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Journal of Cultural Heritage 9 (2008) 269e276http://france.elsevier.com/direct/CULHER/

Original article

Effect of the impregnation treatment with Paraloid B-72 on theproperties of old Portuguese ceramic tiles

M.F. Vaz a,*, J. Pires b, A.P. Carvalho b

a Instituto de Ciencia de Materiais e Superfıcies and Departamento de Engenharia de Materiais, Instituto Superior Tecnico, Av. Rovisco Pais,

1049-001 Lisboa, Portugalb Departamento de Quımica e Bioquımica and Centro de Quımica e Bioquımica, Faculdade de Ciencias da Universidade de Lisboa, Campo Grande Ed. C8,

1749-016 Lisboa, Portugal

Received 11 May 2007; accepted 4 January 2008

Abstract

In this work, the effect of the impregnation with Paraloid B-72, using the protocol commonly followed in museum restoration departments,on the mechanical and water absorption properties of Portuguese tiles from XVI to XXth centuries, was studied. Mineralogical characterizationof the biscuit was made from X-ray diffraction patterns. Microstructural features (pore size) were determined using scanning electron micro-scope (SEM) photographs. Mechanical strength was determined with four-point bending tests. The absorption coefficient and the total amountof water retained were obtained from water absorption essays, which also allowed the estimation of the open porosity.

The impregnation treatment, in general, increases the mechanical resistance, reduces the porosity and modifies the water absorption behav-iour of the tiles.� 2008 Elsevier Masson SAS. All rights reserved.

Keywords: Ancient tiles; Paraloid B-72; Impregnation; Mechanical properties; Porosity; Water absorption

1. Research aims

During the last few decades, the preservation of culturalheritage has merited the attention of scientists, architects andengineers. In this sense, in the literature there are a lot ofworks focused on the conservation and rehabilitation of build-ing materials [1e5]. Nevertheless, relatively modest attentionhas been given to the study of ancient tiles. In fact, there areonly few studies centred on the characterization of the ceramicbody (biscuit) of antique tiles using different approaches andanalytical techniques [6e10]. The analysis of the glazed layerof tiles, also an important aspect of the characterization ofthese artefacts, was recently the object of some studies[11,12]. A less common approach to the characterization ofbiscuit porosity, made on Portuguese tiles from the XVIth to

* Corresponding author. Tel.: þ351 218 418 106; fax: þ351 218 418 132.

E-mail addresses: fatima.vaz@ist.utl.pt (M.F. Vaz), jpsilva@fc.ul.pt

(J. Pires), ana.carvalho@fc.ul.pt (A.P. Carvalho).

1296-2074/$ - see front matter � 2008 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.culher.2008.01.003

XXth century, using low-temperature nitrogen adsorption[13], is also worth mentioning.

Although decorative tiles are an important part of Portu-guese architectural heritage, as they have been used withoutinterruption since the XVth century [14,15], the available in-formation about the development of their characteristics overthe centuries is scarce. In a recent study [16], the present au-thors made an analysis of the microstructural, mechanical,mineralogical and water absorption properties of samples ofPortuguese tiles from the XVIth to XXth centuries. In the pres-ent work, the same set of tiles was impregnated with ParaloidB72 and the effect of the treatment was evaluated on the mi-crostructural, mechanical, and water absorption properties. Itis worth mentioning that, to our knowledge, a study of the ef-fect of consolidation treatment on tiles covering five centuries,presented herein, had not been reported in literature. The re-sults obtained allowed the evaluation of the effect on the im-pregnation of tiles with Paraloid B72, following therestoration procedures commonly used in museums. In fact,

270 M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

the impregnation procedure used in this work was identical tothe one followed by the ‘‘Museu Nacional do Azulejo’’.

Paraloid B72 is an acrylic co-polymer of ethylmethacrylateand methylacrylate (70/30) used frequently in the consolida-tion treatment of tiles. Acrylic polymers and co-polymersare used as protective coatings due to their non-wettability,chemical inertness and environmental stability [17]. Theseproperties have lead to an extensive use of these polymericmaterials in cultural heritage conservation, since the secondhalf of the 1960s. They have been applied to artwork restora-tion as consolidants and/or protectives for, amongst others,stones and porous materials [6,13,16].

2. Introduction

In Portugal, the oldest traces of ceramic glazed productswere found in ‘‘Mosteiro de Alcobaca’’ and date back to theXIIIth century [6]. The products were mosaics used in floordecoration. Mosaics are one of the most durable forms of dec-orative art and were first found in Mesopotamia and spread toEurope (Greece, Italy and Spain) and to the north of Africa(Libya and Syria) [18]. A mosaic is formed by small pieces,with a ceramic body of stone, terracotta, marble or paste, plas-ter layers and a glazed surface [14,15].

In the XIVth century the greatest ceramic producer on theIberian Peninsula was the Arabic kingdom of Granada (Spain).The ceramic products with several coloured glazed coatingswere cut into small pieces, turning into mosaics, and wereplaced on the walls or pavements as geometric drawings.This technique, which is still used in Morocco, requires highlyskilled work. In Portugal, no wall or pavement ceramic deco-rations belonging to the XIVth century have been found [6].

During the XVth century, in Seville, the production of mo-saics was replaced by the manufacturing of tiles. At that time,tiles started to have the same meaning as today. Tiles aresquare ceramic plaques glazed on one side, and have larger di-mensions than mosaics. During the XVth century, the motifswere geometric ones. The techniques used to produce tileswere the ‘‘cuerda seca’’ and the ‘‘cuenca’’. Hispano-Moorishtiles imported from Seville, dating from 1503, have beenfound in the ‘‘Palacio Real de Sintra’’, and in other palacesin Lisbon and Coimbra [6,14,15].

At the beginning of the XVIth century, a new techniquecalled ‘‘majolica’’ was introduced in Italy. This technique con-sists of a first step of firing the ceramic body, painting and fir-ing a second time [19]. This method made possible the use ofnew colour pigments and allowed a variety of figurative andhistorical themes. It was Italian ceramists, who had movedto Flanders, who introduced this new technique to that region.The XVIth century tiles found in Portugal were imported notonly from Spain (Seville and Valencia), but also from Italy andFlanders. After several Portuguese orders to artists in Flanders,some Flemish ceramists decided to establish themselves inLisbon. The first Portuguese tile manufacturing factoriesdate from the second half of the XVIth century and were man-aged by ceramists who came from Antwerp.

From the beginning of the XVIIth century, Portugal hadproduced standard-pattern tiles and therefore did not need toimport tiles. During this century there was widespread useof tiles all over the country. They were applied to both the in-terior and the exterior of buildings, in the covering of facadesof churches, palaces and gardens. These tiles represented hunt-ing, battles, religious and mythological scenes [14,15].

Due to Dutch influence in the late XVIIth century, the poly-chromatic paintings were substituted by paintings in white andblue [20,21]. This tendency, inspired by Chinese porcelain,would last until the first half of the XVIIIth century.

After the earthquake of 1755, which left Lisbon partially de-stroyed, the need to reconstruct led to the large-scale productionof tiles. Tiles were then much simpler and cheaper; paintingscould be applied either mechanically or manually. This manualwork still continues nowadays in some factories. At the end ofthe XVIIIth century, the Church remained faithful to the reli-gious themes. The aristocracy, reflecting the influence of theNeoclassicism, preferred landscapes in the design of tiles.

In the XIXth century, due to the rise of the bourgeoisie, a newuse was found for tiles. All over the country, tiles were used tocover the facades of ordinary buildings. During the XIXth andthe XXth centuries, tiles would follow the aesthetical tenden-cies in Art, such as Art Nouveau and Art Deco [14,15].

At present, apart from the general use of tiles in the interiorand the exterior of buildings, one can find tiles all over thecountry. The Lisbon Underground exhibits tile panels createdby artists, such as Maria Helena Vieira da Silva, Julio Pomar,Manuel Cargaleiro, Sa Nogueira and Eduardo Nery.

The importance of tiles in Portuguese cultural heritage isalso documented in the collection of ‘‘Museu Nacional doAzulejo’’ (‘‘azulejo’’ is the Portuguese word for tiles). Thismuseum has gathered one of the largest collections of tiles(from the XVth to the XXIth centuries), and gratefully pro-vided the samples used in the present study. The samples givento us by the Museum were considered, by the experts, as beingrepresentative of each century as they were the most wide-spread in the country during each century.

3. Experimental

Five samples of Portuguese tiles from the XVIth to XXthcenturies of different origins were studied (Fig. 1). Sampleswill be named by their century. In general, the samples didnot present serious degradation and, with the exception ofsample XVI, the glazed surface was not detached from the ce-ramic body. The thickness of the tiles has changed during thecenturies, decreasing from 20 mm for sample XVI to 8 mm forsample XX. It must also be noted that, in the case of the sam-ples XVI, XVII and XVIII, the thickness of the ceramic bodywas not uniform.

Tiles were cut into pieces of approximately 120 mm �27 mm � h, where h is the tile thickness. Impregnation treat-ments were made with a 10% Paraloid B-72 (a co-polymerof ethylmethacrylate and methylacrylate) solution in acetone,following the protocol used in ‘‘Museu Nacional do Azulejo’’.Briefly, the samples were dried overnight at 105 �C in an oven

Fig. 1. Samples of the tiles used in the present work: (a) XVIth century, (b) XVIIth century, (c) XVIIIth century, (d) XIXeXXth century and (e) XXth century.

271M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

with forced ventilation, after which they were weighed. Thespecimens were firstly immersed in acetone, until no releaseof bubbles was observed (typically 30 min), and then im-mersed in the Paraloid B-72 solution for 3 h. It must be notedthat the samples were placed in a way that only the ceramicbody was in contact with the impregnation solution. After dry-ing at room temperature, the samples were submitted to a sec-ond treatment, in the same conditions as mentioned above.After drying for the necessary time to obtain a constantweight, the final weight of the sample was determined. Theamount of polymer retained per gram of sample was around2.5 � 10�2 g for all the samples, except for the tile of theXIXeXXth century where a lesser amount of polymer wasimpregnated (1.3 � 10�2 g).

The mineralogical composition of the ceramic body of thetiles (crushed to powder in an agate mortar after removingthe glaze) was made from X-ray diffraction (XRD) patternsrecorded on a Philips PW 1710 diffractometer using CuKa(l ¼ 1.5406 A) graphite-monochromatized radiation. Patternswere obtained by step scanning from 15� to 50� 2q, witha step size of 0.05� 2q, a time per step of 0.5 s and 40 kVand 30 mA in the X-ray tube.

Samples, before and after the impregnation treatment, wereobserved under a scanning electron microscope (SEM) (ModelS2400, Hitachi) using electron secondary beams at 20 kV. Im-ages acquired by electron microscopy allowed the microstruc-tural characterization of the tiles with the evaluation of thepore fraction and the pore size distribution. The area of thepores was measured using commercial image analysis soft-ware (SigmaScan Pro 5 [22]). Five photographs were analysedper sample and at least 50 measurements per photograph weremade, which makes a total of 250 measurements of pore areaperformed in each type of tile. The pore fraction was obtainedby the ratio of the area occupied by the pores, i.e., the sum ofpore areas, to the total area of the sample. The pore size dis-tribution is presented through histograms where the frequency,in percentage, of the pores area is plotted against the intervalsof areas.

To evaluate the flexural strength of treated and untreatedtiles, four-point bending tests (inner span of 40 mm and outerspan of 80 mm) were performed. The tests were made in a uni-versal testing machine (Model 4302, Instron Corporation,Canton, USA). The cross-head speed was 0.5 mm min�1 andwe used a load cell of 10 kN. Only three tests were made

272 M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

for each of the five samples, before and after the impregnationtreatment. It is common to perform at least 20 tests for eachcondition and to analyse it using the Weibull statistics [23];however, the nature of the materials assayed (ancient tiles)did not allow us to have such a large number of samples.

The water absorption tests were made at room temperatureon untreated and treated samples, following the UNI 10921/2001 specifications [24]. Before the tests the specimens weredried overnight at 105 �C. After cooling to room temperatureand weighing, the samples were immersed in distilled waterand periodically removed, carefully dried with a lint-freecloth, and weighed in a Metler AE 240 analytical balance.The total immersion time was, typically, 20 h.

4. Results

4.1. Mineralogical composition

The XRD patterns presented in Fig. 2 show the differencesin the mineralogical composition of the ceramic body of thesamples, pointing out possible differences in the raw materialsused in the manufacture of the tiles and also the developmentof the firing technology over the centuries. In fact, during thefiring stage the minerals presented in the raw material sufferwhat is commonly called ‘‘industrial metamorphism’’, i.e.,the development of neoformed mineral phases at the expensesof existing ones. Knowing the temperature at which a newmineral phase is formed, we can have an indication of the tem-perature attained in the kiln. As regards to the raw materials, itis known that until the XIXth century the paste was clay (fromone or several origins) and afterwards it began to be a mixtureof clay and quartz, which improved the mechanical propertiesof the tile [6].

Considering the patterns displayed in Fig. 2 the samplescan be assembled in three groups: (i) sample XVI; (ii) samplesXVII, XVIII and XIXeXX; (iii) sample XX. Quartz

Fig. 2. Scanning electron micrographs of samples before consolidation: (a)

XVI, (b) XIXeXX; and after consolidation: (c) XVI, (d) XIXeXX.

(SiO2, ICDD: 33e1161), calcite (CaCO3, ICDD: 5e586), geh-lenite (SiO2$Al2O3$2CaO, ICDD: 35e755) and diopside(CaMgSiO6, ICDD: 11e654) are identified in the biscuits ofthe samples from XVIth to XIXeXXth centuries. The compar-ison of the intensities of the diffraction peaks corresponding tocalcite and gehlenite shows that this mineral phase, formed attemperatures around 800 �C [1,9,25] and at the expense of cal-cite, became an important component of the ceramic materialsobtained after the XVIth century. On the other hand, traces ofmulite (3Al2O3$2SiO2, ICDD: 15e776) are identified in thepatterns of the samples from XVII to XXth centuries, suggest-ing that the firing technology available could attain tempera-tures above 900 �C. Besides the mentioned mineral phases,in the pattern corresponding to the biscuit of the sampleXVI, traces of feldspars (albite, NaAlSi3O8, ICCD: 10e393;anortite, CaAl2Si2O8, ICCD: 41e1486) are also identified.

The presence of calcite in the samples XVI, XVII, XVIIIand XIXeXX reveals that a calcareous paste was used. Thisis not an unusual finding since the positive effect of carbonateson the properties of ceramics has been known since Romantimes. The presence of this mineral also reveals that the firingprocedures did not permit the total decomposition of calcite.Identical results are reported in the literature, not only forXVIIth century Portuguese tiles [6,26] but also for Spanishtiles from XVIIeXVIIIth centuries [9]. It must be stressedthat studies where the mineral changes of the ceramic pasteswith different experimental parameters of the firing procedurewere monitored revealed that the transformations that occur inthe firing step are dependent, not only on the temperature at-tained, but also on the duration of the firing treatment andalso on the rate of temperature increase [1,9,25]. Moreover,as reported by Riccardi et al. [25], the bulk composition ofthe paste is also a driving force for the formation of new min-eral phases, such as, for instance, gehlenite, favoured by rawmaterials having minerals with high contents in Ca in theircomposition.

The pattern of the sample XX is quite different from all theothers, since quartz is practically the only phase present. Thisis most probably due to advances in technological procedures,such as the firing temperature, leading to an extensivevitrification.

4.2. Microstructure

Samples before and after the consolidation treatments wereobserved by SEM and the images obtained are exemplified inFig. 3 for the samples XVI and XIXeXX. All tiles exhibita typical porous ceramic structure. The simple analysis ofFig. 3a and b allows us to infer that the tile of the XVIth cen-tury has higher porosity than the sample XIXeXX. The struc-ture of treated samples (Fig. 3c and d) is more homogeneousand shows that the co-polymer deposition reduces the porosity.

The pore fraction of the untreated and treated samples isgiven in Table 1. In all the cases, the pore fraction decreasesafter the impregnation treatment. The decrease of the porefraction is 25% in the less favourable case (sample XVI)and reaches 60% in the more favourable one, that is, sample

Table 1

Results of image analysis and mechanical properties of the samples before and

after the consolidation treatment

Sample Pore fraction (%) sf (MPa)

Untreated Treated Untreated Treated

XVI 24.03 � 3.71 18.07 � 2.64 7.38 � 1.21 9.70 � 1.48

XVII 23.17 � 1.12 13.53 � 1.79 10.25 � 1.62 9.28 � 0.32

XVIII 16.86 � 1.09 10.15 � 1.61 2.09 � 0.31a 9.69 � 0.84

XIXeXX 17.32 � 2.00 6.71 � 0.94 11.56 � 1.12 20.84 � 0.98

XX 4.16 � 1.63 2.79 � 0.30 9.74 � 1.79 14.88 � 0.21

a The glazed surface did not break.

273M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

XIXeXX, which, curiously, is the sample where a lesseramount of polymer was retained.

Fig. 4 shows the histograms corresponding to the pore sizedistribution of all samples before and after treatment. All thedistributions of the impregnated samples are shifted to smallerpore sizes, when compared to the distributions of the untreatedtiles. In the XVIth century samples (Fig. 4a), both treated anduntreated samples exhibit the maximum of the distribution forthe same pore size (pore area between around1.75 � 10�6 mm2). In the rest of the samples (Fig. 4bee)the pore size of the maximum frequency of the histograms cor-responding to treated and untreated samples do not coincide.On the other hand, the width of the pore size distribution ofuntreated and treated tiles from the XVIth and XVIIth centu-ries is almost the same, although for the samples XVIII, XIXeXX and XX a narrow pore size distribution is obtained afterthe treatment. From the results of pore fraction and pore sizedistribution, we may conclude that after the impregnationthe samples present fewer and smaller pores.

4.3. Mechanical properties

Fig. 5a shows a four-point bending set-up. Fig. 5b presentstwo loadedisplacement curves obtained with, respectively,treated and untreated XIXeXXth century tiles.

The fracture load Fmax, i.e. the maximum load, is used todetermine the bending strength sf with equation:

sf ¼3

2

FmaxL

bh2ð1Þ

Fig. 3. X-ray diffraction pa

where b and h are the cross-sectional dimensions and L is thedistance between support points. Table 1 indicates the averageand the standard deviations of three tests per sample of thebending strength sf, for samples before and after the consoli-dation treatment.

The bending strength increases with the consolidation treat-ment, except for the XVII tiles for which the value of sf re-mains almost the same before (sf ¼ 10.25 MPa) and after(sf ¼ 9.28 MPa) the treatment. The largest increase in sf,when we compare treated and untreated tiles, occurs in thesamples XIXeXX and XX. It seems then, that in regard ofthe mechanical resistance, consolidation was more effectivein XIXeXX and XX tiles. This is most probably a direct effectof the reduction in porosity due to impregnation, which in thecase of these samples led to considerably smaller pore sizefraction than that presented by the other specimens. However,it must be stressed that the impregnation treatment and the re-lated reduction of the porosity are not the only factors that af-fect the bending resistance. For example, the presence of

tterns of the samples.

Fig. 4. Pore sizes distribution for all untreated (filled bars) and treated samples (open bars).

274 M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

cracks and their propagation during the tests also influence themechanical behaviour of the samples.

4.4. Water absorption tests

The results of water absorption assays in tiles before andafter the impregnation treatment are exemplified for theXXth century sample in Fig. 6, where the weight differencebetween dried and wet samples per unit area of the ceramicbody is plotted against the square root of the immersiontime. The absorption coefficient, A, was estimated from theslope of the initial part of the curve (typically until 2 min).These values are quoted in Table 2, as well as the maximumwater content, CI, defined as the water saturation amount rel-ative to the weight of the dried sample. Open porosity (op) wasevaluated according to the equation:

op¼ ðwsat�wdriedÞ=dwater

Vsample

ð2Þ

where wsat and wdried are the weights of, respectively, the watersaturated and dried sample, dwater is the water density, at theworking temperature, and Vsample the volume of the sample.

The results displayed in Fig. 6 clearly show that the impreg-nation with Paraloid lead to important modifications in the be-haviour of the samples as far as water absorption is concerned.In fact, the curve obtained with the treated sample shows thereduction of the sample porosity and an increase of the hydro-phobic character of the biscuit as a consequence of the impreg-nation treatment with Paraloid. This treatment leads toa partial pore blockage and, at some extension, changes thechemical nature of the surface. The increase of hydrophobiccharacter is expressed by the values of the absorption

Fig. 5. (a) Four-point bending test in a XVIIIth century tile and (b) load-dis-

placement curves of consolidated and non-consolidated XIXeXXth century

tiles.

Table 2

Results of water absorption tests of the samples before and after the consoli-

dation treatment (the error associated to data is less than 1%)

Sample CI (%) A (g cm�2 s�1/2) Open porosity (%)

Untreated Treated Untreated Treated Untreated Treated

XVI 15.5 3.9 8.4 � 10�3 1.0 � 10�4 27 7

XVII 17.0 18.3 12.5 � 10�3 1.0 � 10�4 28 28

XVIII 16.2 14.0 9.0 � 10�3 3.0 � 10�4 28 28

XIXeXX 12.9 10.0 4.0 � 10�3 2.9 � 10�3 22 22

XX 11.2 5.9 6.3 � 10�3 3.3 � 10�5 22 11

275M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

coefficient, A. In fact, the absorption coefficient for treatedspecimens is one or two order(s) of magnitude smaller thanthe value obtained with the untreated tiles. Only for untreatedand treated XIXeXXth century samples the absorption coeffi-cient has the same order of magnitude. However, the value ofA for the impregnated sample is half of the untreated samplevalue.

Analysing the values of the maximum water content ob-tained for the untreated samples, we observe that this param-eter remains almost constant for samples produced up to theXVIIIth century and decreases for the two more recent tiles.The impregnation treatment increased the water protection

Fig. 6. Water absorbed normalized by the area of the ceramic body as a func-

tion of the square root of the immersion time for untreated (closed symbols)

and treated (open symbols) XXth century samples.

efficiency of the tiles, except in the case of the sampleXVII, where no variation of the CI value was observed asa consequence of the treatment.

In general, the open porosity values obtained from waterabsorption are always larger than those estimated from imageanalysis; the only exception is the sample XVI after impregna-tion with Paraloid. Confronting the values of untreated andtreated samples we conclude that only for the specimensXVI and XX there is a decrease in the open porosity assessedby water absorption as a consequence of the impregnation. Forthe other samples (XVII, XVIII and XIXeXX) no alteration inthe open porosity was detected by these measures.

Apparently, the protective effect of the impregnation treat-ment with Paraloid against water absorption was less effectivefor the tiles from the XVIIth, XVIIIth and XIXeXXth centu-ries than for the samples XVI and XX. This may be a probableconsequence of the formation of a less coherent polymer filmin these samples, eventually induced by less favourable inter-actions between the surface of the tiles and the polymer. Infact, as Fig. 2 denotes, these three samples have similar min-eralogical composition that differs from those observed in thetiles from XVIth and XXth centuries. These issues, which arerelated to the spreading of the polymer phase itself in the bis-cuit and, therefore, with the surface tension at the polymerebiscuit surface, are intended to be addressed in future studies.

5. Conclusions

The results reported in the present work allowed us to quan-tify the efficiency of the impregnation with Paraloid B-72, us-ing the protocol commonly followed in museum restorationdepartments, on the microstructural, mechanical and water ab-sorption properties of Portuguese tiles from the XVIth to XXthcenturies.

The set of samples used present different mineralogicalcompositions, in agreement with the manufacturing and tech-nological developments throughout the centuries. Sample XXpresents the most different composition, essentially withquartz and mulite. The mineral composition of the tilesmade in earlier centuries is more complex, showing the pres-ence of quartz, calcite, gehlenite and diopside, the latter onesresulting from the reaction between silica and calciumemag-nesium phases existing in the clay paste.

In general, the impregnation treatment promoted an in-crease of the mechanical resistance of the tiles, which ismore pronounced in the XIXeXXth and XXth centuries tiles.

276 M.F. Vaz et al. / Journal of Cultural Heritage 9 (2008) 269e276

The pore fraction and pore size distribution obtained from im-age analyses show that the treatment leads to a reduction of theporosity associated with the shift in the porous size distribu-tions towards smaller pores.

The impregnation also leads to important modifications in thewater absorption behaviour of the samples. In fact, the water ab-sorption coefficient (amount of water absorbed by the surfaceunit normalized by the square root of time) reduces considerablyas a consequence of the impregnation. The main reduction of wa-ter absorption coefficient is observed for the sample XX where thevalue of 6.3 � 10�3 g cm�2 s�1/2, obtained for the untreated sam-ple, decreases to 3.3 � 10�5 g cm�2 s�1/2 for the impregnatedtile. Except for sample XVII, the total amount of water absorbedby immersion also decreases after the treatment with Paraloid,which is especially important for the samples XVI and XX.

The results obtained allow us to conclude that, in general,the impregnation procedure used in ‘‘Museu Nacional do Azu-lejo’’ accomplishes its objectives, that is, the tiles possesshigher mechanical resistance and the biscuit gains hydropho-bic characteristics. Nevertheless, various exceptions to thisgeneral trend were observed, which can be related to the smallnumber of samples that could be used.

Acknowledgements

The authors would like to thank to the ‘‘Museu Nacional doAzulejo’’ of Lisbon, in particular to Dr Paulo Henriques (Di-rector) and Dra Lurdes Esteves (Conservator) for the samplesand technical information, and to Dr Pedro Amaral (IST) forthe mechanical tests.

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