06 – SEM/EDS CHEMISTRY AND MINERALOGY OF A MILLIMETER-LARGE PIECE OF GLAZE TILE LYING IN THE SIDEWALK FROM ARACATI, CEARÁ, BRAZIL

Ano 07 (2020) – Número 01 Artigos

 10.31419/ISSN.2594-942X.v72020i1a6MLC

 

 

¹Marcondes Lima da Costa*, ¹Glayce Jholy Souza da Silva Valente, ²Thais Alessandra Bastos Caminha Sanjad, ³Milson Edmar da Silva Xavier

¹A Workgroup for Applied Mineralogy and Geochemistry (GMGA)/Geociences Institute/Federal University of Pará (UFPA), Belém (Pará), Brazil, marcondeslc@gmail.com ; glaycej@yahoo.com.br;

² GMGA, Faculty of Conservation and Restoration (FACORE); Graduate Program in Cultural Heritage Sciences (PPGPatri)/Federal University of Pará, Belém (Pará), Brazil, sanjadthais@gmail.com;

³Workgroup for Applied Mineralogy and Geochemistry and a  friend of Museu de Geociências/UFPA, Belém (Pará), Brazil, milsonest@gmail.com

^*Corresponding author.

 

ABSTRACT

It was found that in addition to the cities of Belém, São Luís, Salvador and Rio de Janeiro, that several others are carriers of historical tiles on exterior façades, such as Aracati in Ceará. In this city the tiles are also mostly of Portuguese origin and have similarities with those found in these other main cities, especially Belém. They are also under weathering change, with loss of glaze, with many fragments falling on the sidewalks and being swept by public cleaning. One of these fragments of about 7 mm in the longer length was investigated by SEM / EDS and the results obtained for the white and blue glaze and the biscuit (chamotte) show a chemical composition similar to the Belém equivalents, with basic glaze with Pb and Sn, besides Si, Al, K and Na.  For blue glaze, its color is probably given by cobalt blue, CoAl₂O₄, which used raw material containing As, perhaps Fe. Antimony, observed in a similar glaze in Belém, was not observed in the analyzed sample. The wealth of crystalline micro phases, however, is requiring mineralogical analyzes by XRD / FTIR and a systematic study of the material made available by natural degradation itself, which is large.

Keywords: historical tiles, “azulejo”, Cel. Alexanzito, cassiterite, tin, lead, cobalt blue, arsenic

 

INTRODUCTION

The colonization of Brazil by Europeans, from the 16th century on, left many important and constructive legacies. Among those that stand out are the large civil, urban and residential works. And in these, in particular it is worth registering the beauty and diversity of the tiles as decoration and lining of internal walls, which started in the 17th century, intensifying in the 18th and mainly 19th centuries. This activity seems to have been more notable in the cities of Salvador in Bahia and São Luiz in Maranhão (Alcântara, 1980, 2016), and Salvador (Sanjad et al., 2009; Costa et al., 2013; Holanda & Maia, 2018), Recife in Pernambuco and in the of Rio de Janeiro, then the most important economic and political centers. The tiles were applied both in religious and private buildings, to examples of palaces and residences of the class of greater socioeconomic power. The tiles also arrived in Belém, but practically a century later, intensifying in the 19th century, in the same way in religious and private buildings. As a result of Portuguese colonization, most of the tiles applied in these buildings are of Portuguese manufacture, however, those manufactured in Germany, England, Holand and also in France were also used. The application of tiles also on external façades, however, only happened from the 19th century, both in Portugal and here in Brazil. This apparent new use seems to be related to the aesthetic effects, but also as protection against strong humidity and sunlight, and why not, at the most affordable costs, with production on a larger scale. Therefore, the tile art of an external facade in Brazil is a strong highlight of the historic centers of the cities of Salvador, São Luiz and Belém, in addition to Recife, Olinda, and partly Rio de Janeiro, and other cities in the interior of several states as Ceará, Pará, Pernambuco, Bahia, etc. The tiled historic buildings in these three cities in particular have obviously given rise to many studies of the historical-cultural collection in general, architecture, engineering, construction materials and production technology, such as tiles, and finally their conservation and restoration, since many of them, especially the external ones, were not resisting the dry-humid weathering. In the past decades, thousands of these historic buildings have been listed by IPHAN in these cities. And, from the 1990s, researchers and students from UFPA (Federal University of Pará) and UFBA (Federal University of Bahia) dedicated themselves to the detailed study of tiles and related materials, seeking to know them regarding their chemical, mineral composition, production technology, pigments, application processes, and, mainly its natural degradation in the face of severe tropical conditions, as well as vandalism. The results were auspicious, culminating in the creation and installation of the Faculty of Conservation and Restoration (FACORE), with the first undergraduate course in Conservation and Restoration, and of the Graduate Program in Cultural Heritage Sciences (PPGPatri) by the University Federal do Pará in 2019, located in the Historical set Mercedários UFPA, Rua Gaspar Viana, 125, CEP: 66010-060, Belém, Brazil. He follows, in part, the example already in operation since 2010 in Bahia, “Professional Master’s Degree in Conservation and Restoration of Monuments and Historical Centers” from the Federal University of Bahia (MP-CECRE).

The previous presentation was just to subsidize the surprise of the first author when he came across in 2019, in the company of the second author, with the simple wealth of tiles adorning the external facades of countless residences and public buildings in the city of Aracati, Ceará, especially on Avenida Cel. Alexanzito, apparently recently revitalized. And why? Because the tiles affixed to the facades, at first glance, were very similar in general to those in Belém do Pará and São Luiz, in which the tiles in Belém had already been part of studies carried out by the first author alongside the dedicated professor Dra. Thais Alessandra Bastos Caminha Sanajd, also in the co-authorship of the present work. She still doesn’t know Aracati, but she was already aware of the occurrence of façade tiles in the city, when it was mentioned, but she had no idea of ​​the relative grandeur. Why the surprise? Aracati was not among the cities mentioned previously, perhaps because it is a small city, almost unknown to the public in Brazil, but which is also old, from colonial times. It was founded in 1747 and its historic center, with its tiled buildings, had already been listed by IPHAN in 2000, a long time ago. We were uninformed.

Aracati was an important port in the 18th, 19th and early 20th centuries, initially with a producer and exporter of beef jerky, then cotton, called white gold, and carnauba wax. The latter, exported to the industrialized world, especially the USA, which caused the great trip of industrialist Herbert. F. Johnson, 1935, a wax producer in Racine (Wisconsin), with carnauba as the main raw material (Johnson, 1936). He had come to Brazil to know the region where the carnauba naturally raged and Aracati was part of the visited region, with aquatization in the Jaguaribe River. This city until the end of the 19th century was more important than the state capital, Fortaleza.

In recent years, in the middle of the 21st century, Aracati (Terra dos Bons Ventos:  land of the good winds) has come to be known again, now for its beautiful beaches, like Canoa Quebrada, which awakened national and international tourism, including building an airport with commercial flights on Tuesdays, Thursdays and Saturdays, in the stretch Recife / Aracati / Recife. More recently it has attracted fish farming and shrimp farming, and has mainly become a thriving hub for the production of alternative energy (wind, and partially photovoltaic). It is a prosperous region again.

The Internet provides countless beautiful images of these buildings and their tiles, but very little about the history of their arrival in the city and its historical context, not even on Wikipedia, accessed on April 15, 2020 (https://pt.wikipedia.org / wiki / Aracati) and on the municipality’s official website, also accessed on April 15, 2020 (https://www.aracati.ce.gov.br/). However, a more acute search, with the help of Google, was found the beautiful recent work (a book) by Holanda and Maia (2018), “Do Tejo ao Jaguaribe – The tile in the historical heritage of Aracati”, with 240 pages, richly illustrated, available only in pdf on the internet, although the access is not yet so friendly. But it was a work sponsored by the Instituto do Museu Jaguaribano, in Aracati. The title alone gives us an idea of ​​its broad content. The first part is dedicated to tiles in Portugal, emphasizing its history, artisanal and semi-industrial production, main producers and Arab, French, Dutch and Spanish influences. It then advances on application in façades and exports to Brazil, mainly to Rio de Janeiro, Belém, São Luís, Salvador, Recife, Olinda, João Pessoa, Maceió and cities in the interior of Minas Gerais and Alagoas, among others. The application started in the 17th century, although restricted to the interiors of religious buildings, intensifying in the following centuries, reaching the façades only in the 19th and early 20th centuries, when it then contemplated the exterior façades. The authors conclude that the tiles reach the façades of Aracati at the end of the 19th century, but mainly at the beginning of the 20th.

They were not imported directly from Portugal, as most importing companies did, but probably bought in the markets of Recife, Belém and São Luís, as many of the tiles found in Aracati were also observed on the facades of these cities and some of the interior of Pernambuco. The “Rua Grande”, wide and very long, is the most important in the city, and was later called Avenida Cel. Alexanzito, a prosperous businessman from the city, who has a beautiful townhouse on that street. From time to time, the street was hit by major floods on the Jaguaribe River (one of them occurred in 1917), even before the trendy “extreme events” of global warming in recent days. The last flood occurred in 1985, which motivated the construction of the containment dike around the city, a work carried out by a Pará company – Estacon Engenharia S/A.

Holanda & Maia (2018) advancing in the work identify seventeen types of distinct tile patterns on the facades of Aracati. They are all stamped tiles, semi-industrial, partly with manual touches, probably produced from the second half of the 19th century. Most of the pieces are Portuguese, followed by the French and Catalans, some with no identifiable origin, for now. They also recognized 8 bars of architraves or fences, from Portugal and France, and also found on the façades of Belém, São Luís, Recife, Rio de Janeiro, among others. These 17 tile patterns and 8 bars or fences were isolated or, in small groups found in the 24 different facades cataloged by Holanda & Maia (2018). They highlight Fachada 1 as the main house of the current Jaguaribano Museum Institute (Figure 1), located at Avenida Coronel Alexanzito, number 743. The original construction took place in the 18th century, with architectural transformations in the 19th, with front and side facades all tiled on a regular basis and using only a single pattern, Pattern 1 (Figure 1), and without border (Holanda & Maia, 2018). Pattern 1 is recurrent in Belém, São Luís, Salvador, Recife and Rio Janeiro, in Brazil and Lisbon, Cascais and Santarém, in Portugal (Holanda & Maia, 2018).

 

 

Finally, the book by Holanda & Maia (2018) ends with the chapter “Creative economy: differentiation by cultural value”, highlighting the creativity and diversities present in the Brazilian cultural heritage as a competitive differential in the global context, which can add value to the current coastal tourism.

The book, however, was not dedicated to the raw materials used, its chemical and mineral constitution, pigments and technology used, glaze and ceramics relation; moreover, the deterioration processes in which a large part of them are not addressed. Certainly, these aspects would already be far beyond its objectives, which were great and achieved. In view of this, and considering the experience of the authors of this article with the chemical and mineralogical characterization of facade tiles in Belém do Pará, and their tropical weathering, leading to their natural deterioration with strong microbiological action (Sanjad et al., 2014), preliminary studies were developed of material naturally disintegrated and turned by the cleaning system of the city from the sidewalk to the curb of Avenida Cel. Alexanzito. Who knows, it could be collected, in part, and filed for subsequent studies. It is a recommendation that is registered in this work.

 

MATERIALS AND METHODS

As already mentioned, on a quick walk through the historic center of Aracati, intuitively you reached Avenida Cel. Alexanzito, which draws attention by the extension, houses and townhouses such as residences, public and commercial buildings, banks, etc., well presented, with planters, all with very clean sidewalks and street bodies, in addition to good afforestation. In very good taste. However, the maximum point was the tiles on the facades, due to the diversity of motifs (patterns) according to the symmetry, drawings, shades and arrangements. The façades reminded us of those of Belém do Pará and São Luís, and in particular Belém, as they were studied by team members. And it was soon realized that, just as in those cities, many tiles are in an advanced stage of weathering, with intensive loss of glaze and even biscuits, like the house 936 on Avenida Cel. Alexanzito, which corresponds to Fachada 4 de Holanda & Maia (2018, p. 186). But it is also found in other facades. Millimeter to centimeter fragments accumulated at the foot of the wall and spread, in part, on the sidewalk and curb (Figure 2). From the sidewalk, a fragment of approximately 7 mm (longer length) of glaze was collected, corresponding to that of the tiled wall in the vicinity, for non-destructive analyzes. It would, like the others, soon be swept and lost. In addition, several images were taken of the tiled facades and the avenue, and their houses with the aid of a digital camera and a smartphone camera.

The analyzes performed were simply by Scanning Electron Microscopy with Energy Dispersive X Ray Spectrometry (SEM / EDS) under low vacuum, therefore without the need for metallization, without any physical and chemical damage to the small fragment of glaze collected: a non-destructive analysis procedure. Bench Hitachi SEM TM 3000 and EDS Oxford Instruments / SwiftED3000, from LAMIGA Laboratories, Institute of Geosciences / UFPA, were used. However, in view of the results obtained and the doubts raised, XRD analyzes will be necessary to better determine the crystalline phases.

 

RESULTS AND DISCUSSIONS

The images of the tiles captured by the cameras of this work (Figures 3 and 4) are not very representative compared to the complete work of Holanda & Maia (2018). They are equivalent to the following standards for their classification: Standard 2.1, Standard 6, Standard 16, Standard 3, Standard 7, Standard 4 and Standard 8 (Figure 3) and Standard 2 (Figure 4).

The planar symmetry in Hermann-Mauguin notation is dominated by 4mm for a set of 4 identical tiles, if individual in general m, or 2mm and a 4mm (Figures 3 and 4). The 4mm symmetry is very common, because in general it gives simple beauty to most of the reasons used in the design.

 

The glazed fragment investigated belongs to the Standard 2 tile from Holanda & Maia (2018), found on the type facades 4 and 8 of Aracati. It is a beautiful pattern, Portuguese, and used widely in Brazil and Portugal, with symmetry m for the individual and 4mm for a set of 4 and multiples (Figure 4). The analyzed fragment includes a part of the blue horseshoe (ferradura) and another part of the white enamel for the central zone (Figure 5).

 

Holanda & Maia (2018) give great prominence to Pattern 2 (Figure 4 D), and it is not for nothing, it is simple in its symmetry (combination of motifs and colors), individual m in a set of 4 4mm and its multiples: “Pattern known as horseshoe, fully stamped and quite common in Portugal, especially in the Lisbon region. It was manufactured for many years, with several versions, either from the same factory or from different factories, with changes in both the stamp and the color of the finial. Its popularity crossed the Atlantic, as it is found in almost all Brazilian cities with historic tiled facades …, even in the completely blue version” (Holanda & Maia, 2018). “In Aracati, this version is the most common, being the one that is present in the largest number of houses, all single storey house, some with façade features much later than the other façades of the listed building. In the cities of Belém and São Luís, there is a large number of tiled facades with this pattern, in its various versions” (Holanda & Maia, 2018).

 

The Blue (horseshoe) and White Glaze Domains

The millimeter-sized glaze fragment analyzed comprises two main domains: the blue of the horseshoe and the white interior and the relicts of the ceramic body (chamotte) on back of another small fragment (Figure 5). It was stamped on the dominant white glaze base. In the SEM image, the area of ​​the blue domain (which in the SEM image is light gray) appears more homogeneous, but locally there are aggregates of tabular crystals on the surface of the glaze and also immersed in the glassy mass. Likewise, it contemplates dark gray scattered spots and also oriented close to the contact with the white domain. The contact between the two domains is very sharp. The white domain is distinguished by the reticulate aspect described by irregular white spots assuming a venular aspect on the gray background. The dark spots of the blue domain are also present in this white domain, dispersed, small, but locally relatively large. Don’t forget that the scale is micrometric. The partially adherent side of the biscuit has an irregular gray to dark gray surface (SEM image color, not real), with fine to medium grain size. It highlights white spots (coloration of the SEM image, it is not real), irregular, from punctual to micrometric. All of these features were the subject of occasional and sandy chemical analyzes by SEM/EDS and will be presented and discussed ahead.

 

General chemical composition of the two domains

Chemical analyzes of the two domains, blue and white (Table 1), show differences only in the contents of cobalt, arsenic, lead and tin. Cobalt and exclusive arsenic and lead in part of the blue domain, while tin and lead for white, which is the first basic layer on the ceramic (chamotte). Therefore, lead is found in both domains, which is to be expected, since it was used in this period as flux, and to promote basic glaze with opacity. The tin was used as opacifier and at the time must represent the stamp to give white color, so it seems to portray two different stamps. The concentrations of Si, Al, K, Na and Mg, practically do not depend on the two domains, except Mg, absent in the blue, and show that the fundamental composition of the glaze raw material has not varied, is in the order of 80% and the sum of the other elements Co + As + Sn + Pb in the order of 20% (ranges from 20.5 to 23%) (Tables 1 and 2). The first suggest the use of SiO₂ minerals (quartz? In the form of sand? Ashes?) and potassium and sodium feldspar, or perhaps salts, ash and residues from the production of red wine (Coentro, 2010) in the approximate proportion of 80% of silicates (quartz + feldspars) and 20% opacifiers + pigments, feldspars probably used as fluxes. The blue color has the main chromophore or pigment in cobalt. But what would have been the cobalt-bearing substance? Currently, cobalt blue, CoAl₂O₄, is used, which does not exist as a mineral. Would it be known at the time of the production of these tiles? Everything indicates that empirical Co (II, III) oxides were used, with the formation perhaps of CoAl₂O₄, a blue color promoter. But many historians have concluded that cobalt blue has been known for a long time, even in Ancient Egypt. However, the understanding of production processes began to be unveiled only from the 17th century in Austria and Germany. The blue color of the ceramics glazes from the Alcazar Palace in Seville (Spain) dated between the11th and 15th centuries was correlated to cobalt (Garofano et al., 2015). In the analyzes performed here, everything indicates that it is related to the As levels, which varies between 1 and 1.6% of As (Tables 1 and 2) and also Fe (Table 2). It probably reflects the original geological raw material for the production of cobalt blue, which in order to reach oxides, certainly started from arsenites (skutterudite) or perhaps arsenates (erythrite) in general violets (purple) of this element, the most common minerals. At a given time in the production of tiles, the blue color had as raw material cobalt with As and Ni impurity from Schneeberg from the Mineral Ores Province Erzgebirge in Germany (Coentro, 2010).

 

Table 1 – Chemical composition (SEM/EDS) in Wt. % of the glaze color domains (white and blue). Nd: not detected.

Chemical elements	White	White	White	Blue
Oxygen	45.260	45.602	45.133	42.010
Sodium	1.752	1.676	1.571	2.322
Magnesium	0.373	0.340	0.361	nd
Aluminum	2.330	1.850	1.989	1.221
Silicon	26.072	24.876	25.487	27.121
Potassium	3.665	3.690	3.688	3.663
Cobalt	nd	nd	nd	1.618
Arsenic	nd	nd	nd	0.959
Tin	7.679	8.668	8.816	nd
Lead	12.869	13.296	12.955	21.086
Total	99.627	99.658	99.639	99.041

 

Crystals and their chemical composition in the blue domain

In the domain of the blue glaze, it is possible to recognize random aggregates of crystals locally (Figure 6 A, B, C, D and E) emerging on the surface of the glaze or immersed in it in a radial form (Figure F). They stand out in the gray glaze through their intense white color in the SEM image. Micrometric crystals are tabular, with an apparently hexagonal cross section or rhombic symmetry (Figure 6 C). Some of them have a core of dark gray material (Figure 6 C), visible in longitudinal and transverse section (Figure 6 F) of high contrast, suggesting that these are compounds containing chemical elements of high atomic weight, such as Pb and Sn.

 

The analytical results indicate that the tabular crystals are basically formed by Pb, Ca, As and Cl, perhaps partly Br (Table 2, analyzes 4, 5, 6). The concentration of all elements together vary between 44 and 54%. They do not contain Co, and the other elements Si, Al, K and Na depict the general composition of the glaze, adding up to 12 to 17%. Oxygen varies between 33 and 37%. Although the glazed present as a whole corresponds to part of a Portuguese tile found in Belém and analyzed by Sanjad & Costa (2009), its motif inside the horseshoe is different and its pigmentation is also blue. They also made no mention of possible crystals. The Pb domain is notorious and explains why the crystals stand out in a strong white tint in the glaze gray of the SEM image. Based only on these data from chemical analyzes and discrete crystal morphology, it is assumed that they can be chlor-arsenates of Pb and Ca. Here the analysis by XRD, which could even be non-destructive, directly on the sample, would have been very important, whose thickness and larger dimension are compatible with the equipment’s sample holder. Lead is common in Portuguese tile enamels for facades (Sanjad & Costa, 2009) forming the colorless glaze. On the other hand, As and Cl do not. There is a strong relationship between the contents of Pb, Ca, As and Cl, which suggests that they form a single phase, and the analyzes and images thus allow us to understand. It is suggested as possible: Pb₅ (As³⁺ O₃) 3Cl, finnemanite, hexagonal, P63; Pb₅ (AsO₄) 3Cl, mimetite, (6/m) Space Group P63/m; clinomimetite, (2/m) Space Group: P 21/ b. However, none of them contain Ca. However, Pb²⁺ can be ionically replaced by Ca²⁺, although it is not common in these minerals. In yellow glazed tiles from Portugal Coentro (2010) mentioned in turn lead antimoniate, Pb₂SbO₇, which form crystals with hexagonal cross section. It is possible at a time that the source of Cl is from the salts of Na, NaCl It must have been incorporated as a flux (Coentro, 2010).

The designs and pigments of the glaze of this tile were elaborated using the stamping technique (Holanda and Maia, 2018), which has already been demonstrated in the present study, so it is likely that the origin of As and Cl, perhaps Ca, in this stamped glaze, are related to the pigmentation materials, which were catalyzed by lead and forming the probable minerals mentioned previously, or even from the crandallite group, where plumbogummite is the best representative, but there is not enough aluminum to assert itself with greater security. Due to these chemical and mineralogical characteristics, the glaze and tile as a whole, although with a symmetry and design similar to that of Belém, are not equivalent in what concerns the raw material of production, when compared with the data published by Sanjad et al. (2007), Sanjad & Costa (2009) and Costa et al. (2013).

 

Table 2 – Chemical composition (in Wt.%) of tabular crystals in the domain of the blue glaze.    Nd: not detected.

Chemical Elements:	Blue area		Matrix/Crystal (3)	Crystal (4)	Crystal (5)	Crystal (6)
	(1)	(2)
Oxygen	44.316	41.450	33.583	33.984	32.959	37.647
Sodium	2.438	2.271	1.145	0.831	1.119	0.926
Aluminum	2.060	1.363	2.156	2.029		2.188
Silicon	24.214	27.045	16.784	8.884	10.102	13.571
Potassium	3.306	3.942	2.658	0.962	1.135	1.697
Calcium	0.713		5.467	3.790	3.682	3.312
Chlorine	nd	nd	1.383	2.250	2.173	1.751
Bromine	nd	nd	nd	nd	3.431	nd
Iron	0.944	0.845	nd	nd	nd	nd
Cobalt	1.210	1.780	nd	nd	nd	nd
Arsenic	1.582	1.048	2.174	10.820	10.803	8.747
Lead	19.217	20.257	34.650	36.450	34.596	30.161
Total	98.343	99.156	100.000	98.207	100.000	99.074

 

Whitish area with deep white points in the blue glaze

SEM images also show whitish uneven areas in the blue glaze (Figure 7 B, C), which stand out for being composed, almost invariably, by Sn and Pb, adding up to 29 to 34% (Table 3, analyzes 1 to 4). This corresponds to the basic glaze layer, in which it is normally composed of SiO₂ and compounds of Pb and Sn. Renaissance glazed terracotta (ceramics). Madonna statue kept in the Museum of Fine Arts, Budapest, older than previous ones, from the 14th and 15th century, shows similar chemical composition in terms of Pb and Sn (Bajnóczi et al., 2018). The results presented here are also marked by the constant presence of C, with little variation, 7 to 8%, in the same way Si, Al and Na. Analysis 5 shows the influence of the blue glaze, indicated by the content of Co. Zn was detected in analyzes 1 and 5, and the levels may represent components of the lead material, as they are chemical elements very similar to each other in the minerals of the raw material, mainly sulfides. Analysis 6 stands out for its much higher Sn content, which in the image in Figure 7E represents high-contrast white matter. Analyzes 1 to 4 clearly represent tin-lead enamels, and by correlation with the studies by Sanjad & Costa (2009) and Costa et al. (2013) would be respectively isolated phases of Sn, cassiterite, SnO₂ and Pb, bindheimite, Pb₂Sb₂O₆(O,OH), which contains Sb, not detected in the material investigated here at any point analyzed. Coentro (2010) mentioned this compound in tiles glazed from 17th century Portugal. Here it is suggested that it is probably scrutinyite or plattnerite, polymorphs of PbO₂. The morphology in the image suggests plattnerite, which forms tetragonal prismatic crystals. Analysis 6, by analogy, would be dominated by cassiterite, with plattnerite to a lesser extent. Si, Al and Na should represent the use of minerals of SiO₂ and sodium feldspar as a flux. For carbon, no relation to carbonate was found, for example lead, as there is no direct relationship between them in the analyzes.

 

Table 3 – Chemical composition (SEM/EDS) in Wt.% of the white micrometric area in the blue glaze. Nd: not detected.

Chemical elements	Área (1)	Área (2)	Área (3)	Area (4)	deep white/ matrix (5)	deep White (6)
Carbon	7.265	8.302	6.811	6.827	nd	Nd
Oxygen	41.185	40.531	43.048	42.951	41.079	45.003
Magnesium	nd	nd	nd	nd	0.405	nd
Sodium	1.016	1.343	1.215	1.161	nd	1.419
Aluminum	1.275	1.308	1.301	1.442	1.257	1.184
Silicon	16.897	17.803	17.836	17.706	18.877	15.254
Iron	nd	0.703	nd	nd	nd	nd
Cobalt	nd	nd	nd	nd	0.995	nd
Zinc	0.424	nd	nd	nd	2.491	nd
Tin	19.128	16.961	19.071	19.296	21.440	27.011
Lead	12.811	13.049	10.718	10.617	13.456	10.128
Total	99.577	99.297	100.000	100.000	98.600	99.999

 

The black stains in the blue glaze

The black spots of the blue domain of the glaze (Figure 7 A, B, C; Table 4 analyzes 1, 2 and 3) they present themselves both in forms from dendritic to massive, discoid contours and as if they were also micrometric fragments, including in the white domain of the glaze (Figure 7 D, E). The chemical analyzes (Table 4) show the domain of O, Si and the contents of Al, K and Na, persistent, in the order of magnitude of those verified in the other analyzed materials, in which only analysis 2 disagrees with higher values ​​of Al and the only one with Fe. In turn, all of them are lead, in which analysis 2 continues to differ, presenting the lowest value of Pb, which was offset by Al and Fe. As and Ti were detected in analysis 1, the first being recurrent. Therefore, the black stains seem to reflect the general chemistry of the basic glaze, oscillating between the various layers of stamps and painting base.

 

Table 4 – Chemical composition (in Wt.%) of black stains in the blue glaze. Nd: not detected.

Chemical Elements/ Points analyzed	1	2	3
Oxygen	47.020	51.235	48.444
Sodium	1.521	1.092	1.460
Aluminum	1.597	5.130	1.235
Silicon	34.389	32.161	38.273
Potassium	1.807	1.404	1.540
Titanium	0.867	nd	nd
Iron	nd	2.363	nd
Arsenic	0.710	nd	nd
Lead	12.088	6.615	9.048
Total	98.422	100.000	100.000

 

White crystals and white stains in the white glaze

As already mentioned, the white domain of the glaze in the SEM image is highlighted by a reticule of white venules on a gray background, when white crystals / zones appear in prominent relief, which, as previously, reflect the presence of compounds with high atomic weight chemical elements (Pb and Sn) (Figure 7 D and E). The chemical analyzes of these crystals, or white spots (Table 5, analyzes 1 and 2) then confirm the Sn domain on the base, with restricted Pb, relatively, therefore, we conclude that it is, by correlation, cassiterite with Pb, or intergrowth of cassiterite and plattnerite, the enamels of tin and lead, respectively, stamped. Confirms Sn stamp as enamel whitening agent. The basic composition remains Si, Al and Na, therefore SiO₂ minerals and sodium feldspar, or Na salts, the flux. Coentro (2010) when studying the colored glazes (pictorial layer) of 17th century Portuguese tiles in Portugal mentions that often sodium was inserted as salt. Here comes back the presence of C, which seems partly related to Pb, also added as flux. It suggests that part of the Pb source was in natural oxidation zone minerals, in which cerussite, PbCO₃, is always present. However, here the content of C is relative to Pb, very high stoichiometrically.

The white irregular venules or spots (Figure 7 D and E), according to analysis 3 (Table 5), have a chemical composition that represents intergrowth between the white crystals (analyzes 1, 2, and 3) and the gray background (analysis 4), therefore with cassiterite, plattnerite, sodium feldspar and SiO₂ polymorphs. It would thus be a localized mixture of the stamps, these defects, which micrometric, are not perceived with the naked eye in the glaze.

The dominant gray background is then composed fundamentally of O, Si, K, Na, Al, Mg. It does not contain Sn, but a high content of Pb, but not C, again the colorless lead enamel. So, the raw material dominated by SiO₂, probably quartz or other SiO₂ polymorphs, potassium and sodium feldspars, or SiO₂-rich ashes the fluxes. It would then be the white color and the opacity of the white glass conferred mainly by the compounds of Pb, the lead plumb, which dominated the piece and over which came the others.

 

Table 5 – Chemical composition (in Wt. %) of the white glaze features. Nd: not detected.

Chemical Elements	White Crystal (1)	White Crystal (2)	White Stains (3)	Gray background (4)
Carbon	4.223	3.541	6.287	nd
Oxygen	43.906	45.240	43.458	44.280
Sodium	0.728	0.804	0.962	2.362
Magnesium			0.358	0.538
Aluminum	1.596	1.059	1.508	2.037
Silicon	12.367	9.816	17.881	29.944
Potassium	nd	nd	nd	4.013
Tin	31.760	35.754	20.200	nd
Lead	5.420	3.785	9.346	16.827
Total	99.272	99.195	98.680	99.463

 

Ceramics portion on the back of the glaze

As already mentioned, the back of the small fragment of glaze analyzed detached from the ceramic material, still retaining part of it (Figures 5 B and 7 F), which thus allowed us to have an idea of ​​its chemical composition, and, by deduction, mineralogical (Table 6). The ceramic body, obviously, is very distinct from the glaze, being fine granular, gray (color of the SEM image), with portions of irregular white micrometric aggregates, of high relief, which, as demonstrated in the glaze, depict compounds and/or minerals with the presence of chemical elements of high atomic weight (Figures 5 B and 7 F). This ceramic body devoid of white spots (Table 6, analyzes 1, 2, 3) consists of O, Si, Ca, Al, Fe, Mg, K and Na, in decreasing order, in addition to S, P, Cl in some areas analyzed. Sn, Pb and C were not detected. This chemical composition can be compared only partially to that of the ceramics analyzed by Sanjad & Costa (2009) and, by analogy, it should correspond, in part, to quartz and/or cristobalite, diopside and/or tremolite-actinolite, albite and KAlSi₃O₈ polymorphs (microcline, orthoclase and/or sanidine), in addition to probable apatite (Ca, P, S, Cl); the absence of C eliminates the possible presence of carbonates. Therefore, the raw material of the ceramics, could not have been limestone, admitted by Sanjad & Costa (2009) for the similar tile in Belém. It was probably amphibolite-type rocks, partially weathered, with clay minerals, with the addition of feldspars, and / or ashes, residues from wine production, and even Na salts, as fluxes. This is partly compatible, since Al levels do not justify those of alkalis, being stoichiometrically lower.

In turn, the white spots, with expressive contents of Pb and, in part Sn, represent formations of plattnerite and cassiterite in the ceramics, indicated by the presence of high contents of O, Si, Ca, Al, Fe, Mg, K and Na, but much poorer in Ca and Mg, but richer in K and Na. The larger, more intense white patches, on the other hand, chemically resemble the ceramics, differing only by the presence of Pb content (3 and 5%), compensating for the lower Ca content. Therefore, it would be a mass formed of diopside and/or tremolite. lead actinolite with feldspars (K, Na)? Probably the presence of Pb and Sn, and respective compounds, are demonstrating the proximity of the ceramics to the glaze, and/or its contamination with the materials of the stamps, therefore creating a transitional contact between both.

 

Table 6 – Chemical composition (in Wt.%) of ceramics features: areal ceramic body, white stains and large white stains. Nd: not detected.

Chemical Elements	Areal cer. body (1)	Areal cer. body (2)	Areal cer. body (3)	White stains (4)	White stains (5)	White stains (6)	Large w. stain (7)	Large w. stain (8)
Oxygen	49.707	55.508	55.200	46.671	51.667	47.905	51.068	55.220
Sodium	0.790	nd	1.054	2.116	1.237	2.015	nd	1.264
Magnesium	3.715	3.472	3.254	0.783	0.459	0.448	2.468	2.514
Aluminum	4.865	4.205	4.864	2.268	3.429	1.784	4.214	3.221
Silicon	19.052	21.636	17.927	21.348	26.359	20.401	21.300	23.125
Phosphorus	0.504	nd	nd	nd	nd	nd	nd	nd
Sulfur	nd	nd	0.742	nd	nd	nd	nd	nd
Chlorine	nd	0.825	1.111	0.561	nd	0.626	nd	nd
Potassium	0.667	1.133	1.400	2.249	1.666	2.206	3.376	0.906
Calcium	15.676	8.603	10.141	1.926	1.755	nd	8.810	5.917
Iron	5.025	4.619	4.307	0.843	1.445	nd	3.745	4.706
Tin	nd	nd	nd	5.026	nd	7.820	nd	nd
Lead	nd	nd	nd	16.208	11.983	16.797	5.018	3.128
TOTAL	98.040	99.176	99.258	97.812	99.541	98.928	99.999	99.095

 

CONCLUSIONS

The small glazed fragment investigated illustrates a blue portion of the border and a white portion within its field, which has fallen naturally due to the detachment from ceramics of Standard 2 tile from Facade 4. The correspondent complete single tile has symmetry m and 4mm for the set of four and their multiples, when seated on the facade of the house 936 located on Avenida Cel. Alexanzito. It is a beautiful Portuguese tile, widely used on external facades, both in Portugal and in Brazil, like Aracati. Here it is already in an advanced stage of physical degradation, being the glaze detached from the ceramic body.

Micrometric, prismatic crystals, interpreted as chlor-arsenates of Pb and Ca are frequent in the glazing of the blue domain. It is likely that the arsenic that contributed to the formation of these arsenates came from the raw material used to produce the blue pigment, equivalent to cobalt blue.

Lead (plattnerite), tin (cassiterite) and cobalt enamels, probably cobalt blue, CoAl₂O₄, were identified in the blue domain, in a less elaborate form, and may also contain remnants of As, reflecting the possible use of arsenide or Co arsenates for empirical production cobalt blue, depicting the different stamps. The basic white (or colorless) color appears to be conferred by the lead and tin compounds, which are dominant, that is, the lead-tin enamel. The different enamels resulting from the stamps always had the same basic chemical composition, O, Si, Al, K and Na, and then the addition of the pigment, cobalt blue or tin oxides. The white micrometric portions of high relief usually represent the domain of cassiterite in probable lead enamel.

Although the sample was millimetric and unique, it allowed, at first, to assess the stamped nature of the glaze as a whole, the interpretive chemical and mineralogical composition of the pigment material and also the general mass of the glazes and, also, the possible ceramics. Finally, relate them to tiles of a similar nature, placed on façades, for example, Belém do Pará.

The results, although very rich, are insufficient for more robust conclusions, in view of the little representativeness of the investigated sample, which had not been subjected to XRD analysis for mineral identification. It is highly recommended the systematic collection of fragments listed naturally on the sidewalk to increase the number of analysis and the coverage of the different pigments that enrich the Aracati historical tiles. This material should be collected and archived as soon as possible. In addition, it is possible to expand the studies with on-site analyzes, without the need to collect samples in situ.

 

Acknowledgements

To CNPq for the financial support through Grant Fees and Research Project to the first author (Proc. 305015/2016-8 and 442871/2018-0), to LAMIGA Laboratories at IG / UFPA and to the beautiful city of Aracati with so many stories and beautiful landscapes.

 

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 10.31419/ISSN.2594-942X.v72020i1a6MLC