Low-cost sensor system for online monitoring during laser cleaning

The self-limiting effect during laser cleaning only occurs in a limited amount of specific applications in restoration (e.g. removal of black crust from white marble). In all the other cases, a control of the removal process will be necessary either by the operator himself or by the employment of sensor equipment. Various methods, mainly spectroscopic (e.g. LIBS), have been investigated and proposed by others. Despite the fact that these have been shown to be promising, they all have in common rather high investment cost close to that of the cleaning equipment. Furthermore, this highly sophisticated control equipment is not easy to handle by conservators in practice. As an alternative low-cost method, we employed a simple photodiode to detect the scattered light from the irradiation area on the artwork surface. In many cases, this signal contains several pieces of information on the layer just being removed. The scattered radiation detected by the photodiode originates from the laser-induced plasma as well as reflected laser radiation. A separation, if necessary in order to separate the information, is possible by spectral filters. First applications during laser cleaning of corroded metal, encrusted glass and stone were promising. It has turned out that there is a distinct influence on the scattered light amplitude or even the pulse-bandwidth once the laser has removed the encrustation completely. The corresponding signal can be used in a closed loop control or for online monitoring.     

Biogenic surface layers on historical window glass and the effect of excimer laser cleaning

The ablation of biofilms by excimer laser radiation was systematically examined in a series of model studies during which the biofilms originating from different historical panels were simulated on model glasses. The composition of these model glasses was modelled on that of the original historical glasses. Our studies have shown that glass composition, climatic conditions and biofilm formation are factors which interact synergetically. It could be observed that various biofilms grow differently on the same type of model glass and that the same type of biofilm shows a different development on various model glasses. The decisive factors for the effectiveness of biofilm ablation by laser irradiation is the formation of the biofilm on the one hand and its corrosive potential on account of its ability to accumulate moisture and to produce glass-damaging metabolites on the other. Glasses of low chemical stability promote the growth of dense biofilms and can be cleaned only with a high energy density, whereas glasses of high chemical stability merely allow for a slow growth of a biofilm spreading two-dimensionally on the glass surface which can be gently removed using low energy density.     

Investigations regarding the behaviour of historic glass and its surface layers towards different wavelengths applied for laser cleaning

The application of lasers for the removal of superficial deposits from historic stained glass is a comparatively new field of scientific interest. Experimental studies concerning the behaviour of glass substrates and the corresponding superficial deposits towards different laser wavelengths were carried out. The experiments were performed using wavelengths of λ = 193 nm (ArF-Excimer), 308 nm (XeCl-Excimer), 355 nm (Nd:YAG third harmonic) and 1 064 nm (Nd:YAG fundamental) in comparison to λ = 248 nm (KrF-Excimer). This comparison is due to the fact that the present knowledge is based on the 248 nm wavelength. Specially prepared model glass samples representing the original fragments and samples of organic polymers (formerly used as a protective material for historic stained glasses) were used to study the effects of laser radiation and were subsequently characterised by optical microscopy.     

Assessment of the state of conservation of stone artworks after laser cleaning: comparison with conventional cleaning results on a two-decade follow up

The utilization of laser light to produce fine and selective cleaning of superficial deposits and encrustation from stones was originally proposed more than 20 years ago by John Asmus who performed pioneering trials of laser cleaning on stone façades in Venice, Cremona and Padua. The opportunity to collect stone samples from these sites allowed us to carry out an original study aimed at evaluating the conservation state of stone surfaces cleaned 10–20 years ago by means of three different techniques: Nd:YAG laser radiation, micron sandblasting and chemical pads. Samples of various stone types, as Istria limestone, Botticino limestone and Nanto sandstone were collected and examined by means of optical and SEM microscopy, FT/IR, and EDAX analyses. The results provided preliminary indications that the state of conservation of stone surfaces previously subjected to laser cleaning is generally better in comparison with those which concurrently underwent mechanical and chemical cleaning.     

UV-laser radiation: basic research of the potential for cleaning stained glass

A KrF-excimer laser operating at λ = 248 nm was used for basic studies in cleaning ancient glass surfaces. For irradiation a mask projection technique was applied. A modified optical set-up was designed using cylindrical lenses for large area removal by line scanning. Previous investigations revealed insufficient knowledge about interaction processes between UV-laser radiation and individual surface layers of encrusted historical glass. Thus, first detailed studies were carried out on special model glass samples simulating the behaviour of historical glass. The potential of removing crusts, bio layers as well as layers of different conservation materials (due to former conservation work) were examined. The avoidance of damaging material such as gel layers, paint layers or the bulk glass was of great importance. Removal rates and thresholds for the materials mentioned above were carried out. The collected data enables a comparison and evaluation of the feasibility for the removal of superficial layers from historical glass artefacts. The results indicate that in some cases a closed loop process control will be necessary to avoid over-cleaning.     

Laser cleaning applied in the restoration of a medieval wooden panel chamber at Pirna

A medieval wooden panel chamber was rediscovered during reconstruction of the Tetzelhouse in the Saxon town of Pirna. Ceiling and walls are made from 600-year-old fire wood panels fitted together. The complete removal of layers (limewash, plaster, glue and dirt) from the wall panels was the precondition for conservation of the wood panels and for the restoration of the wooden panel chamber. The cleaning was carried out with a pulsed Nd:YAG laser after basic investigation of ablation mechanisms of the layers on the old wood. All layers of the different materials were removed both from the normal surface and out of depressions and woodworm holes with a special range of laser parameters, without damaging the wood. Within this range of parameters the laser cleaning is a self-limiting process. The health hazard from emitted waste products were determined and checked with work place measurements. The laser cleaning of the wooden wall panels was completed in March 1999. A uniform visual impression of all panels was obtained with a warm wooden colour tone.     

Assessment of laser cleaning rate on limestones and sandstones

Within the framework of a European project, three cleaning laser devices are tested to evaluate the influence of different parameters on stone cleaning rate and efficiency. The project objective is to design a large surface-cleaning laser. One of the devices is a prototype with some technological improvements compared to the lasers used today for restoration. The tests are performed on limestone and sandstone covered with various dirt layers. The influence of fluence, water spraying, spot area, frequency, as well as particle emission rate is investigated. Results allow a comparison of the importance of each parameter and confirm the increase in cleaning speed of the new prototype. Nevertheless, it can be concluded that an automatic device will be suitable to attain a cleaning speed competitive with other traditional cleaning techniques.     

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.     

The effect of Nd:YAG laser radiation on medieval pigments

Very often traces of paint and pigment have been found on medieval sculpture. Presently, little is known about the effects of laser radiation on polychromy. It is important to protect and preserve these traces during any cleaning or restoration process. The absorption properties of polychrome materials are likely to be different from those of the underlying stone or substrate and as a result special care must be taken to avoid any damaging or discolouring phenomena associated with laser cleaning. Spectroscopic methods (XPS, AES, reflectance spectroscopy) together with optical microscopy, digital photography and X-ray diffraction analysis have been used in a series of experiments on simulated samples of common medieval pigments (vermilion, red and white lead and ochres in linseed oil) on limestone plates, in order to study the reaction of these materials to infrared laser radiation.     

Application of a new laser cleaning procedure to the mausoleum of Theodoric

Within the conservation programme of the mausoleum of Theodoric, located in Ravenna, Italy, we applied a new cleaning procedure based on the use of a recently developed Nd:YAG laser system, suitably equipped in this case with long optical fibre cables for the transmission of laser radiation to work sites on the façade. The intervention of the laser technique was required to complete the cleaning of a decoration surrounding the monolithic dome, after conventional cleaning methods had been applied with unsatisfactory results. Preliminary laboratory analyses on stone samples collected from the dome and subjected to laser treatment indicated that this technique was effective and could fully preserve the lime–Ca oxalate film existing below the thick layer of black crusts. Before starting the operative phase on the mausoleum, it was necessary to solve the practical problem of keeping free access to the monument during cleaning operations, which hindered the location of the laser in close proximity to the sites to be cleaned. This was achieved by preparing long fibre cables (up to 50 m) which permitted the successful completion of the cleaning while leaving the laser body far away from the dome.     

An investigation into the effect of wavelength in the laser cleaning of parchment

An investigation has been undertaken into the effect of wavelength in the laser cleaning of parchment. Tests have been carried out using the fundamental (1 064 nm), second harmonic (532 nm) and fourth harmonic (266 nm) outputs from a Q-switched Nd:YAG laser (pulse length 10 ns). Initial testing was carried out on new parchment to characterise the damage caused by overcleaning. The fluence and wavelength of the laser beam were then optimised so that any damage to the parchment or loss of ink during dirt removal from a late eighteenth century parchment document were minimised. The cleaned surfaces were examined by optical and scanning electron microscopy and analysed by measurement of the shrinkage temperature of cleaned collagen fibres. The laser-cleaned areas were compared with uncleaned areas and areas cleaned by traditional techniques.     

Laser cleaning of stained glass windows. Overview on an interdisciplinary project

The German Federal Foundation for the Environment (Deutsche Bundesstiftung Umwelt) is one of Europe's largest organizations of its kind. It supports innovative, exemplary projects for the protection of the environment and of national historic monuments and cultural landscapes. All projects are set up to provide a platform for researchers, craftsmen and restorers to find a common solution to difficult problems in conservation practice. More than 360 projects have been funded since 1990. Two recent projects deal with laser cleaning, one for stone and the other one for stained glass windows, the latter being presented here. This publication gives an overview on cleaning problems on stained glass windows and the project methodology. The interdisciplinary approach of the project provides the possibility of including experts in different fields to examine perspectives and limits of laser cleaning for stained glass windows. Two other publications in this issue (LACONA III) give selected results on cleaning experiments achieved within this project.     

Experimental study on the effect of wavelength in the laser cleaning of silver threads

Laser removal of surface contaminants on silver threads was carried out using Nd:YAG laser radiations from near infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm) produced by frequency harmonic generation. The thread in the museum textile was made of silver and silk in which the silver ribbon wraps the bunch of silk fibres. The goal of this work is to find a feasibility to clean the tarnished silver without any damage of the underlying silk since the conventional chemical treatment is problematic to apply in this specific specimen. From the results, it was found that the laser wavelength of 266 nm is most appropriate to clean the silver surface without causing any damage either to the silver or the silk surfaces while 1064 nm wavelength easily causes damages such as melting and burning to the silver as well as the silk inside.     

Chemical cleaning of soiled deposits and encrustations on archaeological glass: A diagnostic and practical study

The aim of this study is practically to establish a chemical strategy for cleaning soiled deposits and encrustations on archaeological glasses. Investigations were performed on a series of Roman glass samples (Fragments and complete objects) coming from different excavations in northern Jordan. The chemical composition of the glass samples was determined by X-ray fluorescence spectrometer (XRF) analysis technique, whereas X-ray powder diffractometer (XRD) and Energy dispersive X-ray (EDX) methods were used to determine the mineralogical and elemental composition of the soiled deposits and encrustations on the glass surfaces. Furthermore, Scanning Electron Microscopy (SEM) examination and optical assessment were performed before and after cleaning glass. The glass samples were subjected to different cleaning protocols such as Calgon (Sodium hexametaphosphate), ethylenediaminetetraacetic acid (EDTA) at different pH values, citric and tartaric acids and piranha solution (a solution of sulphuric acid and hydrogen peroxide). Sepiolite poultices soaked by chemical agents were the most suitable methods used for applying chemical solutions on the glass surface. It can be concluded that EDTA is generally accepted as the most effective chelating agents recommended for cleaning encrustations on durable glass. It was more effective and safe at neutral pH with low concentrations around 5 to 7%. The calcareous crusts can safely be removed by using a piranha solution. Citric and tartaric acids appeared a moderate efficiency on cleaning weathered and stable glass. Calgon has a tendency to damage corroded and iridescent surfaces, and should be avoided when cleaning weathered glass.     

Controlled laser cleaning of painted artworks using accurate beam manipulation and on-line LIBS-detection

An innovative laser restoration tool for non-contact cleaning of painted artworks is developed. Accurate beam manipulation techniques in combination with on-line detection make the system suitable for selective cleaning of delicate surfaces. The utilisation of lasers obviates the use of various chemicals, and provides a method to remove layers that are untreatable using conventional methods. The first professional laser cleaning station for paintings is equipped with a modern mechatronic engineering tool for accurate beam manipulation (‘optical arm’). An intelligent combination of software and hardware enables accurate control, necessary to deal with the variable properties of the artworks to be treated. An on-line monitoring system is incorporated, using laser-induced breakdown spectroscopy. The user interface plays an important role in simulating the ‘hands-on’ treatment. In January 1999, the 2-year European co-operative research project ‘Advanced workstations for controlled laser cleaning of artworks’ started. The research objective is to define the boundary conditions in which laser cleaning with the present technology can be safely applied.     

An initial investigation into the cleaning of new and naturally aged cotton textiles using laser radiation

A series of tests has been conducted to evaluate the potential for using a Q-switched Nd:YAG laser (wavelengths 1 064, 532 and 266 nm) to clean both new and naturally aged cotton textiles. Tests have been carried out on i) plain weave and velvet cotton and ii) new and naturally aged cotton samples to investigate the possible roles of weave structure and ageing in the cleaning process. Cleaning trials have been carried out on artificially soiled new cotton and compared with results obtained using traditional cleaning techniques: wet cleaning and organic solvent cleaning. Cleaned surfaces were examined using optical and scanning electron microscopy.     

Middle-ultraviolet laser cleaning of particulates from sized ground wood cellulose and pure cellulose paper

Ground wood cellulose paper exhibits a practicable cleaning laser fluence window during middle-UV radiation processing. In this case, a minimum dose volume density should be applied. However, cleaning of bleached cellulose paper is accompanied by strong yellowing and destruction. The presence of charcoal graphite particulates shows substantial influence on the yellowing with increasing coverage.     

Near-UV laser interaction with contaminants and pigments on parchment: laser cleaning diagnostics by SE-microscopy,VIS-, and IR-spectroscopy

Potentials and limitations of the near-UV pulsed laser cleaning of parchment (wavelength 308 nm, pulse duration 17 ns) are demonstrated by the application of scanning electron microscopy (SEM), colour metrics and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) at model contamination/pigment/parchment systems. Pigment-binder systems stable and unstable against near-UV laser treatment could be identified. A chemical degradation threshold fluence of a goat parchment model substrate was determined which practically coincided with its ablation threshold fluence. This indicates that the fluence range of destructionless laser cleaning at 308 nm is almost not impaired by chemical modfications below the ablation limit. Nevertheless, spectroscopic diagnostics are necessary to guarantee destructionless cleaning for practical cases where the chemical conversion threshold fluence deviates from the ablation threshold to lower values.     

A study of the effect of the wavelength in the Q-switched Nd:YAG laser cleaning of gilded wood

The removal of a brass-based paint (purpurin) used in painting gilded wood to cover losses of gold leaf, represents today a difficult task to conservators, who may have to resort to toxic chemical solvents in order to clean the painted surface. This action, due to its nature, is unsuitable for both the conservator and the artwork itself. In this study, a Q-switched Nd:YAG laser system outputting 1 064 nm (infrared), 532 nm (visible, green) and 266 nm (ultra-violet) radiation was used to study the interaction of these wavelengths in a gilded sample surface painted with purpurin. All tested wavelengths interacted differently with the tested surface and, also, the several layers that form the gilded surface (purpurin, varnish and gold) showed different interactions to each wavelength. The ultra-violet radiation (266 nm), in a multi-pulse mode at low fluences, was found to be the most efficient wavelength in cleaning the painted gilded surface.     

St. Stephen's Church in Vienna: criteria for Nd:YAG laser cleaning on an architectural scale

The hewn sandstone and ornamental figures of the Albertino Chancel, built around the mid-14th century, are covered with black encrustations of plaster and carbon deposits, often several millimetres thick. Their removal constitutes one of the main objectives in the conservation of the precious materials of the cathedral. The Nd:YAG laser method cleans the original surface of hardened encrustations without damaging or affecting the old coloured plasters and without affecting the centuries-old patina. Laser-Tech GmbH, founded by two expert restorers from Austria and Italy, is specialised in large-scale projects and, regarding the present project for the chancel, has managed to combine the sensitivity of the preserver with the laser technology applied to large surfaces. The present project concerning St. Stephen's Church comprises over 2000 m² of stone surface treated only with laser systems, by groups of qualified restorers trained at Altech to use such instrumentation on artistic historic surfaces.