Laser removal of water repellent treatments on limestone

Miguel Gomez-Herasa, Monica Alvarez de Buergoa, Esther Rebollarb,Mohamed Oujjab, Marta Castillejob,*, Rafael Forta

aInstituto de Geologıa Economica, CSIC-UCM, Facultad de Ciencias Geologicas,

Universidad Complutense de Madrid, 28040 Madrid, SpainbInstituto de Quımica Fısica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain

Received 25 March 2003; received in revised form 7 May 2003; accepted 7 May 2003

Abstract

Protective and water repellent treatments are applied on stone materials used on buildings or sculptures of artistic value to

reduce water intrusion without limiting the natural permeability to water vapour of the material. The effect of the wavelength

associated with the laser removal of two water repellent treatments applied on limestone, Paraloid B-72, a copolymer of methyl

acrylate and ethyl methacrylate, and Tegosivin HL-100, a modified polysiloxane resin, was investigated by using the four

harmonics of a Q-switched Nd:YAG laser (1064, 532, 355 and 266 nm). The modifications induced on the surface of limestone

samples by laser irradiation were studied using colorimetry, roughness measurements and scanning electron microscopy (SEM).

The removal of the treatments was found to be dependent on the laser irradiation conditions and on the characteristics of the

coatings. The fundamental laser radiation was effective in removing both treatments, but thermal alteration processes were

induced on the constituent calcite crystals. The best results were obtained by irradiation in the near UV at 355 nm.

# 2003 Elsevier B.V. All rights reserved.

PACS: 42.62.-b laser applications; 61.80.Ba ultraviolet, visible, and infrared radiation effects; 79.20.Ds laser-beam impact phenomena

Keywords: Laser ablation; Nd:YAG pulsed laser; Cleaning of stone; Water repellents

1. Introduction

Since water is involved in most forms of stone decay,

water repellent treatments are applied on architectural

stone surfaces to prevent or reduce liquid water intru-

sion. The treatment creates an impermeable barrier to

water, without limiting the natural permeability of the

material, allowing the passage of water vapour through

the stone out of the walls [1–4]. Application of these

protective treatments slows down the deterioration

process and increase the durability of the stone ele-

ments, so that in the future a new treatment can be

applied if necessary [5]. Although the technology of

waterproofing masonry materials has improved con-

siderably in the last few years, the issue of the re-

versibility of the treatments is a subject of debate,

specially when materials used on buildings or sculp-

tures of artistic value are concerned. The Venice Charter

(1964) [6] and subsequent charters on architectural

conservation adopted by ICOMOS, identified a num-

ber of key conservation principles relating to reversi-

bility, repeatability and retreatability. The principle of

Applied Surface Science 219 (2003) 290–299

* Corresponding author. Tel.: þ34-91-5619400;

fax: þ34-91-5642431.

E-mail address: marta.castillejo@iqfr.csic.es (M. Castillejo).

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0169-4332(03)00701-3

reversibility has more recently been replaced by prin-

ciples of compatibility and retreatability [4,5]. Compat-

ibility requires that treatment materials do not induce

negative effects on the substrate, and retreatability

requires that the present conservation treatment will

not preclude or impede future treatments. These prin-

ciples are considered more sustainable because they

are more realistic and enable future treatments to take

advantage of progress in scientific knowledge [7].

Treatments for stone conservation based on acrylic

resins, have been considered to be reversible due to

their solubility in certain solvents [4,8–10]. Even if a

protective treatment is easily soluble, repeated appli-

cations of solvent may cause material damage. The

statement that one material is soluble in another may

be technically correct, but it does not in itself indicate

the conditions required for dissolution (in vitro dis-

solution versus in situ dissolution, like in the case of a

stone-masonry wall). Lately it has been proposed

[10,11] that the term ‘‘treatment reversibility’’ should

be substituted by the term ‘‘treatment removal’’, the

latter referring to the ideal ability to remove a sub-

stance completely from a treated object. It is therefore

highly desirable to propose alternative removal meth-

ods that do not necessarily rely on the use of solvents.

Laser techniques have gained acceptance as ad-

vanced tools to remove black encrustations and con-

taminants from different types of stone [12–14]. A

methodology has been developed for optimal laser

cleaning, that allows the removal of the external

unwanted layers without damage to the underlying

substrate [15–17]. At the infrared wavelength of the

laser radiation delivered by Q-switched Nd:YAG lasers,

the ablation threshold fluenceof the clean stone ishigher

than the threshold fluence of the black crust. For prac-

tical cleaning, a fluence in the ablation threshold gap can

be used (self-limiting effect). Shorter laser wavelengths

in the UV-Vis range delivered by excimer lasers or the

harmonics of the Nd:YAG fundamental radiation have

also been investigated for cleaning of stone [17,18]. In

the case of painting conservation, UVexcimer lasers can

efficiently remove oxidized varnish layers with minimal

light penetration and thermal effects to the underlying

paint layers [19]. When no self-limiting effect ispresent,

on-line diagnostic techniques have to be coupled to the

cleaning process for controlling laser damage of the

substrate. Several techniques have been developed to

this purpose, such as laser-induced breakdown spectro-

scopy (LIBS) [20,21] or monitoring of the acoustic

wave accompanying the ablation process [15].

In this work, we investigate the suitability of laser

irradiation as an adequate procedure to remove water

repellent treatments applied on limestone, an abundant

material used extensively throughout Europe in the

building of monuments. Although this type of stone is

a high quality building material [22,23], the rock

might be susceptible of requiring protective water

repellent treatments under particular circumstances

[2]. In this study, we used the four harmonics of a

Q-switched Nd:YAG laser (1064, 532, 355 and

266 nm). To monitor the effects induced by laser

irradiation, we measured changes on the chromatic

properties of the surface by colorimetry and changes

on the surface roughness. Scanning electron micro-

scopy (SEM) served to verify the degree of elimination

of the treatments, considering only the elimination of

the superficial coating, but not the product which could

eventually penetrate into the stone through its porous

system. Finally, an evaluation of the optimal irradiation

conditions for removal of the protective treatments was

thus performed.

2. Experimental

2.1. Sample description

This study was carried out on samples of a lime-

stone variety called Colmenar, the building rock of

many monuments included in the architectural heri-

tage of the area of Madrid. Limestone is a carbonatic

rock mostly composed of calcium carbonate. This

property, together with the monochromatic appear-

ance of the Colmenar variety, simplify the study of the

effects of laser irradiation. Table 1 lists the petrophy-

sical properties of the Colmenar limestone. Water

repellent treatments were applied on prismatic samples

Table 1

Petrophysical properties of the Colmenar limestone

Real density (g cm�3) 2.685

Bulk density (g cm�3) 2.595

Porosity accessible to water (%) 3.4

Water saturation (%) 1.3

Water absorption after 48 h (%) 0.8

Porosity accessible to Hg (%) 3.6

M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299 291

of 40 mm � 40 mm � 20 mm by immersion during

3 min in a solution of the corresponding product.

The samples were then let to dry until their weight

was found constant. Reference samples without treat-

ment were kept for comparison and to study the effects

of the direct laser irradiation on the stone material.

Two water repellent treatments were selected for this

study. Paraloid B-72 (Rohm & Haas), a copolymer of

methyl acrylate and ethyl methacrylate, was used as a

5 vol.% solution of xylene. Tegosivin HL-100 (Gold-

schmidt) is a low molecular weight modified polysi-

loxane resin (methyl-ethoxy-polysiloxane) that was

dissolved in white spirit at a concentration of 8 wt.%.

The first product (PB-72) has also adhesive and

consolidant properties. It has been widely used in

Europe since the decade of the 1950 of the past

century and has been identified as the causing agent

of chromatic and gloss pathologies on some types of

stone on which it was applied. Therefore, it is impor-

tant to identify methods of elimination of this treat-

ment. The use of the second product (HL-100) as

water repellent coating, eliminates the above-men-

tioned pathologies, but its extensive use in the last

decades of the 20th century compels to investigate the

removal of this product in terms of retreatability or

compatibility with any other treatment.

To further characterize the above two products and

their response to laser irradiation, we measured their

absorption spectra in the 200–800 nm range using a

UV-Vis Cary 3E Varian spectrophotometer. The absor-

bance of a film of each treatment applied on a quartz

plate is shown in Fig. 1. The thickness of the PB-72

film was measured with a Talystep Taylor–Hobson

profilometer to be 1.8 mm. The HL-100 film was too

thin to be properly measured with the profilometer.

Only an estimated value around 0.8 mm could be given.

Light absorption by the two products increases at the

shorter irradiation wavelengths. The absorbance

values, as shown in Fig. 1, give rise to absorption

coefficients for the HL-100 treatment that are two to

three times higher than the corresponding values of PB-

72. This implies that the effective penetration depth of

laser radiation is lower in the HL-100 treatment.

2.2. Laser irradiation

The laser system is a Q-switched Nd:YAG laser

(Quantel Brilliant B) that delivers pulses of 5 ns

Fig. 1. Absorption spectra of water repellent treatments applied on a quartz plate. The film thickness measured for PB-72 is 1.8 mm. For HL-

100, the estimated thickness is 0.8 mm (see text).

292 M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299

(FWHM) with a maximum repetition rate of 10 Hz.

The fundamental radiation at 1064 nm and the second,

third and fourth harmonics at 532, 355 and 266 nm,

respectively were used for irradiation of the samples.

The laser beam had a circular cross-section of 5 mm

radius with a Gaussian profile. The samples were

irradiated by directing the unfocussed laser beam over

the surface by the use of mirrors. The pulse energy was

measured by a joulemeter (Gentec ED-200). At each

wavelength of the laser, various conditions of energy

per pulse and number of pulses were chosen to irradi-

ate circular zones of ca. 0.8 cm2. We irradiated three

type of samples: untreated stone, stone treated with

PB-72 and stone treated with HL-100. Untreated, non-

irradiated samples were kept for comparison.

In order to define the laser irradiation conditions of

the samples, we determined the ablation rate at a range

of laser fluences for each irradiation wavelength, and

the corresponding ablation threshold for the removal

of the untreated limestone. At each fluence a crater

was created by ablation by delivering typically 50

laser pulses on the same spot. The depth of the craters

was measured with the profilometer mentioned before.

The mean ablation rate was obtained by dividing the

measured depth by the number of pulses. The ablation

threshold fluences that were determined for the Col-

menar limestone are reported in Table 2. These values

are consistent with previous reported values in similar

types of stone [17,18]. The ablation threshold fluences

decrease with wavelength as observed by other authors

[18]. Table 2 also includes our measurements of

ablation thresholds for the PB-72 water repellent

treatment. At the four Nd:YAG wavelengths the mea-

sured ablation threshold of this coating is higher than

that of the stone. A good agreement with previous

measurements on this coating [24] is also found. The

ablation thresholds of the HL-100 treatments could

not be determined with the described procedure,

probably due to the fact that the thickness of the

coating is too low (see below). However, at fluences

similar to those used to measure the ablation thresh-

olds of PB-72, resulted in no appreciable crater for-

mation on the HL-100 coating. This indicates that the

ablation thresholds of HL-100 are higher than the

values measured for PB-72.

In each type of sample, four areas, corresponding to

each laser wavelength, were created by pulsed laser

irradiation using the same number of pulses, 1800, and

repetition rate, 10 Hz. Fluences were chosen to be

below the ablation thresholds of the bare stone: 2.3 J/

cm2 for l ¼ 1064 nm; 0.9 J/cm2 for l ¼ 532 nm; 0.4 J/

cm2 for l ¼ 355 nm and 0.2 J/cm2 for l ¼ 266 nm.

2.3. Techniques for assessment of effects

A Minolta CM 2002 spectrocolorimeter served to

measure the chromatic properties of the samples and

the changes induced by application of the treatments

and laser irradiation. The CIE-Lab colour space was

used to measure colour shifts expressed in three vari-

ables, viz. L* luminosity, a* redness/greenness and

b* yellowness/blueness. The chroma is defined as

C� ¼ ða�2 þ b�2Þ1=2and the global colour variation,

DE� ¼ ðDL�2 þ Da�2 þ Db�2Þ1=2, indicates the mag-

nitude of the colour change [25]. The yellow and white

indexes, YI and WI, respectively, were also deter-

mined for each sample. The morphological changes of

the surface of the samples were assessed by means of a

surface roughness instrument (Surtest SJ-201, Mitu-

toyo) that evaluated the parameters Ra, the arithmetic

mean deviation of the roughness profile and Rp, or

mean value of roughness depth of five consecutive

sampling lengths. SEM measurements were per-

formed with a JEOL, JSM 6400 instrument on gra-

phite-sputtered samples.

Attempts of using LIBS for on-line monitoring of

the removal of the treatments probed inconclusive, as

no emission lines, specifically assigned to the coating,

could be identified in the spectra.

3. Results and discussion

Application of water repellent treatments results in

chromatic changes of the stone surface [1,26]. In the

case of PB-72, the global colour variation amounts to

Table 2

Laser ablation threshold measurements (fluence in J/cm2) at four

wavelengths

266 nm 355 nm 532 nm 1064 nm

Untreated stone 0.4 0.9 1.7 3.0

PB-72 0.5 1.3 4.3 >8.3

Fluence used for

irradiation

0.2 0.4 0.9 2.3

M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299 293

4.5 units. As shown in Figs. 2 and 3, that display in

detail the measured changes of chromatic properties,

this is due to the reduction of the luminosity (L*) and

increase of the chroma (C*). The application of this

treatment results in appreciable changes in the white

and yellow indexes of the stone of �18 and

þ7.9 units, respectively. In contrast, coating with

HL-100 does not introduce significant changes in

the chromatic parameters of the stone, DE* being less

that one unit.

Laser irradiation of the untreated samples also

discolours the surface of the stone. Table 3 indicates

the measured values of the total colour variation

parameter (DE*). At the irradiation conditions of

the 1064 nm wavelength, an increase of luminosity

and decrease of chroma (Fig. 2) induce a global colour

change of DE� ¼ 6:3 (Table 3), even if the fluence

used is below the ablation threshold of the material. It

is worth noticing that upon irradiation at 1064 nm, the

yellow index of the untreated stone does not increase

Fig. 2. Changes of the chroma and luminosity parameters of limestone samples. The origin of coordinates represent the values of the untreated

non-irradiated samples.

Fig. 3. Changes of yellow and white indexes, of limestone samples. The origin of coordinates represent the values of the untreated non-

irradiated samples.

294 M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299

(Fig. 3), on the contrary, a decrease of four units was

measured. This is in agreement with previous obser-

vations, as yellowing has been reported to take place

during the laser removal of soiling or encrustations,

and not during clean substrate stone irradiation [27].

Irradiation of the uncoated limestone at shorter wave-

lengths results in small DE* values.

Laser irradiation of the samples treated with the

water repellent coatings introduces various degrees of

change on the chromatic properties of the stone,

depending on the irradiation conditions and the type

of treatment. Irradiation of the treated stone at

1064 nm results in a global colour change DE*, with

respect to the untreated stone, slightly higher than

3 units for both treatments (Table 3). In the case of PB-

72, this value is less than the global colour variation

induced by application of the treatment (DE� ¼ 4:5)

indicating that the chromatic modifications are

reverted by irradiating at this wavelength. For PB-

72, the conditions of the 355 nm irradiation are the

most effective in the recovery of the chromatic proper-

ties displayed by limestone before coating. This

applies to each measured chromatic parameter as

observed in Figs. 2 and 3. On the other hand, it has

been already mentioned that coating with HL-100

only discolours the stone slightly (DE� ¼ 1); however

laser irradiation of the treated surface at 1064 nm in

the conditions of this work results in an increased

discoloration (DE� ¼ 3:5 in Table 3) with respect to

the original untreated stone. The effect of irradiation

of the stone coated with HL-100, at the conditions of

the shorter wavelengths of this study, 266 and 355 nm,

is weaker than the effect at 1064 nm, Irradiation at

these two UV wavelengths causes similar global

colour changes with respect to the untreated stone,

with DE* values between 1 and 2.

As shown in Fig. 3, the yellow index, observed to

increase when PB-72 is applied on the stone samples,

decreases upon irradiation at the conditions defined

for 532, 355 and 266 nm, indicating an approximation

Table 3

Total colour variation (DE�) induced by laser irradiation

Treatments Comparison 266 nm 355 nm 532 nm 1064 nm

Untreated Irradiated-untreated 2.0 1.8 1.0 6.3

PB-72 Irradiated-untreated 3.2 1.5 4.2 3.2

HL-100 Irradiated-untreated 1.4 1.6 1.8 3.5

Fig. 4. Changes of Ra, and Rp roughness parameters induced by laser irradiation. The origin of coordinates represent the values of the

untreated non-irradiated samples.

M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299 295

Fig. 5. SEM images of the surface of limestone samples: (a) untreated, non-irradiated; (b) treated with PB-72, non-irradiated; (c) treated with

HL-100, non-irradiated; (d) untreated, irradiated at 266 nm; (e) treated with PB-72, irradiated at 266 nm; (f) treated with HL-100, irradiated at

266 nm; (g) untreated, irradiated at 355 nm; (h) treated with PB-72, irradiated at 355 nm; (i) treated with HL-100, irradiated at 355 nm; (j)

untreated, irradiated at 532 nm; (k) treated with PB-72, irradiated at 532 nm; (l) treated with HL-100, irradiated at 532 nm; (m) untreated,

irradiated at 1064 nm; (n) treated with PB-72, irradiated at 1064 nm and (o) treated with HL-100, irradiated at 1064 nm.

296 M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299

to the initial chromatic conditions, i.e. irradiation at

these wavelengths causes the loss of yellowing

induced by the application of the treatment to the

stone.

Application of water repellent products changes

slightly the roughness of the surface, but in lesser

extent than laser irradiation, as represented in Fig. 4.

This figure also shows the changes induced in the

roughness by laser irradiation of the bare and treated

stone. The modifications of roughness are consider-

able under irradiation at 1064 nm, whereas irradiation

at the conditions of the shorter wavelengths induce

lesser changes. Irradiation of samples coated with PB-

72 at 532, 355 and 266 nm does not induce significant

changes of roughness, as compared with the untreated

stone, although they are more noticeable in the case of

HL-100 coating.

Fig. 5 shows SEM images of the surface of the

limestone samples and Fig. 6 of transversal sections of

the stone coated with the two products. Differences in

the coating characteristics of the two products are

clearly observed when comparing Fig. 5a (untreated),

b (PB-72) and c (HL-100) and Fig. 6a and b. Treatment

PB-72 covers the surface in an homogeneous and

compact way, hiding completely the calcite crystals.

The measured thickness of this coating is 3.9 mm

(Fig. 6a). In contrast, the HL-100 coating appears

as a thinner film of thickness 1.7 mm (Fig. 6b). The

measured thickness must not be mistaken with the

penetration depth of each product into the stone.

Asthe imagesof thesurfaceclearly show(Fig.5m–o),

irradiation at 1064 nm originates over the rock ther-

mal alteration processes, such as melting of the con-

stituent calcite crystals. These effects are apparent in

both untreated (Fig. 5m) and treated stone samples

(Fig. 5n and o). The measured ablation threshold

of Colmenar limestone at 1064 nm is 3.0 J/cm2.

Although the fluence used for irradiation at this

wavelength, 2.3 J/cm2, is below this value, it may

be high enough to alter the stone crystals. Addition-

ally, the high number of pulses applied on the same

spot (1800) may lower the threshold for ablation

through incubation effects, and contribute to the

observed damage. The conditions of 1064 nm irradia-

tion are effective for the removal of the coatings, even

if the laser fluence is below their ablation threshold,

but the original stone surface is exposed to laser

fluences that are too high for preserving the integrity

of the constituent crystals, causing significant altera-

tion processes.

As shown in Fig. 6e and f, irradiation at the con-

ditions of the shorter wavelength, 266 nm, barely

modify the appearance of the coated stone surface.

Laser irradiation is inefficient for treatment removal,

although partial elimination of the HL-100 coating is

observed (Fig. 6f). Irradiation at 266 nm was made at a

fluence of 0.2 J/cm2, below the threshold of ablation of

both the stone and the coatings. Irradiation under these

conditions, even with a high number of laser pulses,

does not alter the untreated stone, but appears to be

somewhat effective in the elimination of the thin HL-

100 layer.

Irradiation at the intermediate wavelengths of 355

and 532 nm of the samples protected with PB-72, thins

down the treatment (Fig. 5h and k). The thickness of

the protective layer is clearly reduced after irradiation,

Fig. 6. SEM images of transversal sections of limestone samples

coated with (a) PB-72 and (b) HL-100 treatments. The measured

thickness is 3.9 and 1.7 mm for PB-72 and HL-100, respectively.

M. Gomez-Heras et al. / Applied Surface Science 219 (2003) 290–299 297

but it is not completely eliminated. However, irradia-

tion at these two wavelengths of the surfaces coated

with HL-100, results in the partial elimination of the

treatment with little apparent damaging effect to the

stone, according to the SEM images.

The operational mechanisms governing the nano-

second laser ablation of polymeric materials that

constitute the water repellent treatments should be

most importantly determined by the laser wavelength.

In UV laser ablation, electronic excitation results

either in the breaking of polymeric bonds, through

a direct photochemical process (photochemical

model), or thermalisation on picosecond time scales

yielding thermally broken bonds (photothermal

model) [28,29]. IR laser ablation should be preferably

governed by thermal mechanisms, as the photon

energy is not sufficient for electronic excitation.

The results presented here show that irradiation in

the UV at 266 and 355 nm is a clean etching process

leading to partial or to complete elimination of the

treatments, suggesting the dominance of photochemi-

cal mechanisms operating at these wavelengths. As

mentioned before, the fact that removal is effected at

laser fluences below the laser ablation threshold of the

polymeric coating, indicates the participation of incu-

bation processes [30], that transform the material,

through repetitive pulse irradiation, in a strongly

absorbing matrix.

4. Conclusions

Laser removal of PB-72 and HL-100 water repellent

treatments, applied on Colmenar limestone, does not

feature a self-limiting effect at any of the four wave-

lengths of the Nd:YAG laser, the ablation threshold of

the layer of coating being above the threshold of the

bare stone. Determination of safe conditions for treat-

ment removal, ensuring no damage to the stone sub-

strate, has to be performed through careful assessment

of the effects of laser irradiation. By choosing irradia-

tion fluences below the ablation threshold of the lime-

stone and a high number of pulses, we have found that

at the intermediate wavelengths of this study, specially

at 355 nm, the layer of coating is thinned down or even

removed in the case of the HL-100 treatment. Colori-

metric and roughness measurements demonstrate

that laser irradiation under these conditions make

the coated surface to recover the characteristic colour

and roughness parameters of the bare stone, while

SEM measurements indicate that thermal alteration of

the constituent calcite crystals is the main conse-

quence of damage to the underlying substrate. These

results show the potential of the third harmonic of the

Nd:YAG laser for the removal of water repellent

polymeric materials applied on stone. Further studies

on different types of building stone are under way.

Acknowledgements

Support from Project BQU2000-1163-C02-01 is

gratefully acknowledged. Thanks are given to Minis-

try of Culture and Education, Spain (MG), Comunidad

de Madrid (MO) and the Thematic Network on Cul-

tural Heritage of CSIC (MA and ER) for fellowships.

Dr. A. Duran (ICV, CSIC) is acknowledged for the use

of the profilometer. SEM measurements were carried

out at Centro de Microscopıa Electronica Luis Bru of

Universidad Complutense, Madrid.

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