors, where medium to high densities of contaminant particles are present. It is also significant in suggesting processes by which glass may be rendered more readily machinable by laser ablative methods.

Laser induced removal of spherical particles from silicon wafers

Laser-induced removal of spherical silica particles from silicon wafers was investigated. The cleaning efficiency and laser cleaning thresholds for particles with diameters of 0.5, 1.0, 2.5, and 5.0 μm were carefully measured. It is found that the cleaning efficiency is more sensitive to laser fluence than laser pulse number and repetition frequency. The particle ejecting energies were found to increase with laser fluence. The threshold laser fluences for removing particles with sizes of 0.5 and 1.0 μm are 225 and 100 mJ/cm2, respectively, when KrF excimer laser is used. The threshold laser fluences are only a value below 5.0 mJ/cm2 for particles with a size of 2.5 and 5 μm. A model including both linear expansion and elastic deformation model was proposed to explain the experimental results. With this model, the particle movement and deformation in laser cleaning process were calculated. The expressions for threshold laser fluences were derived. The theoretical predictions are found to be greater than the experimental results. The difference can be explained by the enhancement of light intensity near the contacting area, due to the focusing and scattering by spherical particles. This model is useful to the study of laser cleaning as well as particle adhesion and deformation on solid surfaces.

Laser-induced breakdown spectroscopy as a diagnostic technique for the laser cleaning of marble

Laser cleaning studies were performed on samples of ancient Pentelic marble covered with black encrustations by employing a Q-switched Nd:YAG laser. Elemental analysis by laser induced breakdown spectroscopy (LIBS) has been applied as a real-time diagnostic technique for the cleaning process, with the aim of safeguarding against potential damage. In addition to the elemental composition, this technique can also provide information on the depth and morphology of the crust when combined with ablation rate studies. In this way the choice of the optimum laser parameters for effective cleaning is facilitated. The analytical and structural composition obtained by LIBS compares well with the results obtained by traditional analytical techniques, such as scanning electron microscopy, energy dispersive X-ray analysis, Fourier transform infrared spectroscopy and optical microscopy. This indicates that LIBS can be used as an autonomous on-line diagnostic technique, providing information necessary for accomplishing the efficient laser cleaning of marble artefacts.