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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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Heritage. 1, S21 (2000)

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Fort: Appl. Surf. Sci. 219, 290 (2003)

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Prez de Andrs, C. Escudero, M. Barrera, E. Sebastin, C. Rodrguez-Navarro, K.

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(2009)

[11] P. Maravelaki-Kalaitzaki, V. Zafiropulos, C. Fotakis: Appl. Surf. Sci. 148, 92

(1999)


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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)

[13] G. Marakis, P.Pouli, V.Zafiropulos, P. Maravelaki-Kalaitzaki: J.Cult:Heritage

4,83s(2003)

[14] A. J. Lpez, T. Rivas, J. Lamas, A. Yez: Appl. Phys. 100, 733 (2010)

[15] A. Ramil, A. J. Lpez, M. P. Mateo, C. lvarez, A. Yez: Colour changes in

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

(Taylor&Francis London 2004)

[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)

[21] P.H. Ribbe: The Chemistry, Structure and Nomenclature of Feldspars

(Mineralogical Society of America, Short Course Notes Washington DC 1975), Vol. 2.

[22] J. C. H: Spence, J. M. Zuo: Electron Microdiffraction (Plenum Press New York

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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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[28] A. Putnis, R. Hinrichs, C. V. Putnis, U. Golla-Schindler, L. G: Collins: Lithos 95,

10 (2007)


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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

draft-rosa porriño_ana_4

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