Factors Determining Magnetic Enhancement of Burnt Clay from Archaeological Sites

Journal of Archaeological Science (2001) 28, 1137–1148doi:10.1006/jasc.2000.0645, available online at http://www.idealibrary.com on

Factors Determining Magnetic Enhancement of Burnt Clayfrom Archaeological Sites

Neli Jordanova*

Geophysical Institute, BAS, Acad. G. Bonchev Str., bl.3, 1113 Sofia, Bulgaria and Geophysical Institute, ASCR,Bocni II/1401, 141 31 Prague 4, Czech Republic

Eduard Petrovsky

Geophysical Institute, ASCR, Bocni II/1401, 141 31 Prague 4, Czech Republic and CEREGE,Univ. Aix-Marseille III, Europole de l’Arbois, Aix en Provence, France

Mary Kovacheva and Diana Jordanova

Geophysical Institute, BAS, Acad. G. Bonchev Str., bl.3, 1113 Sofia, Bulgaria

(Received 15 March 2000, revised manuscript accepted 5 October 2000)

The magnetic susceptibility of a collection of 212 archaeological samples of burnt clay of different origin (burnt soil,ovens and bricks) taken from 60 archaeological sites from Bulgaria, were studied in order to determine factors whichmay have affected their magnetic enhancement. The highest magentic susceptibility values were obtained for bricks, andthe lowest for burnt soils, suggesting that the firing temperature and the duration of burning are important conditionsfor determining magnetic enhancement. Specific spatial pattern of the low-field magnetic susceptibility was alsoobserved, characterized by separation of sites situated in northern and southern parts of the country, divided by thewest-east trending mountain chain Stara Planina. Maximum susceptibility values are obtained for burnt soils insouthern Bulgaria and minima in northern Bulgaria. In contrast, burnt clay plasters and bricks show the oppositepicture. It is supposed that the observed pattern is due to differences in the outcropped geological formations andvariable climate conditions between the two regions which determine different iron-supply during burning of initial claymaterial. Results from the thermomagnetic analyses show that the dominant ferrimagnetic mineral in all burnt claymaterials studied, is magnetite/titanomagnetite with low Ti content and different grain-sizes. High values offrequency-dependent magnetic susceptibility indicate the presence of significant amount of fine-grained (d<0·015 �m)superparamagnetic magnetite. � 2001 Academic Press

Keywords: MAGNETIC SUSCEPTIBILITY, BURNT CLAY, SOURCE MATERIAL, BULGARIA.

*E-mail: [email protected]; Fax: 359 9 971 30 05; Tel.: 359 2979 39 58

Introduction

O ur study tries to summarize some generalcharacteristics of magnetic susceptibility ofburnt clay materials of different origin, par-

ticularly burnt soil, clay plasters (ovens, kilns, hearths)and bricks. Magnetic studies carried out on archaeo-logical materials of burnt clay are usually focused onobtaining the ancient direction and intensity of thegeomagnetic field at the time of the last burning (e.g.Abrahamsen, 1996; Aitken et al., 1998; Kovacheva,Jordanova & Karloukovski, 1998; Sternberg, 1999).Environmentally oriented investigations are directed to

11370305–4403/01/111137+12 $35.00/0

help archaeological prospection and explore the devel-opment of certain archaeological sites (Fasbinder &Stanjek, 1993; Peters, Church & Coles, 2000; Marmetet al., 1999; Morinaga et al., 1999). The latter aspect isbased on the theory of magnetic enhancement of burntsoils (Le Borgne, 1955) and soils associated withancient human activity.

Magnetic susceptibility and its frequency depen-dence are widely used in studying magnetic enhance-ment. Susceptibility itself mostly depends upon theconcentrations of ferrimagnetic grains (mainly magnet-ite). Measurements of frequency-dependent magneticsusceptibility (difference between magnetic susceptibil-ity measured at low- and high frequency of the induc-tive magnetic field) are now widely used for detection

� 2001 Academic Press

1138 N. Jordanova et al.

of fine magnetite/maghemite grains in soils and rocks(Mullins & Tite, 1973; Maher, 1988; Dearing et al.,1996a,b; Forster, Evans & Heller, 1994). The basis ofthis technique is the Neel’s theory for superapara-magnetic relaxation (Neel, 1949) of fine particles,which have relaxation times, lying between the twomeasuring frequencies. The difference between themeasured magnetic susceptibility at low and highfrequency depends on the concentration of thegrains having relaxation frequencies in this interval.Mass-specific- and percentage frequency dependentsusceptibility are the two parameters most frequentlyused.

Generally, several factors play the most importantrole in determining magnetic enhancement of firedclay. These are the properties of the initial unburntmaterial through its specific iron content (Fe incorpor-ated in clay minerals or Fe-oxide/hydroxides); thedegree of burning which in most cases depends on thekind of the remains (e.g. fired clay, brick, oven . . .) anddetermines the final magnetic mineralogy of burnt claymaterials. In order to evaluate the influence of thesefactors, our study considers the degree of enhancementof magnetic susceptibility in relation to: (1) the type ofmaterial; (2) the geographical location of each archaeo-logical site which in general, predetermines theproperties and type of the initial clay material; (3) themagnetic mineralogy, as revealed by high-temperaturebehaviour of magnetic susceptibility and its frequencydependence.

During the last 30 years a wealth of archaeomagneticcollections were studied in Sofia palaeomagneticlaboratory to create ‘‘master curves’’ of the variations ofthe geomagnetic field characteristics (directionand intensity) for Bulgaria for the last 8000 years(Kovacheva, 1997). A large set of archaeologicalmaterials of different age and origin allowed us to inves-tigate the potential influence of the above listed factorson the magnetic susceptibility of the studied materials.

Sites, Materials and MethodsBulgaria is situated on the Balkan peninsula, at thecrossroads between Asia and Europe, exposed to manycultural influences during the last 8000 years. Thecountry is very rich in archaeological sites. We havestudied samples of burnt clay materials taken from 60different archaeological sites (Figure 1).

Burnt soil samples are taken from the baked ma-terial under the clay plasters or from visibly burnt soillayer at the site. Depending on the degree of firing,these samples could be very well burnt, but in othercases they carry only partial thermoremanence.

In contrast, ovens, kilns and hearths were deliber-ately used by man for domestic or other purposes inhistorical times. In all cases these are structures ofplastered clay material, usually from the floor of thestructure and less frequently from a wall.

Brick samples are taken from bricks of differentsizes, which were used in walls or kilns. The samplesused in our study are standard 2�2�2 cm or2·4�2·4�2·4 cm cubes. Only regular cubes wereselected, without gypsum, which is often used to im-prove the shape when not enough material is available.Two hundred and twelve specimens were included inthis study. Samples were classified, depending on theorigin of the material, as follows: (1) burnt soil/destructions of unclear origin; (2) burnt-clay plastersfrom ovens, kilns and hearths; (3) bricks. All sites arewell dated archaeologically and/or archaeomagneti-cally, the age of the samples studied varies between�8000 years to very recent (Kovacheva, 1997).

Mass-specific magnetic susceptibility � wasmeasured with the Bartington MS2B kappa meter attwo frequencies (�LF at 0·47 kHz and �HF at 4·7 kHz)with measuring accuracy of 1�10�5 SI. Percentagefrequency-dependent magnetic susceptibility (�FD%)defined as: �FD%=(�LF��HF)�100/�LF, as well asmass-specific frequency-dependent susceptibility �FD,were calculated. These parameters reflect the signifi-cance of the contribution of fine superparamagneticmagnetite grains (�0·012–0·023 �m) (e.g., Dearinget al., 1996a; Forster, Evans & Heller, 1994; Worm,1998).

High-temperature behaviour of magnetic suscepti-bility (in air atmosphere) was studied using KLY-3kappa bridge equipped with a high-temperature fur-nace CS-3 (Agico, Czech Republic). The accuracy of asingle susceptibility measurement is 1�10�7 SI.

Results and Discussion

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Figure 1. Sketch map with locations of the sampled archaeologicalsites.

Magnetic susceptibility versus origin of material

Figure 2 represents summarized results for low-frequency magnetic susceptibility (�LF) and �FD%values for the three groups of samples studied. Thedata for each group are shown as a percentageof all values in order to compare the three groups.

Factors of Magnetic Enhancement in Burnt Clays 1139

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Figure 2. Histograms of low-field magnetic susceptibility (�LF) and percentage frequency dependent magnetic susceptibility (�FD%) for burntsoil, clay plasters (ovens, hearths and kilns) and bricks. ‘‘N ’’ denotes the number of samples included in the corresponding histogram (thedifferences are due to the fact that some samples were measured only at low frequency).

Significant part of the samples of burnt soil and ovensshow low �LF, between 0·1–0·5 (10�6 m3/kg), althoughthere are also samples of plastered clay showing veryhigh �LF (15–20�10�6 m3/kg) (Figure 2). Magneticsusceptibility of brick samples is more evenly spreadover a wide interval 3–24 (�10�6 m3/kg), pointing tohigher magnetic enhancement. �FD% values exhibitclose to normal distribution (Figure 2) with maximumaround 6–10%, suggesting the presence of sig-nificant content of superparamagnetic magnetite grains(Dearing et al., 1996a) in the studied materials.

Burnt-clay materials, investigated in the presentstudy, were taken from structures of burnt soil and clayplasters at different archaeological sites. Therefore,their properties are related to the corresponding‘‘source material’’ to a certain extent. In most casesclay materials used for construction of ovens, kilns orhearths were not specially selected by ancient people(unlike the case of potteries) and probably are similarto the soil/clay types present at the surroundings ofeach site. This fact is also evident from the measuredmagnetic susceptibility values of burnt soils and clay


plasters (Figure 2). Higher magnetic susceptibilityvalues obtained for samples from ovens are mostprobably due to the higher degree of firing and mul-tiple usage of these structures, while burnt-soil ma-terials are often connected with fire which marks theend of a certain period in the existence of the archaeo-logical site. High percentages of relatively low suscep-tibility values measured (1–5�10�6 m3/kg) for burntsoil and plasters are most probably connected withnon-uniform and less intense burning of the claymaterial (especially burnt soil), resulting in lowerconcentration of ferrimagnetic grains and, respectively,causing lower susceptibility enhancement. Bricksamples generally show high susceptibility, due to hightemperature achieved during the baking procedure.Since magnetic susceptibility is a magnetic character-istic which depends mainly on the concentrationof ferrimagnetic minerals present (e.g. Thompson& Oldfield, 1986; Dunlop & Ozdemir, 1997), ourmeasurements suggest that brick samples are magneti-cally the most enhanced materials in terms of con-centration and degree of crystallinity of magnetite.�FD% for the three groups of samples is quite high(Figure 2). Greater percentage of samples with�FD%>10 for clay plasters and bricks is most probablydue to higher firing temperature, causing formation ofa bigger amount of fine (SP) iron oxide particles(Murad & Wagner, 1998).

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Burnt Soil χLF (× 10E–6 m3/kg)

Ovens, Hearths, Kilns χLF (× 10E–6 m3/kg)

Bricks χLF (× 10E–6 m3/kg)

Figure 3. Spatial distribution of low-field magnetic susceptibilitydata obtained for burnt soils (a), burnt clay plasters—ovens, hearths,kilns; (b) and bricks; (c). Black points denote the location of thearchaeological sites from which the corresponding materials arestudied.

Magnetic susceptibility versus geographical positionSpatial distribution of the magnetic susceptibility forall samples is shown in Figure 3(a)–(c). In these plots,�LF for all samples from the same site, is averaged. Theisolines are drawn applying the Kriging method. The‘‘hot spots’’ seen on the maps represent in all cases datafrom at least 4–5 samples. In addition, in many casessamples from one site represent burnt clay of differentages.

Magnetic susceptibility of samples from burnt soilshows specific distribution, characterized by highervalues for the south of Bulgaria, and minimum valuesin the north of Bulgaria (Figure 3(a)). In contrast, clayplasters (ovens, kilns, hearths) and bricks show thehighest susceptibility for sites in the north of Bulgaria,while low values come from sites in the southern partof the country (Figure 3(b), (c)).

Since the archaeological sites included in this study,are situated in different parts of the country, theoriginal unbaked material differs significantly. This isconnected with the influence of several factors on themagnetic properties of the burnt clay materials—(1)different initial concentration of ferrimagnetic grains;(2) different potential for iron supply (Fe sources)during heating from iron-bearing phyllosilicates; (3)geology, climatic influence and corresponding vari-ability in soil/clay types. In order to consider all ofthese factors, it is necessary to give short outline of themain lithologies found in the different regions ofBulgaria.

The territory of the country is naturally divided innorthern and southern parts by the Stara Planinamountain approximately at 42·7�N (Figure 1). Recentgeomorphological relief in northern Bulgaria is gener-ally covered by loess/soil sediments (Minkov, 1968;Evlogiev, 1993), while the outcropped rocks and thegeology of the southern part of the country is morecomplex. It is represented mainly by different-grademetamorphic and volcanic rocks. Consequently,weathering products used by man for constructions aremuch more complex. Moreover, the latter determines


formation of very different and widely varying claytypes on a local scale, suitable for production of burntclay structures by ancient people.

Because of the specific role of the Stara Planinamountains, there are also climatic differences betweenthe southern and northern parts. Continental climate(relatively hot dry summers and cold winters) is typicalfor the northern Bulgaria, while in the south ofBulgaria climatic conditions are more humid (Velev,1990). These circumstances determine different weath-ering rates of the parent material in the two regions.The combination of metamorphic/volcanic ‘‘source’’material (thermodynamically unstable in the surfaceconditions) and high precipitation in southern part ofthe country most probably leads to an advancedweathering of the primary materials in rocks. Conse-quently, there are also differences in the main phyllo-silicate clay minerals, developed as a result ofweathering. In the loess complex in northern Bulgariathe dominant clay minerals in soils are montmoriloniteand illite (Minkov, 1968), while stronger weatheringrates in southern Bulgaria lead to abundance of kao-linite as the common clay mineral. Differences in themain soil types developed in the two parts of thecountry should be also noted, as far as the loam is quitefrequently used material for construction of hearths.Chernozems and luvisols are the dominant topsoilsin the north of Bulgaria, while wide variety of soiltypes is found in southern part—cambisols, vertisols,chernozems, luvisols (Boyadjiev, 1994).

Considering the complex of the above pedologicaland geological factors, the following potential sourcesin the initial clay material for magnetic enhancement inthe north and south of Bulgaria can be considered:

(1) North Bulgaria: Loess sediments in northernBulgaria are weakly magnetic, magnetic suscepti-bility of the loess material is about 2–3�10�7 m3/kg (Jordanova & Petersen, 1999). There-fore the primary ferrimagnetic concentration islow. However, the prevailing montmoriloniteclays are characterized by high Fe-for-Al substi-tutions (Ganev, 1990; Goodman, 1994) and thereis a very high potential for Fe-supply duringmultiple heating.

(2) South Bulgaria: The primary minerals in rocks(metamorphic/volcanic) are more strongly mag-netic. At the same time, kaolinites are phylo-silicates with minor possibilities for cationsubstitutions (Ganev, 1990; Young, Mohamed &Warkentin, 1992) and, correspondingly, little ifany Fe ions are included in their structure. Thisdetermines low Fe-supply from phyllosilicatesduring heating.

On the basis of the above considerations, the follow-ing interpretation of the obtained spatial pattern ofmagnetic susceptibility of the three groups of materials(Figure 3) is proposed.

In most cases, burnt-soil materials for archaeo-magnetic studies are taken from remains of fire at thearchaeological site, or from the baked material belowthe well burnt ovens/kilns. This suggests that the firingtemperature was moderate and/or the burning was asingle event. Thus, one significant source for magneticsusceptibility enhancement is the primary ferrimagneticcontent, because thermal destruction of clay mineralsstarts at �400�C (Murad & Wagner, 1998; Wagneret al., 1998). Since the primary detrital ferrimagneticsare present in a higher concentration in southern partof the country, magnetic susceptibility of burnt soilmaterials is more enhanced there (Figure 3(a)). On thecontrary, multiple burning of clay plasters (ovens,kilns, hearths) and very high temperatures necessaryfor bricks preparation allow complete use of all poten-tial Fe-sources (including also Fe cations in clay min-erals). Since this Fe-supply potential is higher in thenorthern Bulgaria, higher enhancement for the burntplasters and bricks from these sites was obtained(Figure 3(b), (c)). Our results are in agreement with thehypothesis of Dearing et al. (1996b) that the magneticsusceptibility enhancement of top soils at the regionalscale (101–102 km) is linked to geology and soilprocesses, although our study deals with burnt-claymaterials.

Summarized average magnetic susceptibility dataseparately for north and south Bulgaria are shown inTable 1. It is again evident that for the sites in northernpart mass-specific values (�LF and �FD) increase fromburnt soil to brick materials, while �FD% is rather highbut slightly decreases. This is evidence for graduallyincreasing concentration of ferrimagnetic grains andslightly decreasing portion of SP fraction as firingbecomes more intense and higher temperatures arereached. However, for the sites in southern Bulgaria,this pattern is not observed (Table 1). �FD and �LF aremore similar for burnt soils and bricks, which is notusual. One possible reason is the presence of extremelyfine SP grains (of sizes below �0·012 �m) which do notcontribute to �LF (Dearing et al., 1996a). In general,the spatial distribution of magnetic susceptibility(Figure 3) is confirmed also by the calculated averagevalues (Table 1). The ‘‘outlayers’’ could be due to localsmall-scale variations in geology and/or particular claymaterial used for corresponding construction. On theother hand, combining all the data (Table 1, down),magnetic susceptibility of burnt soils and plasters issimilar, while bricks show mean value almost twicehigher.

Magnetic mineralogy versus type of materialMagnetic properties and mineralogy of burnt-clay ma-terials seem simple and easy to determine. However,this simplicity is only apparent. The main reason is thecomplex nature of the system ‘‘phyllosilicate clayminerals-iron oxides’’ because of its colloidal nature(e.g. Osipov, 1978; Murad & Wagner, 1998; Wagner


Table 1. Summary of minimum, maximum and calculated average values of magnetic susceptibility (�LF), mass-specific frequency-dependentsusceptibility (�FD=�LF��HF) and percentage frequency-dependent susceptibility (�FD%) separately for the sites from northern and southernBulgaria, as well as taking into account the data of the whole collection

Material�LF (min)

(�10�6 m3/kg)�LF (max)

(�10�6 m3/kg)�LF aver.

(�10�6 m3/kg)�FD (min)

(�10�7 m3/kg)�FD (max)

(�10�7 m3/kg)�FD aver.

(�10�7 m3/kg)�FD %aver.

North Bulgaria (Latitude >42·7�N)Burnt soil 0·11 7·77 3·04 0·18 7·20 2·42 9·92Ovens 0·27 17·87 6·66 0·19 17·60 6·47 9·34Bricks 0·14 24·00 8·21 0·09 29·20 7·35 8·35

South Bulgaria (Latitude <42·7�N)Burnt soil 0·28 10·45 3·23 0·18 10·60 2·50 7·41Ovens 0·25 13·32 2·31 0·05 12·20 1·76 7·70Bricks 0·41 11·26 3·53 0·20 13·50 3·05 8·07

All dataBurnt soil 0·11 10·45 3·17 0·18 10·60 2·47 7·80Ovens 0·25 17·87 3·77 0·05 17·60 3·18 8·23Bricks 0·14 24·00 6·64 0·09 29·20 5·85 8·09

et al., 1998), and most often processes occurringamong nanometer-sized particles should be considered.Fine iron oxide grains are strongly associated withalluminosilicates and that is why their magnetic separ-ation is very difficult or impossible (Osipov, 1978).Coarser magnetic grains are not mechanically attachedto clay minerals, usually they are derived from theparent rocks and comprise a major part of the sandand silt fractions (Young, Mohamed & Warkentin,1992). Consequently, there are several possible pro-cesses occurring during firing clays: (1) at the super-paramagnetic level—‘‘paramagnetic’’ relaxation untilthe blocking temperature is reached; growth of grainsand phase transformations in the original particles(Osipov, 1978); (2) processes in coarser single domainand multidomain grains—ferri/paramagnetic trans-ition at the Curie/Neel temperature and recrystalliz-ation during cooling, as well as phase- and chemicaltransformations.

In general, magnetic mineralogy of burnt clay re-mains depends mainly on the composition of the initialclay material, firing temperature and atmosphere(reducing or oxidizing) (Osipov, 1978; Wagner et al.,1998; Murad & Wagner, 1998). We have studiedmaterials from burnt soils, clay plasters and bricks, andwe assume that in most cases firing atmosphere hasbeen oxidizing, judging also from the reddish colourof majority of the samples. Reducing conditions couldbe supposed more often for pottery materials (e.g.,Wagner et al., 1998).

High-temperature behaviour of magnetic suscepti-bility is examined in order to determine the mainferromagnetic minerals, responsible for magneticenhancement of the burnt clays. Examples of thermo-magnetic curves of burnt soil samples from differentsites and age are shown in Figure 4. Quite diverseshapes of the curves are obtained, although in all casesmagnetite Curie temperature (Tc�580�C) is dominant.Generally, the heating and cooling branches do not

coincide, which supposes that certain mineralogicalchanges occur during heating to 700�C. Similarly,thermomagnetic curves for materials of clay plasters(Figure 5) confirm the presence of magnetite as themain ferrimagnetic mineral. In some cases low-temperature phases are also detected (e.g. samples2110d, 2124g), which disappear on cooling. Non-reversible behaviour is again characteristic for thesematerials. Hematite Curie temperature was not de-tected (with an exception of sample SLN21 only),which suggests that this mineral is not present in wellcrystalline form in the studied baked materials. Wecould draw such a conclusion because thermomagneticanalysis of magnetic susceptibility is a powerfulmethod for detection of hematite, even when it ispresent in minor quantity. The main reason is thatafter heating the material above 580�C, magnetite doesnot contribute any more to the measured susceptibilitysignal, therefore allowing detection of hematite, ifpresent. The absence of hematite is not very surprising,since Wagner et al.’s (1998) experiments on heatingclays showed that crystallized hematite appears uponheating above 800–1100�C. Such high temperatures arerarely achieved during burning of soils and ovens (e.g.Hathaway, 1990). On the other hand, the reddishcolour of the samples suggests that hematite should bepresent. One possible explanation is that hematiteexists in amorphous state, causing the pigmentation ofthe burnt clay but not contributing to the magneticproperties. There is no uniform behaviour of thethermomagnetic curves with regards to the coolingbranch (Figures 4 & 5). In some cases (e.g. samplesRs60 from burnt soil and DB19v from oven) magneticsusceptibility enormously increases on cooling, whichis most probably due to breakdown of clay mineralsand formation of new strong ferrimagnetic phase.Hence, these materials were probably not burnt to hightemperatures in ancient time. The observed decreaseof magnetic susceptibility (e.g. samples 2136g, Pl12v


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Figure 4. High-temperature behaviour of magnetic susceptibility for selected burnt soil samples from different sites. Sample number, site nameand midpoint of the dating interval are shown for each example. Heating curve is plotted by thick line, cooling, by thin line.


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Figure 5. High-temperature behaviour of magnetic susceptibility for selected samples of clay plasters from different sites. Sample number, sitename and midpoint of the dating interval are shown for each example. Heating curve is plotted by thick line, cooling, by thin line.


(burnt soil), 2124g D83 (ovens), etc.) could indicateadvancing oxidation which is the most characteristicprocess in clays during heating in air (Murad &Wagner, 1998).

In contrast, all brick samples show stable suscepti-bility behaviour (Figure 6), which proves that suf-ficiently high temperatures were achieved during theirpreparation. Nevertheless, magnetite (or titanomagnet-ite) is the dominant ferrimagnetic phase and no hema-tite is detected by K(T) measurements. Minor changesduring cooling (Figure 6) suggest that the magneticmineralogy is practically stabilized and probably nophase changes occur during heating on 700�C. Likeother burnt clay materials, bricks also show differentshape of the thermomagnetic curves (Figure 6), evi-dencing a range of grain-size distributions in separatesites. Similar conclusions about the main ferrimagneticmineralogy of different archaeological materials(although deduced from the results of other magneticmeasurements) were reported by Evans & Jiang(1996), Jordanova, Petrovsky & Kovacheva (1997) andKovacheva, Jordanova & Karloukovski (1998).

High-temperature behaviour of magnetic suscepti-bility of the studied burnt clay materials reveal that: (1)magnetite or titanomagnetite with low Ti content is adominant ferrimagnetic mineral in all samples; (2)there is a great diversity of grain-size distribution in thematerials from different archaeological sites; (3) min-eral composition of burnt soils and plasters duringheating is more unstable as compared to bricks. Thelast result suggests that lower temperatures (below�700�C) were achieved during burning of soilsand plasters, while firing temperature during brickspreparation was higher.

Frequency dependent magnetic susceptibility of burntclay materialsMeasurements of frequency-dependent magneticsusceptibility are very useful for detection of finesuperparamagnetic magnetite grains in different en-vironmental materials. Mass-specific- and percentagefrequency dependent magnetic susceptibility are calcu-lated for the samples used in the present study, andplotted as a function of low-field magnetic susceptibil-ity (�LF) (Figure 7(a), (b)). Evidently, unlike �FD%,mass-specific �FD=�LF��HF shows very good lineardependence on the low-field susceptibility �LF. Thisconfirms the idea (Dearing et al., 1997; Eyre, 1997) thatthe mass-specific frequency dependent magnetic sus-ceptibility is a good estimate of the SP concentration.However, for one and the same values of �LF and �FDa range of �FD% are obtained (Figure 8(a)), which ismost probably a result of different relative amount ofthe SP-fraction in the total ferrimagnetic assemblageand varying grain-size distribution in the superpara-magnetic range for different samples/sites. Theoreticalcalculations (e.g. Stephenson, 1971; Worm, 1998)showed that low-field magnetic susceptibility of

magnetite grains increases significantly with decreasinggrain volume from SD to SP, while �FD% remainsconstant for high �LF. Therefore, the large data scatteron the �LF versus �FD% graph (Figure 7(a)) could bedue to both increasing ferrimagnetic concentration andgrain-size changes. Dearing et al. (1997) showed in amodel mixing experiment that addition of increasingamount of MD-magnetite to soil, containing predomi-nantly SP magnetite grains (�FD%=10·5), causes �FD%to decrease to <2, while �LF increases with concen-tration. Another factor, which should be considered, isvarying contribution of the para- and diamagneticminerals (Worm, 1998), since the studied collectionconsists of samples from 60 different sites. This effect,however, would be significant for weak samples (�LF�0·1 �m3/kg (Dearing, 1994)) which are only few. Allof the samples measured (except three cases) showed�FD%>2 (Figure 7(a)), which according to the Dearinget al.’s (1997) model, suggests that archaeologicalremains of burnt clay contain significant amountof fine superparamagnetic (SP) magnetite grains(�0·012–0·023 �m). About half of the samples show�FD% between 8 to 11, which would correspond to>50% SP content (Dearing et al., 1997). Interestingly,in spite of the argued upper limit of �15% for �FD%for natural samples (Worm, 1998), archaeologicalsamples included in our study also show maximum�FD%=14 (Figure 2, the value of 16% is most probablydue to measuring error, since �LF for this sample is thelowest one), as shown by Dearing et al. (1996b) fortop-soil samples from England. This fact is not incontradition with the Worm’s (1998) model if we takeinto account that archaeomagnetic materials mostoften contain ferrimagnetic particles of wide grain-sizespectrum (Cui et al., 1997; Jordanova, Petrovsky& Kovacheva, 1997; Kovacheva, Jordanova &Karloukovski, 1998). The non-unique relationship be-tween �LF��HF and �FD% (Figure 7(c)) may be due tovariations in the grain-size distribution among differentsamples and materials, but also due to the presenceof very fine SP grains of sizes <0·012 �m, which donot contribute to the frequency-dependent fraction(Dearing et al., 1997). The possibility of the presenceof such ultra-fine particles is not excluded, because ingeneral clay minerals, as well as minerals newly formedduring firing of iron-oxide grains are mostly ofsub-micron size and superparamagnetic at roomtemperature (Murad & Wagner, 1998).

Conclusions

(1) Enhancement of magnetic susceptibility of burntclay depends mainly on the Fe-sources availablefrom the parent unbaked clays, as well as on thefiring temperature. For the studied collection ofarchaeological remains of burnt clay the strongestmagnetic susceptibility was observed for samplesfrom bricks.


0 600

100

T (°C)

K (

× 10

–6 S

I)/g

20

60

400200

1331b, V. Tarnovo, church "Sv. 40machenici"; 625 y. BP

0 600

600

T (°C)

K (

× 10

–6 S

I)/g

200

400

400200

1307b, V. Tarnovo church "Sv.Dimitar"; 875 y. BP

Bricks

0 600

1500

T (°C)

K (

× 10

–6 S

I)/g

500

1000

400200

908v, Preslav, 1075 y. BP

0 600

1200

T (°C)

K (

× 10

–6 S

I)/g

400

800

400200

1855a, Kipra, 1185 y. BP

0 600

800

T (°C)

K (

× 10

–6 S

I)/g

200

600

400200

1861b, Kipra, 1185 y. BP

0 600

600

T (°C)

K (

× 10

–6 S

I)/g

200

400

400200

1132v, Drianovo monastery,1385 y. BP

0 600

300

T (°C)

K (

× 10

–6 S

I)/g

100

200

400200

1094v, Abritus, 1720 y. BP

0 600

400

T (°C)

K (

× 10

–6 S

I)/g

100

200

400200

377v, Nessebar, 1945 y. BP

80

40

400

300

Figure 6. High-temperature behaviour of magnetic susceptibility for different brick samples. Sample number, site name and midpoint of thedating interval are shown for each example. Heating curve is plotted by thick line, cooling, by thin line.


(2) The main ferrimagnetic iron oxide mineral, re-sponsible for the magnetic susceptibility of burntclays is magnetite or titanomagnetite with low Ticontent, of different grain-sizes.

(3) Obtained spatial distribution of magnetic suscep-tibility of burnt clay samples from different sitesin Bulgaria, is most probably due to differentgeology and climatic conditions and, hence,potential of the material for Fe-supply duringfiring process.

(4) Burnt clay archaeological materials containsignificant amount of fine superparamagneticmagnetite particles of varying grain-size, whichdetermine high magnetic susceptibility of theseremains.

The obtained results, linking magnetic susceptibilityenhancement and high-temperature behaviour to theinitial clay material could be used for application ofmagnetic susceptibility mapping of archaeologicalsites. Further experimental work is intended to studyproperties and behaviour of different initial claymaterials during heating to successively higher tem-peratures, in order to better explain the observed inthis study relations.

025

16

χLF (× 10–6 m3/kg)

χ FD

%

0

2

4

6

8

10

12

14

5 10 15 200.001

100.00

χLF (× 10–6 m3/kg)χ L

F –

χH

F (

× 10

–6 m

3 /kg)

0.01

0.010

0.100

0.10 1.00 10.000

3.5

16

χLF – χHF (× 10–6 m3/kg)

χ FD

%

0.0

2

4

6

8

10

12

14

0.5 1.0 2.0 3.0

1.000

2.51.5

Figure 7. Plots of different relations between mass-specific low-field magnetic susceptibility (�LF), mass-specific frequency-dependentsusceptibility (�LF��HF) and percentage frequency-dependent susceptibility (�FD%). Discussion in the text. �, Burnt soil; �, ovens;�, bricks.

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Factors Determining Magnetic Enhancement of Burnt Clay from Archaeological Sites

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