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Research Paper

Bromine volatilization during firing of calcareous and non-calcareous clays:Archaeometric implications

M.J. Trindade a,d,⁎, M.I. Dias a,d, F. Rocha b,d, M.I. Prudêncio a,d, J. Coroado c,d

a Instituto Tecnológico e Nuclear, EN 10, 2686-953 Sacavém, Portugalb Departamento de Geociências, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugalc Departamento de Arte, Conservação e Restauro, Instituto Politécnico de Tomar, 2300-313 Tomar, Portugald GeoBioTec–GeoBiociências, GeoTecnologias e GeoEngenharias, Universidade de Aveiro, Portugal

a b s t r a c ta r t i c l e i n f o

Article history:Received 21 October 2009Received in revised form 30 June 2010Accepted 1 July 2010Available online 16 July 2010

Keywords:Bromine volatilizationFiring productsCalcareous claysNon-calcareous claysAlgarve Basin

The influence of carbonates in the temperature of Br volatilization during firing of clays and its implicationsin archaeometric studies are investigated. The main goal is to determine the circumstances in which Brcontent of fired products (“pottery”) can be considered inherited from raw material or added to ceramicpastes after production. Seven samples representing different types of clays (non-calcareous clays, calcite-rich clays and dolomite-rich clays) from Algarve region (South Portugal) were studied before and after firingat temperatures ranging from 300 °C to 1100 °C by steps of 100 °C. Original clays were characterized bymajor element geochemistry, obtained by X-ray fluorescence, and by mineralogy of bulk rock and b2 μmfraction, using X-ray diffraction. The chemical composition of the test pieces (unfired and fired at varioustemperatures) of each clay was determined by instrumental neutron activation analysis, as it enablesobtaining Br concentration with high precision and accuracy. Thermogravimetric analysis was done to bettercharacterize mass changes after firing. The results confirm the influence of clays composition, especially thepresence of carbonates rather than the clay minerals associations, on temperature of Br volatilization: i) innon-calcareous clays Br volatilized more intensely up to 600 °C, suggesting its association with combustionof organic matter and dehydroxylation of clay minerals; ii) in calcareous clays Br volatilized more stronglyafter 800―900 °C, suggesting that at least part of the process is associated with decarbonation reactions. As aresult, this study contributes to elucidate that not all Br existing in pottery can be interpreted as acontamination product. Instead, Br can be inherited from raw material and its presence in pottery,particularly in carbonate-rich pastes, may simply indicate that the temperature attained in the kiln was notenough for its complete volatilization. Extending the Algarve study to other clay materials used in ancientceramics, the conclusions of this study may assist broad archaeometric studies.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Aims of the study

The compositional study of clays can contribute to the resolution ofarchaeometric problems related with provenance of ancient pottery(Taylor et al., 1997; Hein et al., 2004; Prudêncio et al., 2006).Particularly, the firing of clays, simulating the ceramic manufactureprocess, and further characterization of the compositional transfor-mations with temperature, may help identifying the clay materialsused for pottery production and the maximum firing temperatureattained in the kiln (Peters and Iberg, 1978; Duminuco et al., 1998;Riccardi et al., 1999; Cultrone et al., 2001).

Most provenance studies are based on the compositional compar-ison between pottery and clay materials, using multivariate statisticalanalysis of geochemical data. However, several works (Perlman andAsaro, 1969; Rye and Duerden, 1982; Cogswell et al., 1996) indicatethat Br volatilizes from ceramic bodies during firing, thus it should notbe used in such studies.

Even though Br is not a provenance indicator element, itsdetermination can be a useful tool for archaeologists, as it enablesinterpreting the history of a ceramic object, because the alteration andcontamination that may take place during the phases of manufacture,use, burial, excavation and conservation can be sources for thecompositional variability of pottery. Particularly, Br can be a goodindicator of the post depositional environment or informing about theuse given to pottery, as it can be fixed in the ceramic pastes during itsmanagement and utilization or during its burial (Aloupi et al., 2000;Prudêncio et al., 2005, 2006). In general, Br offers a very powerfuldiscriminating criterion betweenmarine and terrestrial environments(Maravelaki-Kalaitzaki and Kallithrakas-Kontos, 2003) and a research

Applied Clay Science 53 (2011) 489–499

⁎ Corresponding author. Instituto Tecnológico e Nuclear, EN 10, 2686-953 Sacavém,Portugal. Tel.: +351 21 9946215; fax: +351 21 9946185.

E-mail address: mjtrindade@itn.pt (M.J. Trindade).

0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.clay.2010.07.001

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project is ongoing of Greek investigators (Aloupi and co-workers)about the use of Br as a tracer of sea and salt routes in ancient cultures.

This study is focused in the Algarve region (South Portugal). Thechoice of this region lays on the fact that several archaeological sitesof Lusitanian–Roman age with or without kilns were identified andthat this region played an important role in amphorae trade inRoman times (Fabião and Arruda, 1990; Fabião, 2000, 2007; Fabiãoet al., 2008; Fabião, 2009). A few archaeometric studies werealready done (Dias et al., 2009) and a research project is ongoing inorder to better characterize pottery from the known Roman sites(Fig. 1). Preliminary results point to the use of local raw materials(Dias et al., 2009). The archaeological findings are being studied byinstrumental neutron activation analysis at Instituto Tecnológico eNuclear (Portugal), and the results are included in a database. Inrecent years, this database has been complemented not only withthe mineralogical and geochemical characterization of differenttypes of clays of Algarve, but also with the characterization of theircompositional transformations with temperature, after firing(Trindade, 2007; Trindade et al., 2009; Trindade et al., 2010a).Due to this abundance of data the Algarve region is a promisingprovenance case-study, therefore it is important understandinghow chemical element contents are affected by firing and theirrelationship with composition of available clays.

Our main goal in this work is to investigate the geochemicalpattern of Br, through firing of clays at different temperatures, andstudy the influence of the original composition. Bromine is commonlyincorporated in several clays components, such as organic matter, Aland Fe oxyhydroxides and clay minerals. This element is, in general,present in deep-sea clays and carbonates. In this work the possibilityof a relationship between temperature of Br volatilization and theabundance and type of carbonates in clays is explored. This approachis useful in compositional studies of archaeological pottery pastes,principally when discussing provenance issues. Thus, the study of Brvolatilization during firing will also contribute to verify in whichcircumstances the presence of Br in fired clays (“pottery”) can beconsidered inherited from the raw materials or added to the ceramicpastes after production, as have been suggested in the abovementioned works. The establishment of the origin of Br in ceramicpastes can be particularly important in the case of Roman amphoraelike those from Algarve that were used as salt and fish based foodcontainers.

1.2. Bromine occurrences

The geochemistry of Br is yet little known and understood.Reviews of its general geochemistry have been provided by Fuge(1974, 1988) and Reimann and Caritat (1998). In addition, thedistribution of the halogen in soils and plants is reviewed by Kabata-Pendias (2001).

Bromine has been described as a typical lithophile elementevidenced by its concentration in silicate minerals (Fuge, 1988). Ithas also been classified as a biophile, hydrophile and atmophileelement (Fuge, 1988), because it is strongly associated with organicmatter, it is highly concentrated in the hydrosphere, and it istransported through the atmosphere in gaseous form, respectively.

Bromine is rarely found in nature in its free elemental state as it isextremely reactive, easily achieving an inert gas structure, so itcommonly occurs as the Br− ion (Goldberg, 1963; Fuge, 1988).

Table 1 presents a compilation of the average Br concentrations inorganic and inorganic components based on several works. Much datawas taken from Vassilev et al. (2000).

The oceans are the main Br containers of the Earth (Fuge, 1988). Bris usually accumulated in the sea water and in marine plants.Particularly, red and brown algae are enriched in Br, showingconcentration factors of about 50 and 9 respectively, over the seawater.

Bromine is volatilized from the sea into the atmosphere in theform of aerosols and gases (Duce et al., 1965) and then transported onto land surfaces by precipitation. Much of the particulate Br content ofthe atmosphere precipitates in near coastal areas, however, gaseousBr could be carried relatively large distances inland so also enrichingthe continental environment. Hence, it is probable that most of theenvironmental Br in continental and coastal areas is derived from theoceans (Fuge, 1988).

Bromine concentrations in river water are much lower than in seawater and are controlled by precipitation, chemistry of tributaries anddrainage basin geology. In some areas, high Br content in surfacerunoff is derived from weathering of marine sedimentary rocks andfrom waters draining areas of Br-rich soils (Konovalov, 1959;Perel'man, 1977). The subsurface waters can also be enriched in Brwhere waters circulate through marine-deposited sedimentary rocks(Fuge, 1974), as occurs in Algarve, where climatic conditions favorevapotranspiration, increasing salts in solution (Salminem, 2005).

Fig. 1. A) Localization of Algarve region (rectangle) and Algarve Basin (grey area) in South Portugal; B) Known Roman amphora production sites in Algarve region: MRT—Martinhal,QLA—Quinta do Lago, SJV—São João da Venda, MR—Manta Rota and SBCM—São Bartolomeu de Castro Marim.

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Other natural or pollutant sources of Br in the environment areassociated with volcanism, hydrothermal mineralization, fossil fuelburning and industrial processes involving coal and oil, vehicle exhaustfumes due to the use of ethylene dibromide in petrol and agriculturalpractices (Fuge, 1988), due to thewidespreaduseof organic bromides asfumigants of soils and plants (Kabata-Pendias, 2001).

Estimations of Br concentration in the continental crust are variable,being suggested (Fuge, 1988) that themost recent ones aremore correctdue to the use of more sensitive analytical techniques.

Bromine with its large ionic radius is unlikely to fit into the latticesites of rock forming minerals to any great extent (Lieberman, 1966),its occurrence being most commonly associated with substitution ofCl− ions for Br− ions in chlorine-rich minerals such as sodalite(Kogarko and Gulyayeva, 1965). It also forms various highly solublesalts with both alkali and alkaline-earth metals in evaporate deposits,mainly NaBr and KBr. A small number of these alkali bromides, inaddition to silver bromides, occur naturally (Fuge, 1974; Salminem,2005). Natural brines and thermal waters have high Br concentra-tions, thus the geochemistry of Br is closely related to evaporatedeposits and water chemistry (Kabata-Pendias, 2001).

In igneous rocks, Br concentrations rarely exceed 4 μg/g, volcanicrocks retaining more Br than their intrusive counterparts (Salminem,2005). Most frequently Br is present in igneous rocks in fluidinclusions and adsorbed on crystal surfaces and in lattice defects(Fuge, 1988).

Sedimentary rocks and recent sediments contain relatively highconcentrations of Br, which is thought to be adsorbed on grainsurfaces and detrital organic material, showing correlation withorganic carbon (Cosgrove, 1970; Price et al., 1970). Although,appreciable Br in sediments is water soluble (Fuge, 1988). Marinedeposited rocks and sediments, especially carbonates, are markedlyricher in Br than non-marine deposits. Sediments formed under salineconditions have been found to contain higher concentrations of Brwhen compared to those from fresh water settings (Salminem, 2005).Bromine correlation with organic carbon content is somewhatobscured by post depositional mobility of Br through diageneticalteration of organic matter. Br concentrations within sediments tendto decrease with depth of burial, leading to Br accumulation in porefluids (Fuge, 1974; Von Breymann et al., 1990).

The available data of Br concentration in soils show a very broadrange, but point to higher Br content in soils than the rocks fromwhich they derive (Fuge, 1988). This Br enrichment in top soilhorizons is largely an effect of its precipitation with rain (Kabata-Pendias, 2001). Much of the Br in soils derives from the oceans, thussoils in coastal regions are richer in Br than inland soils (Fuge,1988).

Bromine in soil shows a strong tendency to concentrate in the humicand fine fractions (Vinogradov, 1959), and a strong positive correlationof Br with organic carbon has been reported (Gerzabek et al., 1999;Kabata-Pendias, 2001). It is also suggested that Br retention in soils is

Table 1Bromine average concentration (μg/g, unless expressed otherwise) in several organic and inorganic components.

Components Br content References

Sea water 65; 67 (mg/l) Bowen (1966); Fuge (1974)Precipitation 0.015 (mg/l) Fuge (1974)Surface runoff 0.030 (mg/l) Fuge (1974)River water 0.021(mg/l) Bowen (1966)Red algae 3293 Fuge (1988)Brown algae 740; 606 Bowen (1966); Fuge (1988)Continental crust 2.5; 0.37; 1.0; 0.88 Taylor (1964), Mason (1982); Bowen (1979); Wedepohl (1995); Rudnick and Gao (2003)Chlorine-rich minerals (such as sodalite) 34 Salminem (2005)Felsic rocks 0.71; 1.3; 1.7 Fuge (1974); Beus and Grigorian (1975); Vinogradov (1962)Intermediate rocks 0.81; 4.5 Fuge (1974); Vinogradov (1962), Beus and Grigorian (1975)Basic rocks 0.61; 3; 3.6 Fuge (1974); Vinogradov (1962); Beus and Grigorian (1975)Shales 14.8; 4; 6 Fuge (1974); Turekian and Wedepohl (1961), Beus and Grigorian; (1975); Vinogradov (1962)Deep-sea clays 70 Turekian and Wedepohl (1961)Sanstones 6.7; 1 Fuge (1974); Turekian and Wedepohl (1961), Beus and Grigorian (1975)Carbonates 10.2; 6.2 Fuge (1974); Turekian and Wedepohl (1961), Beus and Grigorian (1975)Deep-sea carbonates 70 Turekian and Wedepohl (1961)Soils 5–40; 10 Kabata-Pendias (2001); Bowen (1982)Coals 17; 5–17 Valkovic (1983), Finkelman (1994); Yudovich et al. (1985)Terrestrial plants 40 Kabata-Pendias (2001)

Table 2Summary of the age, geologic unit (according to the geological chart of Algarve region, Manuppella, 1992) and number of samples studied in each of the three groups of clays definedfor Algarve.

Type of clay Age Geologic unit Number ofsamples

Group 1: Non-calcareous clays Quaternary Holocene Gravel and terraces (Qb) 5Pleistocene Faro-Quarteira gravel and sands (Qa) 7

Neogene Miocene Cacela Formation (MC) 4Lower Cretaceous Barremian Shales, sandstones and conglomerates of the wealdien facies (C1) 7

Berriasian Sobral Formation (C1) 17Carboniferous Westephalian Brejeira Formation (HBr) 4

Namurian Mira Formation (Hmi) 3Group 2: Calcareous clays Group 2A: Calcite-rich clays Lower Cretaceous Aptian Luz Formation (C2) 2

Upper Jurassic Oxfordian Peral argillous limestones and marls (JP3) 3Middle Jurassic Callovian Telheiro limestones nad marls (JT2) 6

Bathonian Praia de Mareta marls and limestones with Zoophycos (JZ2) 1Group 2B: Dolomite-rich clays Lower Jurassic Hettangian Volcano-sedimentary complex (JV1) 6

Lower Jurassic–Upper Triassic Silves shales, limestones and evaporites (JS1) 22Upper Triassic Silves sandstones (TS) 6Lower–Middle (?) Triassic S. Bartolomeu de Messines shales (TM) 1

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governed by the Al and Fe oxide therein (Whitehead, 1973). Despite theobserved sorption capacity of Br byAl and Fehydroxides, organicmatterand clays, it is easily leached from soil profiles during weathering andtransported to marine basins in large amounts (Salminem, 2005).

Significant positive correlation of Br with exchangeable Ca,especially in calcareous soil, has been observed (Gerzabek et al.,1999; Kabata-Pendias, 2001). The Br–Ca correlation found in apedogenic calcrete (Grevenitz and Chivas, 2005) was explained bythe process of evaporation causing salt accumulation and storage of Brconcurrently with Ca within the calcrete profile.

Bromine content is generally low in terrestrial plants and showslittle correlation with soil contents, thus it is likely that appreciablequantities are derived from the atmosphere (Fuge, 1988).

Accumulation of Br in coal has also been reported (Vassilev et al.,2000). In coals, small amounts of Br were found linked with Fe oxidesand clay minerals, especially with illite and to a lesser extent withkaolinite (Vassilev et al., 2000).

2. Materials and methods

The Algarve basin (South Portugal) comprises Triassic to Holocenesediments over Carboniferous low-grade metamorphic basement

formed by alternating slates and greywackes. The mineralogical andgeochemical composition of about 100 clay samples of Carboniferousto Holocene age from Algarve have been studied (Trindade, 2007;Trindade et al., 2010b), and theywere grouped in three different typesof clays (Table 2). Considering the geological diversity andcorresponding clay deposits occurrence, seven samples of clay-richmaterials representative of each mineralogical association werechosen for firing tests, simulating the ceramic manufacture process,and evaluation of related chemical elements behavior. The maincompositional criterion to sample selection was related to thepresence/absence of carbonates, and their abundance and composi-tion (calcite or dolomite). Sampling locations are shown in thegeologic map of Algarve (Fig. 2).

Each samplewaspulverized inanagatemortar and then tenportionsof 10 g each were pressed at 1200 kg/cm2 in a Specac hydraulic press,whichproduced cylindrical test pieces of 4×0.3 cm(diameter×height).After dried at 110 °C, batches of 9 test pieces of one samplewere fired totemperatures ranging from 300 °C to 1100 °C by steps of 100 °C, in anelectric furnacewith oxidizing atmosphere. The firing cycle was of 5 °C/min, keeping maximum temperature for 30 min. This procedureoriginated one unfired specimen and nine heated specimens for eachsample.

Fig. 2. Geologic map of Algarve showing the localization of the seven samples selected for firing experiments.

Table 3List of reflection powers used for semiquantification of minerals in non-oriented mounts (bulk mineralogy) and oriented mounts (clay mineralogy).

Mineral Reflection Powers References

Bulk Mineralogy Anatase (Ant) 1 Martins et al. (2007)Anhydrite (Anh) 1.5 Martins et al. (2007)Ankerite (Ank) 1 a

Calcite (Cal) 1 Schultz (1964), Barahona (1974)Dolomite (Dol) 1 Schultz (1964), Barahona (1974)Goethite (Gt) 1 Schultz (1964)Gypsum (Gp) 1.5 Schultz (1964)Hematite (Hem) 1.3 Martins et al. (2007)K-Feldspar (Kfs) 1 Schultz (1964), Barahona (1974)Phylossilicates (Phy) 0.1 Schultz (1964), Galhano et al. (1999)Plagioclase (Pl) 1 Schultz (1964)Pyrite (Pr) 1 Martins et al. (2007)Quartz (Qtz) 2 Schultz (1964)Rutile (Rt) 1 *

Clay Min. Chlorite (Chl) 1.25 Martins et al. (2007)Illite (Ill) 0.5 Barahona (1974), Rocha (1993), Oliveira et al. (2002)Kaolinite (Kln) 1 Barahona (1974), Rocha (1993), Oliveira et al. (2002)Smectite (Sme) 4 Rocha (1993), Martins et al. (2007), Oliveira et al. (2002)

a No reference found in literature. In such cases, a reflection power of 1 was used.

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Geochemical and mineralogical characterization of the originalclays were done by X-ray fluorescence (XRF), for determining majorelements concentrations, and by X-ray diffraction (XRD) consideringbulk rock and clay (b2 μm) fraction. The test pieces (unfired and firedspecimens) were entirely powdered and chemically analyzed byinstrumental neutron activation analysis (INAA), as it enablesdetermining Br concentration with high precision and accuracy dueto its low detection limit. Thermogravimetric analysis (TGA) and losson ignition (LOI) were performed to better characterize samplesafter firing as it enables to determine a material's thermal stabilityand its fraction of volatile compounds by monitoring the weightchange that occurs as a specimen is heated.

XRF was performed using a 1:9 sample to flux (Spectromelt A12)ratio, whichwas fused to a glass bead and then analyzedwith a PhilipsPW 1410/00 spectrometer, using CrKα radiation. The Na2O and K2O

contents were determined by flame photometry, using a Corning 400spectrometer.

The bulk mineralogy of clays was determined by XRD using non-oriented powdered samples. It was used a Philips X'Pert Prodiffractometer, with a PW 3050/6x goniometer, CuKα radiation,and operating at 45 kV and 40 mA. Scans were done using a step sizeof 0.02º 2θ and a scan time of 1.25 s per step. The identification ofclay minerals was performed on oriented b2 μm fraction in the 4–20º 2θ range, using the same conditions as previously. The clayfraction was separated by sedimentation according to Stokes lawand the preferentially oriented clay mounts were prepared byplacing the suspension on a thin glass plate and air-dried.Identification of crystalline phases by XRD was carried out usingthe International Centre for Diffraction Data Powder Diffraction Files(ICDD PDF).

Fig. 3. SiO2–Al2O3–(CaO+MgO) and CaO–MgO relationships for Algarve clays, defining three main types of clays (1—non-calcareous clays; 2A—Ca-rich calcareous clays; 2B—Mg-rich calcareous clays) (data from Trindade, 2007).

Fig. 4. Mineralogical composition of Algarve clays, considering average values for the three main chemical types of raw materials (data from Trindade, 2007).

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The estimation of both clay and non-clay mineral abundances wasdone by measuring the diagnostic peak area, considering the fullwidth at half maximum (FWHM), of each mineral and then weightedby empirically estimated factors or reflection powers (Table 3),according to criteria recommended by Schultz (1964) and Thorez(1976). Given the uncertainties involved in the semi-quantitativemethod, the results obtained should only be taken as rough estimatesof mineral percentages.

INAAwas performed at the Portuguese Research Reactor (RPI). Afterbeing dried at 110 °C for 24 h, 200–300 mg of powdered sample wasweighed into small high-purity polyethylene vials, which were thenheat-sealed. Samples together with two multi-elemental referencematerials (GSD-9 and GSS-1) from the Institute of Geophysical andGeochemical Prospecting (IGGE) were irradiated in the core grid of theRPI for 6 h at a thermal flux of 3.589×1012 ncm2−s−1,∅th/∅fast=14.1,∅ep/∅th=1.4. The reference values were taken from data tabulated byGovindaraju (1994). The γ-spectra was recorded by high-resolutiondetector. Two γ-ray spectrometers were used: (1) a 150 cm3 coaxial Gedetector connected through a Canberra 2020 amplifier to an Accuspec B(Canberra) multichannel analyzer. This system has a FWHM of 1.9 keVat 1.33 MeV; and (2) a low energy photon detector (LEPD) connectedthrough a Canberra 2020 amplifier to an Accuspec B (Canberra)multichannel analyzer. This system has a FWHM of 300 eV at 5.9 keVand of 550 eV at 122 keV. In general, the relative precision and accuracyare within 5%, and occasionally within 10%. More details of the methodcan be found in Dai Kin et al. (1999), Gouveia and Prudêncio (2000) andDias and Prudêncio (2007).

TGA was carried out in powdered samples dried at 110 °C, using aNetzsch Júpiter STA 449 balance. The heating rate was 10 °C/min, upto 1100 °C. The LOI was determined by heating the various test piecesof each sample at 1000 °C for 3 h.

3. Results and discussion

3.1. Algarve clays

Algarve clays have been already characterized (Trindade, 2007;Trindade et al., 2010b). Thus, in this work, just the main aspects oftheir mineralogy and geochemistry are summarized based on datareported in such works.

Major elements enable defining two basic types of clay materials(Fig. 3): i) a first one (Group 1) with very low percentages of CaO andMgO, high percentage of SiO2 and variable Al2O3 content, indicatingthey are non-calcareous clays. The clays of this group are mainly ofNeogene, Cretaceous and Carboniferous age (Table 2); and ii) Group 2comprising samples with variable percentages of CaO and MgO,suggesting the presence of different amounts of carbonates. Aninverse correlation is observed between CaO+MgO contents andSiO2–Al2O3 contents. Thus calcareous clays could be separated on thebasis of differences in the CaO/MgO ratio, into a group rich in CaO(Group 2A), suggesting clays are calcite-rich, and another group withhigh MgO content (Group 2B), suggesting the presence of dolomite asthe main carbonate. Calcite-rich clays are mainly of Middle–UpperJurassic and Cretaceous (Aptian) age, whereas dolomite-rich clays aregenerally of Triassic to Hettangian age (Table 2).

Mineralogical study (Fig. 4) corroborates the geochemical obser-vations: i) non-calcareous clays (Group 1) consist mainly of quartzand phyllosilicates; carbonates are very rare; ii) calcite-rich clays(Group 2A) contain high percentage of calcite and phyllosilicates,followed by quartz; and iii) dolomite-rich clays (Group 2B) have ahigh phyllosilicate content, followed by quartz and carbonates, whichare mainly dolomite, sometimes accompanied by calcite.

The concentration of Br in Algarve clays is widely variable (0–14 μg/g), with 4 μg/g average (Trindade, 2007), being in accordancewith Br contents in sedimentary rocks found in literature (Table 1). No

Fig. 5. Boxplots of Br distribution in non-calcareous clays (Group 1), calcite rich-clays(Group 2A) and dolomite-rich clays (Group 2B) of Algarve region (data from Trindade,2007).

Fig. 6. Hierarchical cluster analysis of the chemical elements contents for Algarve clays,using unweighted pair group average as amalgamation rule and the Pearson correlationcoefficient (data from Trindade, 2007).

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significant distinction is observed in Br concentrations for the varioustypes of clays (Fig. 5).

Linear correlations are not observed in scatter plots of Br versusCaO, MgO, Al2O3, carbonates, phyllosilicates, etc., indicating Br has nosignificant relationship with mineralogical components and isprobably mostly linked with organic matter. However, in non-calcareous clays (Group 1) Br appears to be very weakly correlatedwith Mn, whereas in calcareous clays Br show correlation with Ca andLOI, for calcite-rich samples (Group 2A), and with Ca, LOI and Mg, forsamples containing dolomite (Group 2B). These observations suggestthat beside the general association of Br with organic matter, some Brcan be considered to be linked somehow with carbonates incalcareous clays, probably bromides are included in carbonaticfractions. These correlations could be clearly determined in amultivariate analysis (Fig. 6) using the chemical contents of a muchlarger number of variables (data from Trindade, 2007).

3.2. Firing experiments

For firing tests, three samples (Va2, Va7 and MM3) from Group 1,two samples (Te2 and Al2) from Group 2A and two samples (FS6 andBSJ1) from Group 2B of clay materials were selected. The mineralog-ical and geochemical results for these samples are presented inTables 4 and 5.

Non-calcareous samples (Group 1) consist mainly of quartz andphyllosilicates (illite and kaolinite) and minor amounts of feldspars,titanium oxides and goethite. In sample Te2, calcite and phyllosilicatespredominate, which are mainly illite and minor kaolinite, smectiteand chlorite; it contains quartz, plagioclase, goethite and anatase intrace amounts, whereas in Al2 quartz is the most abundant phasefollowed by phyllosilicates (illite andminor kaolinite) and with tracesof calcite, feldspars, hematite and anhydrite. Group 2B samples holdphyllosilicates, quartz and carbonates in variable proportions;hematite, feldspars, anatase and anhydrite may exist as accessoryphases; concerning clay minerals, illite dominates in these samples.Traces of smectite and chlorite occur in BSJ1.

The chemical composition (Table 5) shows that non-calcareousclays have higher SiO2, Al2O3 and TiO2 contents than calcareous clays.CaO and MgO contents are significant only for calcareous clays and

depend on the calcite/dolomite proportion, explaining higher LOIvalues. Fe2O3T content is relatively high, except for sample MM3; K2Oand P2O5 percentages are higher in Group 2B.

Bromine content is widely variable (0.54–5.1 μg/g) in the studiedsamples. The lower amounts are found in MM3 (Group 1) and insamples Al2 (Group 2A) and BSJ1 (Group 2B). The higher Br contentsare observed for samples Va2 and Va7 (Group 1).Therefore, acorrelation between Br concentration and mineralogy (percentageof carbonates and phyllosilicates) it is not clear, suggesting also theintervention of other components hosting Br, such as organic matter.

Thermogravimetric analysis of non-calcareous (Group 1) andcalcareous (Group 2) clays is shown in Figs. 7 and 8, respectively. Themain endothermic reactions, the temperature of maximum endo-thermic peak and the corresponding mass loss are shown in Table 6.

The mass losses for non-calcareous clays (5–7.3%) resulted fromevaporation of adsorbed water at very low temperature, dehydrox-ylation of goethite (≈320 °C) for Va2 and Va7, and dehydroxylation ofclay minerals (≈500 °C) that caused the main mass losses in thesesamples. In fact, the clay minerals present in these samples, kaoliniteand illite, generally dehydroxylate at 400–550 °C (Murad andWagner, 1998), although in the case of illite, especially if dioctahedral,it may persist up to higher temperatures. The combustion of organicmatter may also contribute to the endothermic peak at 500 °C, as itoxidizes between 200 and 600 °C (Velde and Druc, 1999). Nosignificant mass losses occurred after 600 °C in non-calcareous clays.

In calcareous clays, despite mass losses caused by loss of adsorbedwater and dehydroxylation of clay minerals, the thermogravimetriccurves show the effect of decarbonation reactions above 700 °C, whichaccount for the most significant mass losses in these samples,especially for the very calcite-rich Te2 sample. Because sampleshave different carbonate contents they show distinctive endothermicpeaks (Moropoulou et al., 1995; Samtani et al., 2002) at about 740 °C(dolomite) and/or 830 °C (calcite).

The loss on ignition (LOI) decreased for test piecesfired at increasingtemperature, being almost absent for specimens fired above 900 °C(Fig. 9A). In addition to the weight loss caused by dehydroxylation ofclay minerals, volatilization of water and oxidation of organic matter,calcareous clays (Group2) evidence strongpositive correlationbetweenLOI and percentage of carbonates (Fig. 9B).

Table 4Mineralogical composition of bulk rock and clay fraction for the clay samples selected for firing experiments. Mineral abbreviations according to Table 3.

Sample Bulk mineralogy Clay fraction

Qtz Cal Dol Phy Kfs Pl Hem Gt Ant Rt Anh Ill Kln Sme Chl

Group 1 Va2 55 – – 30 2 2 – 7 2 2 – 58 42 – –

Va7 64 – – 22 2 2 – 4 2 4 – 71 29 – –

MM3 54 – – 42 1 1 – – 2 – – 59 41 – –

Group 2A Te2 6 52 – 38 – 2 – 1 1 – – 61 19 13 7Al2 63 7 – 21 3 1 3 – – – 2 91 9 – –

Group 2B FS6 17 3 30 44 3 – 2 – 1 – – 100 – – –

BSJ1 34 8 10 37 3 4 2 – 2 – – 87 – 7 6

Table 5Major elements concentrations (wt.%), obtained by XRF, and Br content (μg/g), obtained by INAA, of the samples selected for firing experiments.

Sample Fe2O3T MnO TiO2 CaO K2O P2O5 SiO2 Al2O3 MgO Na2O LOI Br

Group 1 Va2 7.0 0.06 1.2 0.17 2.5 0.03 62 18 0.76 0.33 6.3 5.1Va7 5.6 0.02 1.3 0.11 1.9 0.04 66 16 0.50 0.76 6.0 4.0MM3 1.7 0.01 1.2 0.06 2.4 0.05 72 16 0.44 0.77 4.5 0.77

Group 2A Te2 5.4 0.05 0.49 28 2.1 0.11 26 9.4 1.8 0.06 26 2.2Al2 7.5 0.02 0.95 1.9 3.2 0.15 63 16 0.64 0.47 5.3 1.3

Group 2B FS6 6.0 0.08 0.72 6.5 5.4 0.18 46 15 5.8 0.24 13 2.5BSJ1 6.5 0.14 0.76 5.1 4.0 0.19 49 13 8.8 0.47 11 0.54

LOI: Loss on Ignition.

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Due to differences in weight loss for test pieces heated at differenttemperatures, the final concentration of chemical elements isdepleted in the samples heated at lower temperatures. This is mostevident for carbonate-rich samples where LOI was greater. In order tominimize these effects and to enable the comparison of chemicalcomposition of the various fired test pieces, the chemical contentswere recalculated to a null value of LOI, according to the expressions:CE=CEi CT/CTi and CT=100 CTi/(100−LOI), where CE is the calcu-lated element concentration, CEi is the initial element concentration,CT is the calculated total concentration of chemical elements and CTi isthe initial total concentration of chemical elements (Trindade, 2007).

This transformation allows solving the apparent dilution effect for thedetermined concentrations, consequently permitting addressing theeffect of firing temperatures on elemental concentrations.

As test pieces were fired in an oxidizing atmosphere and whollyanalyzed, the free removal of gaseous by-products from the piecesinto the atmosphere through volatilization is expected. Bromine, witha relative low boiling point (59 °C), presents a significant change of itsconcentrations in each sample, being absent or present in very lowamounts in the test pieces heated to higher temperatures, evidencingits volatilization through firing (Table 7, Fig. 10A). Although Br is theonly element released to the atmosphere in the gaseous phase, the

Fig. 7. Thermogravimetric curves (solid line) and respective derivate (dotted line) for non-calcareous clays (Group 1). Numbers 1, 2 and 3 are described in Table 6.

Fig. 8. Thermogravimetric curves (solid line) and respective derivate (dotted line) for calcareous clays (Group 2). Numbers 1, 2, 3 and 4 are described in Table 6.

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variation in elements concentration from centre to border of thepieces was not observed because test pieces were entirely analyzed.However, some studies point to migration of elements withoutescaping from the ceramic body (Schwedt and Mommsen, 2007).

The finding of Br volatilization during firing is not new and hasbeen reported in several works, in addition to volatilization of otherelements not analyzed in this work, such as chlorine and sulfur(Perlman and Asaro, 1969; Rye and Duerden, 1982; Cogswell et al.,1996).

Bromine volatilization occurred differently for non-calcareousclays and for calcareous clays, as can be seen in Fig. 10 (B and C): i)in non-calcareous clays Br content decreased rapidly up to 600 °C andthen decreased more gradually up to 800–900 °C. Above thattemperature no Br has been detected; ii) in calcareous clays,significant decrease in Br content for low temperatures of firing wasnot detected, it just decreased considerably at high temperatures(800–1000 °C), disappearing in all samples at 1100 °C only. Theseresults suggest that volatilization of Br is somehow dependent ondecarbonation reactions that occur around 800 °C, as a consequence ofthe partial association of Br with carbonates.

The abrupt decrease of Br concentration in dolomite-rich samples(FS6 and BSJ1) occurred between 800 and 900 °C, accompanyingdolomite decomposition and CO2 liberation to the atmosphere(Fig. 10D). On the other hand, in samples containing calcite as theonly carbonate (Te2 and Al2) the large decrease in Br contentoccurred at higher temperature (900–1000 °C), accompanying calcitedecomposition (Fig. 10E). Once again these observations suggest arelationship between Br volatilization and decarbonation reactions,thus its dependence on carbonate content in calcareous clays.

The obtained results emphasize that the temperature of Brvolatilization depends on absence/presence of different carbonates.In non-calcareous clays, Br volatilized more intensely up to 600 °C and

at 800–900 °C all Br was released from the samples as gaseous by-product into the atmosphere. This suggests that in the original clays Bris mostly associated with organic matter and clay minerals. Thus, itsvolatilization may be essentially related with organic matter com-bustion and dehydroxylation of clay minerals (illite and kaolinite). Incalcareous clays, the Br volatilization occurred more extremely after800 °C for dolomite-rich samples, and after 900 °C for samplescontaining calcite only. This suggests that in the original clays, someBr was probably associated with carbonates and its volatilizationlinked with CO2 release during decarbonation. It should be noted thatillite is the dominant clay mineral in these clays, however, nosignificant correlation appears to exist between the illite content andthe Br volatilization temperature.

4. Concluding remarks

The study of Br behavior during firing of three main types of claysoccurring in Algarve region (non-calcareous clays, calcite-rich clays

Table 6Details of thermogravimetric curves: main endothermic reactions, temperature of maximum endothermic peak, corresponding mass loss, and total mass loss.

Sample 1. Loss of adsorbedwater

2. Dehydroxylation ofgoethite

3. Dehydroxylation ofclay minerals

4. Decarbonationreactions

Total mass loss (%)

T (°C) Mass (%) T (°C) Mass (%) T (°C) Mass (%) T (°C) Mass (%)

Te2 70 −2.9 270 −1.4 510 −2.9 800 −20.4 −27.7FS6 70 −4.0 – – 540 −3.5 740 −7.1 −14.6BSJ1 70 −4.8 – – 590 −3.0 730 −2.8 −11.1

830 −0.5Al2 60 −0.8 – – 520 −1.7 730 −3.3 −5.8Va2 50 −2.5 320 −1 490 −3.8 – – −7.3Va7 60 −1.7 320 −0.8 490 −2.5 – – −5.0MM3 50 −1.4 – – 500 −4.0 – – −5.4

Fig. 9. Variation of loss on ignition (LOI) with firing temperature (A) and its variation with the percentage of carbonates in the samples (B). Legend: 1—non-calcareous clays; 2—calcareous clays.

Table 7Bromine concentrations (μg/g) in the fired test pieces of studied samples afterrecalculated to a null value of LOI as described in the text.

Temperature BrTe2 BrFS6 BrBSJ1 BrAl2 BrVa2 BrVa7 BrMM3

Unfired 3.0 2.8 0.60 1.4 5.4 4.2 0.81300 °C 3.3 2.6 n.d. 0.92 4.9 4.3 n.d.500 °C 3.4 2.9 1.0 1.1 3.6 2.7 n.d.600 °C 4.4 2.7 1.4 1.3 1.5 1.0 n.d.700 °C 4.0 3.1 1.2 1.5 0.97 0.73 n.d.800 °C 4.2 3.4 1.1 0.98 1.3 n.d. n.d.900 °C 3.7 0.95 n.d. 0.69 n.d. n.d. n.d.1000 °C 0.45 n.d. n.d. n.d. n.d. n.d. n.d.1100 °C n.d. 0.53 n.d. n.d. n.d. n.d. n.d.

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and dolomite-rich clays) showed that Br volatilizes at varyingtemperature depending on the composition of clays, thus corrobo-rating the general idea of previous works that Br should not be usedfor provenance studies of ancient ceramics.

As a consequence of Br volatilization during firing, its presence inpottery pastes has been interpreted as a product added to potteryafter production, indicative of the utilization of the ceramic piecesand/or the post depositional processes. This study emphasizes,however, that not all Br existing in pottery can be interpreted as acontamination product; instead Br can be inherited from the rawmaterial, particularly if pottery pastes are rich in carbonates. In suchcases, it can preserve its original Br concentration (with minorvariations) up to 800–900 °C, depending on the composition ofcarbonates. For that reason, the presence of Br in fired products couldsimply indicate that temperature attained in the kiln was not enough(b900 °C) to produce the volatilization of Br, especially in the case ofceramic pastes rich in carbonates. Hence, results obtained in this workhighlight that Br content in high temperature fired ceramic bodies canbe indicative of the original carbonate amount in clays, rather thandependent on the clay minerals associations.

Considering that some percentage of Br is associated withcarbonates in calcareous clays, thus prolonging its volatilization upto temperatures higher than in non-calcareous clays, the Algarve case-study could be extended to other regions and the firing behavior of Brgeneralized and applied in archaeometric interpretations of prove-nance and production technology studies of ceramic materials.

Acknowledgements

Financial support for this workwas provided by the Foundation forScience and Technology as a grant (SFRH/BD/11020/2002) to M.J.Trindade, which is gratefully acknowledged. The authors would like tothank anonymous reviewers for their careful and constructive reviewsthat considerably improved the original manuscript.

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