Nd : YAG láser in art works restoration

Láser cleaning in works of art has a number of advantages over traditional techniques of restoration. In this article, the technique used and the physical mechanisms that explain the process of ablation of pollutants are described. The results obtained in the cleaning of statues of marble and alabaster are exposed as well as oil-painting restoration. In this last specific case, the Nd:YAG láser is used with successful results.


INTRODUCTION
The earliest record of the use of láser in art conservation is to be found at the beginning of the decade of the 70's (1).From the preliminary research works of Asmus (2) up to the present date, the láser cleaning have been made in marble and limestone, textiles, tapestries, leather, pottery, colored glass, bronze and aluminum, etc.In all cases, the cleaning consists of removing the superficial encrustations with the minimum damage for the base material (3).

ADVANTAGES OF LÁSER CLEANING
In the abrasive, chemical and manual-mechanical techniques of cleaning, environmental pollution and variation in the superficial profile of art work are provoked (3).Compared with other techniques, and based on the versatility, accurate control and minimum environmental damage of láser; it permits (*) IMRE.Univ.de la Habana.(Cuba), (lasertec@ffuh.fmq.uh.edu.cu).(**) Centro de Restauración, Conservación y Museología CENCREM (Cuba).(***) Universidad Autónoma Metropolitana UAM-Iztapalapa, México a selective elimination of dirtiness without mechanical contact with the surface, preservation of superficial relief and avoid continued action after the cleaning has finished.

Main physical mechanisms
In previous research works (2) the mechanisms of cleaning are related to the characteristic of láser.Watkings et al (3) put forward the mechanisms that involves selective evaporation, photo and thermal decomposition and ablation by shock wave in Q-Switch regime.In painting, láser excimer has been used (4 and 5), and this article gives a report on the application of Nd: YAG láser by using an ultraviolet wavelength.Radiation power, pulse length and wavelength make Nd:YAG láser the most used, specially in the cleaning of all kinds of stones.Table I shows the lasers used in this work and their more important features, as well as the right concrete application in art cleaning.The samples were treated and analyzed by means of techniques as optical microscopy with digitalization of image (NEOPHOT-21 microscope) and color pictures.

CARRARA MARBLE AND ALABASTER
The láser used possesses a high pulse power which permits to clean big áreas with one pulse.When using free generation (1 ms) the cleaning mechanism would be the selective vaporization of encrustations.For the case of Carrara Marble, the absorvance was for the pollutant layer = 0.6.This strong difference of absorption ((3) between the pollutant and the surface for 1,064 nm (0.2 and 0.6) enables to make the selective evaporation of the dirtiness without damaging the surface (3).In the case of alabaster, two pieces were treated: a dirty ornamental vase also covered with a thick layer of soot; and a Dante bust.The high energy and diameter of the láser beam used permitted to clean the bust in a 4 h work session.

Oilpaintings
Fotakis (4) reported the láser excimer cleaning of oil-paintings.In this case, two experiments were made.Removal of the grime layer at the back of an oíd portrait and removal of the varnish layer and dirtiness over the painting.The first was made with an anonymous 18th century portrait of the Mexican Guadalupe Virgin.The first step to take in the process of restpration is to elimínate the grime layer which covers the back of the cloth and reclothing again.The cleaning was very well done without any affection at a speed higher than that of the traditional methods and without using chemicals, thus contributing to the preservation of the portrait (Fig. 1).
The second experiment consists of cleaning over the portrait and it was made with a Mexican paint  from the beginning of the century.The most appropriate regime was achieved with 266 nm, pulse energy 100 mJ.For this wavelength, the cleaning process relies on non-thermal photoablation of contaminants due the strong absorption.Changing the energy density, the amount of material removing can be controlled.However, the generalization of Nd láser cleaning of paints may lead to complications similar to the difficulties with excimer lasers (6).

SPECTROPHOTOMETRY REFLECTANCE OF SURFACE
In all measured range (500-2500 nm) the reflectance of treated surface was higher than the reflectance of untreated área (Fig. 2).These results confirm that the cleaning process can be controlled by real time feedback reflectance measurements (5).

CONCLUSIONS
-The cleaning of Carrara marble due to environmental pollution and of alabaster due to soot pollution was made successfully by means of a Nd láser.-A layer of dirtiness from the back cloth of a painting using Nd láser in Q-Switch mode was removed.-The cleaning varnish layer and dirtiness adhered to the painting can be removed by means of cjoing the treatment with wavelength 266 nm without damaging the art work.Láser cleaning of art works permits to improve the process of restoration once it decreases the damage to the art work and it increases the speed of the process.
Table II resumes the lasers, samples and results of the cleaning.