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MICROSTRUCTURAL STUDY OF THE ORIGIN OF COLOR IN ROSA PORRIO
GRANITE AND LASER CLEANING EFFECTS
E. Urones-Garrote1,a, A. J. Lpez2, A. Ramil2, L. C. Otero-Daz1,3
1. Centro de Microscopa y Citometra, Universidad Complutense, E-28040, Madrid,
Spain
2. Departamento de Enxeara Industrial II, Centro de Investigacins Tecnolxicas,
Universidade da Corua, E-15403, Ferrol, Spain.
3. Departamento de Qumica Inorgnica, Facultad Ciencias Qumicas, Universidad
Complutense, E-28040, Madrid, Spain
1a. Corresponding author Dr. Esteban Urones-Garrote
fax number: +34 91 394 4191
e-mail address: [email protected]
Dr. Ana J. Lpez
fax number: +34 981 33 74 10
e-mail address: [email protected]
Dr. Alberto Ramil
fax number: +34 981 33 74 10
e-mail address: [email protected].
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Introduction
Laser cleaning of stonework has become a well established technique in the field of
Cultural Heritage, offering unique attributes such as localized action and high spatial
control and feedback [1-3] An important part of the published work concerns limestone
and marbles, and to a lesser extent, silicate rocks such as which were found particularly
sensitive to laser radiation resulting into discoloration of the original surface
The effects of laser cleaning depend on the type of the stone and crust, the laser
parameters, and the characteristics of application [4,5]. Different works have shown that
the color of the stone is a characteristic particularly sensitive to laser irradiation [6-8].
The chemical and mineralogical composition of the stone has an influence on the
absorption of laser radiation and, therefore, possible chemical and physical changes can
occur together with their concomitant color-related behaviour. Color variations to
yellowish, grayish or blackening under Nd:YAG laser both in the fundamental
wavelength ( = 1064 nm) and the 3rd harmonic ( = 355 nm) have been reported for
some carbonate substrates and sandstones (see [9] and references therein).
Our interest is focused on granite, a rock widely used in the Spanish architectural
heritage, especially in western and central areas. Besides, the current development of
transport and construction techniques has led to the widespread use of granite for
cladding, even in areas located far away from the product source, which enlarge the
interest on this topic. In this sense, Rosa Porrio, a granite showing pink hue, is one of
the most marketed Spanish ornamental stones and numerous buildings around the world
are constructed with this material.
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Although granite can be considered one of the most durable construction materials,
humid environments favour the development of biogenic crusts which can be one of the
main causes of decay and blackening of exterior surfaces [10].
The use of laser wavelengths in the UV range has demonstrated to be more effective
in removing biological encrustation than the most commonly used 1064 nm Nd:YAG,
specially in the case of compact or thick layers [11-13]. Klein et al. [12] removed dense
biogenic crust on marble by means of the 3rd harmonic of the Nd:YAG at the fluence of
0.5 J/cm2, Marakis et al.[13] reported ablation thresholds for biogenic crusts in the
range 0.3-0.5 J/cm2. In a previous work [14] the removal of a continuos black patina of
biological origin in Vilachn granite by means of 355 nm Nd:YVO4 laser was analyzed,
and the range of fluences established between 0.5 - 1.5 J/cm2 to ensure efficient
cleaning with minimal damage to the stone surface. On the other hand, the response of
different colored granites to 355 nm Nd:YAG laser was studied at fluences
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cause of chromatic changes in Rosa Porrio granite but no direct experimental evidence
was given[16].
In this paper we are reporting the study of the 355nm laser effects on Rosa
Porrio granite by relating the fading of pink hue of feldspar mineral in the stone with
its micro and nanostructure. Transmission electron microscopy (TEM) and related
techniques are especially useful for the purpose of this work due to the high spatial and
point resolutions achieved [17]. The likely source of the pink hue of this granite has
been identified through TEM observations by comparing the feldspar composition and
microstructure before and after the laser irradiation process.
1. Experimental Techniques
Rosa Porrio is a medium to coarse grained rock whose essential minerals are
quartz, K-feldspar, plagioclase and biotite. As a result of this mineralogy and crystal
size, the rock is polychromatic, and the pink hue in the stone areas consisting of
feldspars is the dominant tone. Further information about its petrographic
characteristics, modal analysis, crystal size and mineral colors can be found elsewhere
[16]. Polished slabs of Rosa Porrio of around 1010 cm2 were irradiated with the third
harmonic, = 355 nm, of a Q-switched Nd:YAG (Quantel, model Brilliant B) at a
repetition frequency f= 10 Hz. The spot diameter was d 8 mm, pulse duration 6 ns
and maximum pulse energy varying between 10 and 60 mJ. Under these conditions,
fluences in the range 0.2-1.0 J/cm2 were applied. As it has been said in Introduction
section, this range includes values reported for the removal of biological deposits in
different types of stone, including granites. Moreover, laser irradiation in this range
ensures chromatic changes in Rosa Porrio granite.
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Granite slabs were set on a 3D translation stage Newport ILS-CC coupled to
Newport MM4006 controller. The laser beam was aimed approximately normal to the
sample and the surface was submitted to 3 laser scans at a scan speed v = 2 mm / s;
between each scan, the slab was shifted slightly to ensure an uniform treatment
throughout the granite sample. The degree of overlapping k given byv
k df
was
approximately 40. These irradiation conditions are a bit far from real operative
conditions owing that our interest is focused on investigating possible causes of fading
the pink hue, and therefore we aim to provoke the color change in the stone surface.
2.1- Color measurements
Color variations of stones associated with different operative laser fluence are
usually measured by means of standard colorimeters or spectrophotometers which
integrate very limited fields of investigation (typically 1 cm2). Therefore, they are
largely irrelevant for monitoring color variations in materials like granites whose poly-
mineral composition and grain size, results in high chromatic heterogeneity. During the
last decade different methods based on digital image analysis have been developed to
characterize or control the quality of ornamental stones and to evaluate their
degradation [18,19]. Since this work is focused on the quantification of relative shade
differences caused by laser, rather than absolute color values; a method based on the
analysis of digital images was used. In brief, images of the granite surface irradiated at
different fluences were captured by a digital camera under proper lighting (experimental
details can be found in [15]). Each pixel of the image is assigned a specific location and
color, (RGB) value, which was transformed into CIE L a b coordinates by means of
the adequate software [20]: L
is the lightness or luminosity (0 black, 100 white);a
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and b are the chromatic coordinates ( a red, a green, b yellow and b blue).;
the attributes of chroma (*abC : saturation or color purity) and hue (
*abh : referring to the
color wheel) can be obtained by the equations: 1 22 2
abC a b
and
* 1 *tan 180abh b a . Color differences between irradiated and non irradiated
samples * *, , , ,ab ab L a b C h can be obtained and the total color change abE
estimated by the expression: 1 22 2 2
ab E L a b
.
2.2- Electron Microscopy
TEM observations were performed with a Philips CM200FEG microscope
(operating at 200 kV, point resolution of 0.23 nm), fitted with an EDAX DX-4 detector
for XEDS (X-Ray energy dispersive spectroscopy) analyses and with a GIF 200 for
EELS (Electron energy-loss spectroscopy) and EFTEM (Energy-filtered TEM)
experiments. High-resolution TEM (HRTEM) and Scanning-TEM (STEM)
observations were carried out with a JEM3000F microscope (acceleration voltage of
300 kV, point resolution of 0.17 nm in TEM mode). TEM specimens were prepared
from powdered parts of the granite feldspar areas, suspended in n-butanol. A drop of the
suspension was deposited on a copper grid covered with a holey carbon film. Scanning
electron microscopy (SEM) images and wave-length energy dispersive spectroscopy
(WEDS) analyses were obtained with a JEOL JXA 8900 electron microprobe operating
at 15 kV (emission of 20 nA), with a beam size of 5 m.
2. Results and discussion
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Rosa Porrio slabs irradiated at 355 nm at fluences ranging 0.2-1 J/cm2 show
chromatic alterations which can be visually identified as a loss of pink coloration in the
feldspar grains, which become paler, and consequently the mean color of the rock
surface turns greyish. Fig. 1 depicts color parameters obtained by means of digital
images as a function of the laser fluence. Error bars represent the dispersion over the
entire number of pixels in each image. As depicted in Fig. 1b and Fig. 1c, there are
variation in chromatic coordinates a and b even at the minimum fluence applied, 0.2
J/cm2; consequently the chroma abC
(Fig. 1d) decreases, showing its highest rate of
change at fluences below 0.5 J/cm2. This decrease in abC
indicates that the color of the
surface is approaching to gray. The increase in hue*ab
h (Fig. 1e) indicates a separation
from the red-green axis; i.e., a loss of red coloration. Finally, from the measurements of
luminosity, L (Fig. 1a) no discernible trend can be appreciated. In coated stones
changes in color after laser irradiation are usually associated to the L parameter
(related to the amount of the layer which remains on the surface); in the case of
uncoated stones (as is the case we are studying), changes in luminosity may be related
to changes in surface roughness [14-16].
From the point of view of color variations, we can establish the damage threshold, at
the wavelength of 355 nm in Rosa Porrio, at fluences below 0.2 J/cm2, and therefore
below 0.5 J/cm2, which has been previously established as minimum value necessary
for the complete removal of the biological crusts in Vilachn granite [14]. Despite the
differences (mineralogical, surface finish,) between granite samples used in both
studies, these results suggests that for the cleaning of biological black crust in Rosa
Porrio granite it will be probably necessary to work at fluences above the damage
threshold.
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WEDS chemical microanalyses of the stone feldspar areas before and after
irradiation (fluence of 1.0 J/cm2) were acquired. In both cases, K-feldspar and
plagioclase [21] were found, without appreciable variation of composition:
(K,Na)AlSi3O8 (K-feldspar) with a K/Na ratio ~26 and NaAlSi3O8+CaAlSi2O8
(plagioclase) with a Na/Ca ratio ~6. No extra elements were detected. The
corresponding SEM images from the original and the irradiated samples are shown in
Fig.2. Apparently, we can observe that the clean and polished surface of the original
granite is affected by the laser irradiation, suffering severe damage (Fig. 2b).Further
analysis of the damage caused by aggressive laser cleaning in the granite surface was
previously reported [14].
The micro and nanostructure of the feldspar areas of the original Rosa Porrio
granite were studied by means of TEM and associated techniques. Typical feldspar
crystals were found, with a very similar composition (measured with XEDS) to the
previously commented WEDS measurements. However, a high number of
nanoparticles, with diameters in the range of 15-40 nm, embedded in a vitreous matrix
of KAlSi3O8, were also observed. A typical example is included in Fig. 3a. XEDS
analyses indicate that the nanoparticles consist of zinc ferrite (ZnFe2O4), which presents
the spinel-type structure (space group Fd-3m and a = 0.84409 nm), as it is also
confirmed through selected-area electron diffraction (SAED) patterns (see Figure 3b)
and HRTEM images (see Fig.4). By means of the micro-diffraction technique [22] we
can obtain spot patterns from a single nanoparticle, which offers a direct confirmation
of its structure type. An electron microdiffraction pattern from a nanoparticle (diameter
~ 20 nm) oriented along the [-112] zone axis of the spinel-type structure is included in
Figure 5, along with the HRTEM image.
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patterns and HRTEM (spinel-type structure). The Fe3+ oxidation state of the zinc ferrite
is not affected by laser irradiation either, as observed with EELS. Fig. 8 shows the Fe-
L2,3 edge EEL spectra from the nanoparticles before and after laser treatment and, in
both cases, the L2/L3 intensity ratio and the ELNES (Energy-Loss Near Edge Structure)
are consistent with Fe3+ oxidation state [25]. In addition to the decrease of zinc ferrite
content, another observed effect of laser irradiation is the occasional agglomeration of
nanoparticles, as shown in the TEM image of Fig. 9a, which was not found in the
original granite sample.
According to the TEM observations, ZnFe2O4 nanoparticles can be considered as
responsible for the pink color of Rosa Porrio. The dispersion of nanoparticles in the
feldspar areas generates the uniform hue of the stone. Apparently, laser irradiation ( =
355 nm) of the granite generates a serious surface damage and elimination of a high
content of ZnFe2O4 as a thermal effect, which is typical of laser treatment on materials
[1,26].
3. Conclusions
In this work, we find a direct relationship between the pink hue fading in Rosa
Porrio granite due to laser irradiation and the microstructure and composition of
feldspar areas. Apparently, the pink color of the stone is originated by ZnFe2O4
nanoparticles, dispersed in the feldspar areas. TEM studies show them embedded in
vitreous KAl2Si3O8. In a previous work [16], as we indicated in the introduction section,
the possible origin of the colour of this granite was attributed to other phases (Fe 2O3).
Besides, non-ferrous minerals were also proposed as responsible for the color [27]. In
both cases, no direct experimental evidence was supplied, since high spatial and energy
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resolution techniques, such as TEM and related, were not employed to study the
microstructure of feldspar. On the other hand, Putnis et al [28] studied the red-coloured
feldspar of different natural granites and observed the existence of needle-like Fe2O3
crystals embedded in the pores of the mineral through TEM.
Therefore, with this work, we consider that the physical elimination of the ZnFe2O4
particles from the surface of Rosa Porrio granite sample, due to thermal effects
induced by the laser irradiation, is the main reason for the total fading of the pink color
of the stone. Besides, the surface damage observed through SEM micrographs can also
have an important influence on the observed color tone. The laser-induced thermal
effects on the nanoparticles also generate the occasional agglomeration of the remaining
ones, which also degrade the color intensity.
Acknowledgements. The authors would like to thank the financial support through
the projects with reference MAT2007-63497 and PGIDIT06CCP00901CT.
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List of References
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Preservation of Cultural Heritage. Principles and Applications (Taylor & Francis Boca
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[3] S. Georgiou, D. Anglos, C. Fotakis: Contemporary Physics 49, 1 (2008)
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[12] S. Klein, F. Fekrsanati, J. Hildenhagen, K. Dickmann, H. Uphoff, Y. Marakis, V.
Zafiropulos: Appl. Surf. Sci, 171, 242 (2001)
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Galician granitic stones induced by UV Nd:YAG laser irradiation. In: M. Castillejo et
al. (eds.): LACONA VII Proceedings. 199 (CRC Press New York 2008)
[16] C. M Grossi, F. J. Alonso, R. M. Esbert, A. Rojo: Color Res. App. 32(2), 152
(2007)
[17] D. B. Williams, C. B. Carter: Transmission electron microscopy (Plenum Press
New York 1996)
[18] V. Lebrun, C. Toussaint, E. Pirard: Monitoring color alteration of ornamental
flagstones using digital image analysis. In R. Prikryl (ed.): Dimension stone
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[19] P. Kapsalas, P. Maravelaki-Kalaitzaki, M. Zervakis, E. T. Delegou, A.
Moropoulou: NDT&E Int. 40, 2 (2007)
[20] S. W. Westland, C. Ripamonti: Computational colour science using Matlab (John
Wiley & Sons England 2004)
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(Mineralogical Society of America, Short Course Notes Washington DC 1975), Vol. 2.
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[24] P. D. Nellist: Scanning Transmission Electron Microscopy. In P. W. Hawkes, J. C.
H. Spence (eds.): Science of Microscopy (Springer New York 2007), Vol. 1.
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List of figures
Figure 1. Variation of color parameters related to the laser fluence: a) lightness, b) red-
green axis, c) yellow-blue axis, d) chroma, e) hue and f) total color variation.
Figure 2. SEM micrographs of a fresh Rosa Porrio sample (a) and the irradiated one
(b)
Figure 3. a)TEM image from a typical vitreous matrix, containing embedded ZnFe2O4
nanoparticles found in the not-irradiated granite sample. b) Corresponding SAED
pattern from the nanoparticles, with some indexed ring reflections according to the zinc
ferrite spinel-type structure.
Figure 4. HRTEM image of a ZnFe2O4 nanoparticle close to the [-110] orientation
respect to the electron beam (see the corresponding Fast-Fourier Transform inset).
Figure 5. Electron microdiffraction pattern along the [-112] zone axis from the zinc
ferrite particle seen on the HRTEM image.
Figure 6. a) Fe-L2,3 EFTEM elemental map and b) Zn-L2,3 jump-ratio image from a
particle containing ZnFe2O4.
Figure 7. STEM-HAADF image of zinc ferrite nanoparticles embedded in a vitreous
matrix from the original Rosa Porrio sample, together with STEM-XEDS elemental
maps.
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Figure 8. Fe-L2,3 edge EEL spectra from: a) irradiated sample and b) original sample.
Figure 9. a) TEM micrograph of agglomerated zinc ferrite particles found in the
irradiated Rosa Porrio sample. b) SAED pattern with some ring reflections indexed
according to ZnFe2O4 spinel-type structure.
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-0.5 0 0.5 1 1.520
40
60
L*
a)
-0.5 0 0.5 1 1.5-10
0
10
a*
b)
-0.5 0 0.5 1 1.50
10
20
b*
c)
-0.5 0 0.5 1 1.50
10
20
C * a
b
d)
-0.5 0 0.5 1 1.540
60
80
100
fluence /(J/cm2)
hab
e)
-0.5 0 0.5 1 1.50
5
10
fluence / (J/cm2)
E * a
b
f)
FIGURE 1
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FIGURE 2
500 500
a b
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FIGURE 3
100 nm
VitreousKAlSi3O8 matrix
ZnFe2O4nanoparticles
111220
311
a b
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FIGURE 4
220
111
[-110]
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FIGURE 5
1-11 220
[-112]
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FIGURE 6
50
a b
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FIGURE 7
Fe Zn
Si A
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FIGURE 8
700 710 720 730 740 750
Intensity
(a.u.)
Energy-Loss (eV)
Fe-L3 Fe-L2
b
a
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FIGURE 9
a
111 220
311 400
b
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