Salvatore Siano

Cleaning processes of encrusted marbles by Nd:YAG lasers operating in free-running and Q-switching regimes.

The removal process of degraded superficial layers from marble samples by Nd:YAG lasers was studied while simulating operative conditions of stone artwork restoration. The effects of laser irradiation at 1064 nm with three different pulse durations of 6 ns, 20 micros, and 200 micros were investigated by time-resolved shadowgraphy and emission spectroscopy of the ejection plume to characterize the specific interaction regimes, with particular concern given to the occurrence of side effects, such as thermal and mechanical damages to the substrate, that could affect the laser cleaning procedure.

Tests of laser cleaning on archeological metal artefacts

Abstract Previously reported experiences on laser cleaning of metal surfaces of artworks such as statues and decorations altered by atmospheric pollution demonstrated the potential advantages of this technique. Here we present a study on the use of laser cleaning on excavated metal objects that typically show alterations and corrosion processes quite different with respect to exposed metal artefacts. Tests were performed on a selection of archaeological samples, such as coins, plates, clamps and ornaments collected from Italian sites. Metals considered in this study are bronze, copper, silver and lead. Different laser systems and different irradiation conditions were employed and compared. The results, evaluated by metallurgic diagnostics and objective observations, indicated that the laser cleaning process was selective and highly precise, allowing the preservation of fine surface details.

Determination of damage thresholds to prevent side effects in laser cleaning of pliocene sandstone of Siena

Abstract The present work reports a basic investigation aimed at optimizing laser cleaning interventions on pliocene sandstone of Siena. Irradiation trials were performed with a short free running Nd:YAG laser, on quarry samples and on samples from Palazzo Spannocchi presenting black crust. The physical parameterization associated with a detailed petrographic and mineralogic diagnostics of the induced effects, allowed the irradiation thresholds for damage phenomena to be derived and their nature understood. The experimental results reported here allow the definition of the best irradiation conditions and the laser parameter ranges for a safe and controlled black crust removal from pliocene sandstone.

Advances in laser cleaning of artwork and objects of historical interest: the optimized pulse duration approach.

Laser ablation has found numerous applications in biomedical and industrial settings but has not spread as quickly as a means of cleaning artwork. In this Account, we report recent advances in the study and application of laser cleaning to the conservation of cultural heritage. We focus on the solution of representative cleaning problems of encrusted stones, metals, and wall paintings that were achieved through the optimization of laser pulse duration. We begin by introducing the basic mechanisms involved in the laser ablation of stratified materials and the criteria for preventing undesired side effects to the substrate and then briefly present case studies for each of these materials. Laser interaction effects are reviewed in a schematic way, with a concise overview of the physical models needed to support intuitive interpretations of the phenomenology observed, both in laboratory tests and in practical applications on important artifacts. This approach aims to provide keys of generalization that will favor the rigorous application of laser cleaning, repeatability of the successful results reported in this work, and further dissemination and acceptance of the technique. The topics treated examine the ablation mechanisms along with the efficiency, gradualness, selectivity, and effectiveness of the technique as a function of the pulse duration of neodymium laser systems and the operating conditions. Physical modeling and experimental evidence support the selection of pulse durations of between several tens of nanoseconds and several tens of microseconds, making it possible to minimize the risk of photothermal and photomechanical effects and maximize the selectivity of the ablation process. The sections dedicated to stones and metals also deal with the important problem of discoloration, which has significantly slowed the spread of the laser cleaning technique. The well-known problem of a yellowish appearance after laser cleaning is shown to be closely related to the ablation process; it can therefore be prevented by a suitable selection of irradiation parameters. The metal surfaces investigated are amalgam gilding, gold leaf gilding, and, for the first time, silver artifacts. We also describe the criteria used for applying laser ablation techniques to restoring unique masterpieces, such as Lorenzo Ghibertis Porta del Paradiso and Donatellos David. Furthermore, a novel and unusual cleaning approach for archaeological silver is reported. Based on underwater laser irradiation, it provides a way to prevent oxidative effects and amplify the photomechanical coupling to the hard, thick concretions that usually accompany archaeological pieces. Finally, the experimental extension of the laser cleaning approach to wall painting and its practical use in important restoration works is presented. The practical examples reveal a significant advance in perspective for the application, which was unthinkable until recently. In sum, this Account describes novel technological and methodological contributions of laser cleaning that are having a significant impact in the field of cultural heritage conservation.

Laser cleaning methodologies for stone façades and monuments: laboratory analyses on lithotypes of Siena architecture

Abstract The present study was conceived for an evaluation of the effectiveness of laser cleaning techniques applied to samples collected from numerous monumental buildings in the city of Siena (Palazzo Pubblico, Cappella di Piazza, Baptistery, Logge del Papa and Palazzo Spannocchi). All the characteristic lithotypes of Sienese architecture are represented: Montagnola Senese marble, Cavernous limestone, Pliocene sandstone and Red Ammonitic limestone. The tests were devised so as to have the widest variability of situations in relation to the colour and composition of the lithotypes, the state of deterioration of the samples and the level at which to stop the cleaning. The tests were preceded and followed by detailed petrographic analyses. In the introduction to the work, we underline the importance of an accurate reconstruction of the stratigraphy of the external horizon of deterioration of the stone for an appropriate choice of the cleaning intervention; the potential advantages of laser techniques are consequently further emphasized.

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

Abstract 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 facades 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.

A variable pulse width Nd:YAG laser for conservation

Abstract Photoablation is a physical process employed for the cleaning of artefacts. While several authors have reported investigations concerning the dependence of the effects induced on the material for different laser wavelengths, no similar investigations have been reported so far about the dependence on the pulse duration, in spite of its importance being recognised. In order to approach this issue and optimise the interaction process on different materials, a new Q-switched Nd:YAG laser system with variable pulse width for cleaning applications was designed and built at the Quantum Electronics Institute in Florence. It provides an extended pulse duration in respect of the standard Q-switched regime, by means of interchangeable optical fibres in the resonator. The new laser allows to change the pulse duration in the range between tens and hundreds of nanoseconds up to several microseconds. According to our experience, the adjustment of the pulse duration in this range may avoid the possibility of strong mechanical forces and excessive heating in the substrate, as it has been observed with standard Nd:YAG laser systems. The laser may be proposed for an accurate cleaning in a variety of restoration problems and materials.

Laser cleaning methodology for the preservation of the Porta del Paradiso by Lorenzo Ghiberti

Abstract We investigated the optimisation of the laser cleaning parameters for the conservation of the Porta del Paradiso, gilded bronze masterpiece of the Florentine Renaissance. Photothermal and photoacoustic regimes, involved in the ablation dynamics by Nd:YAG lasers with different pulse durations, were modelled on the basis of preliminary stratigraphic characterisation and irradiation trials. The physical analysis was aimed at understanding the nature of the ablative process and at estimating the thermal wave coupled into the gold film. This allowed the selection of a suitable pulse duration range to achieve a high discrimination level, thus optimising the cleaning result. The analytical description reported here also contains general results, which could be exploited in other conservation cases of gilded bronzes.

SMART CLEAN: a new laser system with improved emission characteristics and transmission through long optical fibres

Abstract SMART CLEAN is an innovative Nd:YAG laser system that has been designed to optimize laser cleaning procedures, especially for the treatment of altered stone surfaces. The project, originated by the co-operation of researchers and enterprises involved in optoelectronics system development, was aimed at improving the intrinsic features of the laser source, as well as some practical aspects, in order to facilitate laser application in the restoration yard. Emission characteristics were suitably tailored to obtain effective removal of alteration layers, and to minimize possible side effects. In particular, the pulse duration of the SMART CLEAN laser was set at 20 μs, by means of a proprietary design of the power supply. This was in order to reduce the risk of both mechanical and thermal damage to the artwork substrate, which is more likely to occur with short and long laser pulses, respectively. Moreover, this pulse duration permitted a reliable transmission of high laser energy through long optical fibres (50 m), which allowed easy cleaning operations on facades. The laser system was tested on a large variety of lithotypes and in operative cleaning interventions on Italian monuments.

Analysis of blast waves induced by Q-switched Nd:YAG laser photodisruption of absorbing targets

Abstract An experimental investigation on photoacoustic effects originated in plasma-mediated laser photodisruption by a Q -switched Nd:YAG was done by means of a pump-and-probe set up providing time-resolved shadowgraphy of the shock wave produced onto the target surface. Estimations of coupled specific momentum and of the explosion energy were obtained by modelling the process on the basis of the variable energy blast wave theory. A comparison with the instantaneous (constant energy) explosion model is also discussed.