aterials Science Forum Vols. 514-516 (2006) pp. 1648-1652 online at http://www.scientific.net (2006) Trans Tech Publications, Switzerland Online available since 2006/05/15 Characterisation of Ceramic Pastes of Portuguese Ancient Tiles A. P. Carvalho 1,a,. Fátima Vaz 2,b,. J. Samora 1,c and J. Pires 1,d 1 Departamento de uímica e Bioquímica and CB, Faculdade de Ciências da Universidade de Lisboa, Campo Grande C8,1749-016 Lisboa, Portugal. 2 Instituto de Ciência de ateriais e Superfícies and Departamento de Engenharia de ateriais, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal a ana.carvalho@fc.ul.pt, b fatima.vaz@ist.utl.pt, c mjsamora@netcabo.pt, d jpsilva@fc.ul.pt Keywords: Tiles, Water Absorption, X-ray Diffraction, icrostructure and echanical Properties Abstract. Portuguese ceramic tiles of different origins from XVIth century to XXth century were studied. In this work we describe microstructural, mechanical and mineralogical characterisation and water absorption studies. icrostructural features (pore size) were determined using Scanning Electron icroscope (SE) photographs. echanical tests (four point bending) were performed and the bending strength was determined from the fracture loads. From water absorption essays the absorption coefficient and the total amount of water retained were obtained and the open porosity was estimated. The maximum water absorbed tends to decrease from XVIth century until XXth century, showing a more pronounced drop for the tiles of the XIXth and XXth centuries. This evolution is also observed in the open porosity values. From image analysis data we also observed that porosity decreases progressively with time and an important drop is observed for XXth century tiles. The bending resistance of tiles from XVIIth to XXth century is almost constant and higher than that of XVIth century samples. This evolution does not correlate with porosity. Advances in ceramic processing, such as higher firing temperatures may lead to the appearance of quartz as almost the only crystalline phase. We consider that an increase in the bending strength is due to a more effective vitrification and a lower porosity. Introduction During the last decades, heritage preservation has drawn the attention of scientists, engineers and architects due to interdisciplinary character of this subject. Decorative tiles are an important part of Portugal architectural heritage, as they have been widely used since the XVth century. Although many works have been published regarding the preservation/conservation and rehabilitation of building materials [1-6], little attention has been given to tiles. However, some studies, restricted to XVIIth century tiles, used X-ray powder diffraction, ion chromatography and scanning electron microscopy to characterise the chemical and mineralogical composition [7-9]. To our knowledge a broaden study of tiles characterisation from the XVIth to XXth tiles was only focused on porosity [10]. In the present work we intended to make microstructural, mechanical, mineralogical and water absorption properties characterisation extended to some samples of tiles, from XVIth to XXth centuries, belonging to the collection of useu Nacional do Azulejo. Experimental Six samples of Portuguese decorative tiles from XVIth century to XXth century of different origins were studied. The samples are designated by their century, followed by a, or b to distinguish different specimens corresponding to the same century. Tiles are formed by a ceramic body covered by a glazed surface. With the exception of one of the tiles from XVIth century all the samples All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 193.136.128.7-05/11/07,15:22:22)
aterials Science Forum Vols. 514-516 1649 present glazed surfaces in a good state of conservation, although the thickness of the ceramic body was not uniform in most of the cases. The microstructural characterisation included the determination of the pore fraction and the pore size distribution. The pore fraction was determined through the ratio of the area occupied by the pores to the total area measured. These morphological aspects were obtained from images acquired by electron microscopy. Observations were carried out on a scanning electron microscope (odel S2400, Hitachi) using electron secondary beams at 20 kv. The area of the pores was directly measured using a commercial image analysis software (ocha [11]). A minimum of 200 measurements was carried out for each type of tile. Four point bending tests (inner span of 40 mm and outer span of 80 mm) were performed in a universal testing machine (odel 4302, Instron Corporation, Canton, USA). We used a load cell of 10kN and the cross-head speed was 0.5 mm/min. Each tile was cut into three pieces of approximately 120mm 27mm h, where h is the tile thickness. The thickness of tiles changes along the centuries and can vary between 8 (XXth century) and 20mm (XVIth century). We performed 3 tests in each of the six samples. Fig. 1a) shows a four bending test. (a) (b) 1000 750 Load (N) 500 250 0 0 0.1 0.2 0.3 0.4 0.5 Displacement (mm) Fig. 1.a) Four point bending test in a XVIIIth century tile and b) the load-displacement curve of a XVIIth century tile. The mineralogical composition of the ceramic body of the tiles (crushed to powder after removing the glaze) was made from X-ray diffraction (XRD) patterns obtained on a Philips PX 1820 diffractometer using CuKα radiation. The water absorption tests were carried out at room temperature on samples with dimensions of 30mm 30mm h, where h is the tile thickness. Before the tests, the specimens were dried at 105ºC. After weighting, the samples were immersed in distilled water and periodically removed, carefully dried with a lint free cloth, and weighed in a ettler AE 240 analytical balance. The total immersion time was typically 20h. Results Fig. 2 shows the SE micrographs of two ceramic tiles (samples XVIa and XIX-XX). Data of the pore fraction obtained with image analysis is given in Table 1. We observed that porosity is slightly higher than 20% for the XVIth and XVIIth century tiles, decreases to 17% for the XVIIIth and XIX- XXth centuries and an important drop is observed for XXth century tiles. The pore size distribution (not shown) reveals almost the same peak values and the same distribution along the various centuries.
1650 Advanced aterials Forum III a) b) Figure 2- Scanning electron micrographs of a) sample XVIa and b) sample XIX-XX. In Fig 1b) we present a load displacement curve of a bending test. We used the point where the load reached its maximum, the fracture load Fmax, to determine the bending strength σf with equation: σf = 3 Fmax L 2 bh 2 (1) where L is the distance between support points and b and h are the cross-sectional dimensions. In Table 1 we indicate the average and the standard deviation of the three tests bending strength σf for each of the six types of tiles used. The bending resistance of tiles from the XVIth century samples is the lowest one. Nevertheless, from the XVIIth to the XXth century there seems to be no variation of the bending resistance, as σf is almost constant. During the bending test we observed the presence of cracks and at the crushing point, propagation of these defects lead to the failure of the tile. XRD patterns (exemplified in Fig. 3) reveal differences in the mineralogical composition of the ceramic body of this set of samples. In samples from XVIth to XIX-XXth centuries the presence of quartz, feldspars and calcite are identified, as well as phases formed during the firing step of tiles production. These phases correspond to gehlenite and diopside, resulting from the reaction between silica and calcium-magnesium phases existing in the clay paste [1]. A semi-quantitative analysis indicates that quartz and calcite predominate in pieces from the XVIth and XIX-XXth centuries whereas diopside and, specially, gehlenite are the most abundant phase for tiles of XVIIth and XVIIIth centuries. The pattern of the sample XX is quite different from all others, since quartz is practically the only phase present. This is most probably due to advances in technological procedures, such as the firing temperature, leading to an extensive vitrification. In Fig. 4 the results of water absorption essays are exemplified for tiles XVIIth and XIX-XXth centuries, representing the weight difference between dried and wet samples per unit area of the ceramic body as a function of the square root of the immersion time. The absorption coefficient, A, was estimated from the slope of the initial part of the curve (typically until 2 min.). These values are displayed in Table 1, as well as the maximum water content, CI, defined as the water saturation amount relative to the weight of the dried sample. Open porosity was evaluated according to the equation: op = ( wsat wdried ) / d water Vsample (2)
aterials Science Forum Vols. 514-516 1651 where w sat and w dried are the weights of, respectively, the water saturated and dried sample, d water is the water density, at the working temperature, and V sample the volume of the sample. 0.18 0.16 XVII a.u. G G C D A G G C D G A G D D - uartz G - Ghelinite C - Calcite D - Diopside - ullite A - Albite 15 20 25 30 35 40 45 50 2θ (degrees) D D c (w-wdried)/a (g cm -2 ) 0.14 0.12 0.1 0.08 0.06 b 0.04 C C 0.02 a C C 0 XIX-XX 0 10 20 30 40 50 60 t (min 1/2 ) Fig. 3. XRD patterns of samples XVIa (a), XVII(b) and XX (c). Fig. 4. Weight difference between dried and wet samples per unit area of the ceramic body as a function of the square root of the immersion time. Table 1- Results of image analysis, mechanical and water absorption tests. Sample Pore fraction σ f [Pa] A [g cm -2 s -1/2 ] CI Open porosity XVIa 24.03±3.71 7.38±1.21 8.4 10-3 15. 5 27 XVIb 21.51±1.96 8.97±1.63 4.4 10-3 16.2 32 XVII 23.17±1.12 10.25±1.62 12.5 10-3 17.0 28 XVIII 16.86±1.09 2.09±0.31* 9.0 10-3 16.2 28 XIX-XX 17.32±2.00 11.56±1.12 4.0 10-3 12.9 22 XX 4.16±1.63 9.74±1.79 6.3 10-3 11.2 22 *the glazed surface did not break The maximum water content is almost constant for samples produced until the XVIIIth century and decreases for the two more recent tiles. The open porosity values obtained from water absorption are always larger than those estimated from image analysis (the major difference is for XXth century) and show a less pronounced variation along the centuries. Discussion and Conclusions For most ceramic porous materials very complex relationships between microstructure and mechanical properties are observed. In the case of the studied tiles a straight correlation between porosity and bending strengths was not observed. However if we compare the bending strength of tiles of XVIth and XIX-XXth centuries which present similar mineralogical composition we are able to conclude that an increase in porosity (assessed by image analysis and water absorption) favours a decrease in the bending strength. The result of the XXth century sample reflects an extensive vitrification, as shown in the X-ray powder diffraction patterns (Fig.3 c) where quartz is the only phase present. This is most probably due to advances in processing conditions, namely, higher firing temperatures. A combination of lower porosity with high vitrification leads to an increase in the bending strength. The set of results presented in this paper are the initial part of a broaden work aiming to contribute to the characterization and conservation of an important Portuguese cultural heritage. In the
1652 Advanced aterials Forum III sequence we will focus the study on the modifications of water absorption and mechanical properties of tiles after blocking the pores with different consolidants. Acknowledgements This study was supported and by the funding of FCT to CB and ICES. The authors thank useu Nacional do Azulejo, especially its Director, Dr. Paulo Henriques, as well as Dra Lurdes Esteves (Conservator), for provision of samples and Dr. Pedro Amaral for the mechanical tests. References [1] P. López-Arce, J. Garcia-Guinea,. Gracia and J. Obis: ater. Charact. Vol. 50 (2003), p.59. [2] D. Benedetti, S. Valetti, E. Bontempi, C. Piccioli and L.E. Depero: Appl. Phys. A Vol. 79 (2004), p.341 [3] A.G.Bueno and V.J.. Flórez: J. Cult. Herit. Vol. 5 (2004), p. 75 [4] K. Zoghlami, D. Gómez-Gras and A. Álvarez: Proceedings of the 6 th Internationational Symposium on the Conservation of onuments in the editerranean Basin (Ed. L. Aires de Barros and F. Zezza, Portugal, 2004) p.501. [5] K. Zoghlami, D. Gómez-Gras and A. Álvarez: Proceedings of the 6 th Internationational Symposium on the Conservation of onuments in the editerranean Basin (Ed. L. Aires de Barros and F. Zezza, Portugal, 2004) p.385. [6] C. Figueiredo, P. Figueiredo, P. Pina, L. Aires-Barros, V. Ramos and P. achaqueiro: Proceedings of the 6 th Internationational Symposium on the Conservation of onuments in the editerranean Basin (Ed. L. Aires de Barros and F. Zezza, Portugal, 2004) p. 248. [7] J. L. Farinha Antunes: Sc Thesis, Instituto Superior Técnico, Lisbon, 1992. [8] J. L. Farinha Antunes, J. Costa Pessoa,. O. Figueiredo and. Amaral Fortes: Studies in Conservation, Vol. 41 (1996), p.153. [9] J. L. Farinha Antunes,. O. Figueiredo, J. Costa Pessoa, and. Amaral Fortes: Proceedings of the World Ceramics Congress, (Ed. P. Vincenzini, Techner F.R.L., Faenza, Italy,1995). [10]..L. Ribeiro Carrott, J. L. Farinha Antunes and P.J.. Carrot, Proceedings of the 4 th Internationational Symposium on the Conservation of onuments in the editerranean Basin, (Ed. A. oropoulou, F. Zezza, E. Kollias and I. Papachristodoulou, Greece, 1997) p.79. [11] http://www.mediacy.com