Presentado en International Conference Lacona VII, Madrid, Spain, 17-21
September 2007[Resumen]: estudio sobre el cambio de color del granito gallego al limpiarlo mediante láser ultravioleta[Abstract] Laser cleaning has become a high profile and accepted specialized cleaning technique for most stone types. Given that local natural building stone in our region, Galicia (North West of Spain) is granite, our interest was focused on the response of this stone type to laser cleaning. The aim of this work is to determine the change in colour as index of the response of galician granites to UV-Nd:YAG laser exposure. For this purpose samples of commercial granites from different quarries were irradiated at increasing fluences. Colour changes were determined through the analysis of digital images and expressed in the L*a*b* and L*h*C* colour models. The results enable the establishment of application limits of laser cleaning to these material

López, Ana

Mateo Orenes, M. Paz

Ramil, Alberto

Yáñez, Armando

Álvarez-Feal, José-Carlos

Mateo, María Paz

Repositorio da Universidade da Coruña

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE LACONA VII, MADRID, SPAIN, 17-21 SEPTEMBER 2007 Lasers in the Conservation of Artworks Editors Marta Castillejo Instituto de Química Física Rocasolano, CSJC, Madrid, Spain Pablo Moreno Laser Facility, University of Salamanca, Salamanca, Spain Mohamed Oujj a Instituto de Química Física Rocasolano, CSJC, Madrid, Spain Roxana Radvan National Institute of Research and Development far Optoelectronics, Bucharest, Rumania Javier Ruiz Department of Applied Physics 1, University of Málaga, Málaga, Spain 0 ~y~~ ,:.~~~~ocp Boca Raton London New York Leiden CRC Press is an imprint of the Taylor & Francis Group, an informa business A BALKEMA BOOK CRC Press/Balkema is an imprint of the Taylor & Francis Group, an ú?forma business © 2008 Taylor & Francis Group, London, UK Typeset by Charon Tec Ltd (A Macmillan Company), Chennai, India Printed and bound in Great Britain by Cromwell Press Ltd, Towbridge, Wiltshire Ali rights reserved. No part ofthis publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publishers. Although ali care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assurned by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. Published by: CRC Press/Balkema P.O. Box 447, 2300 AK Leiden, The Netherlands e-mail: Pub.NL@taylorandfrancis.com www.crcpress.com - www.taylorandfrancis.co.uk - www.balkema.nl ISBN 13: 978-0-415-47596-9 1.1.1 Colour changes in Galician granitic stones induced by UV Nd:YAG laser irradiation 2 INTRODUCTION  Laser cleaning of stones has become a high profile and accepted specialized technique in restoration. The majority of the reported laser stone cleaning has been for limestone and marble and to a lesser extent for silicate rocks such as sandstone; experience with granite and other crystalline stones is rare at pre-sent (Fotakis et al. 2007). Given that Galicia (North West of Spain) has a rich heritage of buildings and monuments constructed from locally obtained gran-ite, our interest was focused on the response of this stone type to laser cleaning. Besides, the current de-velopment of transport and construction techniques has led to the widespread use of granite in facing tiles, even in areas located far away from the product source, which enlarge the interest on this topic. Because of the wet climate in Galicia, the granite is almost permanently damp, which made it highly bioreceptive. Biological colonization and blackening of exterior surfaces can be observed in many build-ings (Prieto et al. 1999, Aira et al. 2007); the main objective of the cleaning process is the removal of biological contamination. The effects of laser cleaning on stone depend on the laser parameters, the type of the stone and the characteristics of application. Between the undesira-ble effects, changes in colour have been observed for different stone types at different working parame-ters (Klein et al. 2001, Lee et al. 2001). In the case of granites, changes in coloration of "Rosa Porriño" under 1064 nm Nd:YAG laser irradiation, and differ-ent Scottish granites under 1064, 532 and 355 nm were reported (Wakefield et al. 1997, Esbert et al. 2003, McStay et al. 2005, Grossi et al. 2007). The aim of this work is to determine the change in colour of Galician granites under UV laser expo-sure. For this purpose samples of commercial gran-ites showing different colours were irradiated with a Nd:YAG laser source at the wavelength of 355 nm and different fluences. The colour changes were de-termined through the analysis of digital images of the irradiated and non irradiated samples. The results can help to establish the application limits of the la-ser cleaning technique for different types of Galician building granites. 3 EXPERIMENTAL 3.1 Materials The stone types used in this study are specifically those of Galician origin and comprised samples of pink (Rosa Porriño), pinkish-gray (Mondariz), and gray (Albero, Gris Morrazo) granites. All of them are commercial stones being Rosa Porriño, one of the most marketed ornamental stones in Spain and outside Spain.  Fresh samples from stores were cut in tablets of around 10×10 cm2. Surface finishes were polished for all the samples except for Albero which was sew. Table 1 summarizes some relevant characteristics of these stones (Quiroga-Calviño et al. 1998). Colour changes in Galician granitic stones induced by UV Nd:YAG laser irradiation   A. Ramil, A.J. López, M.P. Mateo, C. Álvarez & A. Yáñez Departamento de Enxeñaría Industrial II, CIT, Universidade da Coruña, E-15403, Ferrol (A Coruña) Spain   ABSTRACT: Laser cleaning has become a high profile and accepted specialized cleaning technique for most stone types. Given that local natural building stone in our region, Galicia (North West of Spain) is granite, our interest was focused on the response of this stone type to laser cleaning. The aim of this work is to determine the change in colour as index of the response of galician granites to UV-Nd:YAG laser exposure. For this purpose samples of commercial granites from different quarries were irradiated at increasing fluences. Colour changes were determined through the analysis of digital images and expressed in the L*a*b* and L*h*C* colour models. The results enable the establishment of application limits of laser cleaning to these materials.     Table 1. Granites analyzed in this work.  Granite  Classification  Macroscopic Description   Porriño  Biotite granite  Pink colour due to the             coloration of the feldspars.            Medium to coarse grained.  Mondariz Biotite granite Pinkish-gray with mega-             crystals of feldspar.  Albero  Muscovite   Light gray, with a yellow      granite    to brown shade, medium-           grained, rich in muscovite.  Morrazo Biotite granite Gray, fine-grained. 3.2 Laser irradiation Experiments were carried out using a Q-switched Nd:YAG laser source; operating at the third harmon-ic, 355 nm, 10 Hz repetition rate, spot diameter around 8 mm; pulse duration 6 ns and maximum pulse energy varying between 10 and 60 mJ. The samples were irradiated at different fluences between 0.18 and 1.03 J/cm2. The energy density was calculated from measured energy data and the spot size of the laser beam on heat sensitive paper.  Samples were mounted onto a computer-controlled X−Y translation stage. Each sample was submitted to 3 scans at a scan speed of v = 2 mm/s. Under these conditions the average degree of over-lapping given by the relation (d  f / v) was 40 laser pulses in each scan, being f the frequency, d the di-ameter of the laser beam and v the speed of scan. Then, an average of 120 laser pulses was delivered over each point of the sample surface. 3.3 Colour measurements Although the measure of colour used to be per-formed by conventional spectrophotometric tech-niques; at present available commercial colorimeters measure colour only over a very few square centi-meters, and thus their measurements could not be very representative in heterogeneous materials such as granite items. For this reason, in order to quantify the colour changes induced by laser irradiation, a simple method based on computer vision techniques was applied (Yam et al. 2004, Thornbush et al. 2004, Leon et al. 2006). This method uses a digital camera to capture the colour image of the granite tablet un-der proper lighting. The captured image is a bitmap image consisting of many pixels; each pixel assign a specific location and colour (RGB) value which can be transformed into different colour coordinates by means of the adequate software (Westland and Ri-pamonti 2004).   The digital camera was a Nikon D100 with expo-sure times ranging from 30 s to 1/4000 s and an ob-jective Nikon micro 60/2.8. The CCD of the camera consists of 6.1×106 pixels and presents an active area of 23.7×15.6 mm2. The images were stored in non-compressed file (TIF format) to avoid loss of image quality. The standard light source D65 (http://www.cie.co.at/) which mimic variations of daylight and with a colour temperature of 6500 K was used.  The position of the camera, sample, and light sources was arranged in order to capture the diffuse reflection responsible for the colour, which occurs at 45ο from the incident light, and to ensure a uniform distribution of light intensity over the sample sur-face. A standard colored chart GretagMacbeth Col-ourChecker was used to calibrate and verify the ex-perimental settings prior to actual measurements. The digital images were taken out of focus to obtain a fuzzy image which simulates the effect of the standard CIE viewing angle geometry (http://www.cie.co.at/). 4 RESULTS AND DISCUSSION In order to quantify the colour changes, digital im-ages of all the samples irradiated were analyzed and colour measurements were expressed using the CIE L*a*b* system. Here L* is the variable lightness or luminosity, which varies from 0 (black) to 100 (white); a* and b* are the chromatic coordinates. The attributes of chroma (C*ab: saturation or colour puri-ty) and hue (h*ab: referring to the colour wheel) can be calculated by the equations: 2/12*2** )( baC ab +=  and )/180*)(/*(tan 1* πabh ab −= . Colour differences ( *L∆ , *a∆ , *b∆ , abC*∆ , abh*∆ ) were calculated and the total colour change ( abE*∆ ) was estimated by the expression: 2/12*2*2** ])()()[( baLE ab ∆+∆+∆=∆ .  The colour of the granite stones results from a combination of colours of the granite rock-forming minerals; thus, the selected stones present different colour coordinates, which are summarized in Ta-ble 2. Different responses to UV-Nd:YAG laser ir-radiation of these stones are appreciable under naked eye examination showing changes in colour espe-cially in the case of Rosa Porriño in which a clear loss of pink colouration was observed. Gris Mondar-iz, as well as Albero, present more subtle variations, however in the case of Gris Morrazo no changes were noticed. On the whole, colour changes seem to be higher at higher laser fluences. Data of color variations, in terms of colour coor-dinates, as a function of the laser fluence are dis-cussed below for each granite type:      Table 2: Values of colour coordinates of granites. Granite Albero  Morrazo Mondariz  Porriño  L*    44±2   47±3   45±6     46±4  a*   1.0±0.9 0.1±0.6  0.6±1.0   5.7±3  b*   4.7±1.0 1.0±0.8 3.4±1.4   8.7±3  C*  4.8±1.0 1.2±0.8 3.5±1.4  10.5±4  h*   79±11   68±59   80±19     58±8    -0.5 0 0.5 1 1.5204060L*a)-0.5 0 0.5 1 1.5-10010a*b)-0.5 0 0.5 1 1.501020b*c)-0.5 0 0.5 1 1.501020C* abd)-0.5 0 0.5 1 1.56080100fluence /(J/cm2)h abe)-0.5 0 0.5 1 1.50510fluence /(J/cm2)∆E* abf) Figure 1: Rosa Porriño L*, a*, b*, C*, h* and abE*∆ .  Rosa Porriño  As can be appreciated in Figure 1a and Figure 1b, there are strong changes  in chromatic coordinates a*   -0.5 0 0.5 1 1.530405060L*a)-0.5 0 0.5 1 1.50246C* abb)-0.5 0 0.5 1 1.5050100150fluence /(J/cm2)h abc)0 0.5 1 1.5-50510fluence /(J/cm2)∆E* abd)  Figure 2. Mondariz L*, C*, h*  and abE*∆ .  and b* even at the lowest value of laser fluence. As a consequence, the chroma C* (Fig. 1c) decreases showing a higher rate of change at the lower values of fluence (between 0 and 0.5 J/cm2). This decrease in C* indicates that the colour of the surface was ap-proaching the values corresponding to gray coloura-tion. At the same time, the increase in hue h* (Fig. 1d) indicates a separation from the red-green axis; i.e. a loss of red colouration. Finally, from the measurements of L* (Fig. 1e), no discernible trend can be appreciated confirming that the loss in red colouration was not accompanied by a correspond-ing lightening or bleaching of the stone surface which was measurable using the L* component. Gris Mondariz  As in the case of Rosa Porriño, a loss of pink col-ouration is appreciable at all the fluences analyzed giving a decrease in C* with increasing fluence (Fig. 2b). Values of L* and h* do not show an appre-ciable trend (Fig. 2a and 2c). Albero  This granite presents a light gray colour with a shad-ow of yellow-brown. Changes in colour were dis-cernible by eye at the higher fluences but they are very subtle. Figure 3b shows appreciable changes in C* for fluence values of 0.5 J/cm2 onwards. Coordi-nate h* shows a subtle decrease (Fig. 3c). Values of L* do not evidence lightening or bleaching caused by the UV laser irradiation.  -0.5 0 0.5 1 1.5404550L*a)-0.5 0 0.5 1 1.5246C* abb)-0.5 0 0.5 1 1.56080100fluence /(J/cm2)h abc)0 0.5 1 1.5-2024fluence /(J/cm2)∆E* abd)  Figure 3: Albero L*, C*, h* and abE*∆ . Morrazo  This gray granite does not show changes in coloura-tion caused by the laser irradiation. Then, UV-Nd:YAG laser irradiation has caused colour changes in pinkish granites (Rosa Porriño and Gris Mondariz) even at the lowest fluences applied, probably associated to changes in the pink feldspars which contain iron oxides. In the case of Albero, gray granite with a yellow-brown shadow, changes were appreciated when the laser fluence reaches around 0.2 and 0.5 J/cm2. In all cases chroma, i.e. C* coordinate, is sensitive to the laser fluence, showing a clear decrease when the fluence increases. This de-crease indicates a progressive approach to the gray axis in L*C*h* space. The lightness, L*, does not show any trend indicating that no bleaching is pro-duced by the laser. The behavior of hue, h* coordi-nate, under laser irradiation depends on the granite type but does not exhibit a great variation with laser fluence. As a result, colour difference abE*∆  shows a clear increase with increasing fluences for all the granites analyzed, giving a maximum value of 5 for Rosa Porriño and around 2 for Mondariz and Alber-to. 5 CONCLUSIONS Different coloured granite stones were exposed to ir-radiation of a Q-switched Nd:YAG laser in the UV range (355 nm) at different energy densities to de-termine the effect of the laser on the colour of fresh stone surfaces. Except for the case of gray granite Gris Morrazo, a loss of colour with increasing flu-ence is visible with the naked eye.  In order to obtain quantitative values of changes in coloration, a method based on the analysis of digi-tal images was used, and coordinates of the L*C*h* space were selected for quantification. The more sensitive coordinate to the laser energy density was the chroma C* which decreases with the increase of laser fluence. For granites containing red colored minerals such as Rosa Porriño and Gris Mondariz, colour loss occurred at the lowest fluence applied, 0.18 J/cm2. Exposure of these stone types to UV causes a loss in pink coloration from the feldspar grains which turn gray. In the case of gray granites, Albero, a light gray stone with a yellow-brown shadow, changes occurred at values around 0.5 J/cm2, probably associated to changes in the minerals responsible of the yellow-brown shadow.  ACKNOWLEDGEMENTS Special thanks are given to Dr. Francisco Fernández Martínez from Dept. Química Industrial y Políme-ros, Universidad Politécnica de Madrid. 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Colour changes in Galician granitic stones induced by UV Nd:YAG laser irradiation

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Colour changes in Galician granitic stones induced by UV Nd:YAG laser irradiation

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