Bulgarian archaeomagnetic studies: A review of methodologicalprogress and applications in...

Journal of Radioanalytical and Nuclear Chemistry, Vol. 247, No. 3 (2001) 685�696

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M. Kovacheva,* N. Jordanova**

Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl.3, 1113 Sofia, Bulgaria

(Received August 25, 2000)

Archaeomagnetic studies in Bulgaria have a long history and the well established secular variation curves of the three elements of the ancient

geomagnetic field (declination, inclination and intensity) for the last 8000 years enable the dating of archaeological features of burnt clay,

independently of other methods. The determination of ancient palaeointensity is the most difficult characteristic and requires very cautious

evaluation of the suitability of the burnt clay material. The present paper is an overview of the methodological progress in studying the suitability

of the materials for archaeomagnetic investigations. The main rock-magnetic methods and summary of the most common results are presented

involving the archaeomagnetic practice in the palaeomagnetic laboratory in Sofia. In addition, supplementary information obtained by magnetic

measurements, which can be helpful for archaeology, are discussed. An example of archaeomagnetic dating procedure is also presented.

Introduction problem based on our experience. The great importance

of archaeomagnetic studies for geophysics is discussed

elsewhere.5,2Archaeomagnetic studies have undergone an

extensive development during the last decades mainly as

an unique method to discover the past geomagnetic field

behavior. The geomagnetic field is defined by two

angles declination (D) and inclination (I), both measured

in degrees and the absolute value of the intensity F,

measured in µT. The difficulty of determining both the

direction and intensity of the geomagnetic field reveals

the existence of many �spot� readings of one or the other

in different parts of the world. The Bulgarian studies

cover the longest time period (8000 y)1,2 using dated

archaeological materials. When the geomagnetic

variations for a given territory are established, they can

be used for dating purposes. Some other applications in

archaeology are also possible such as the verification of

�in situ� baked archaeological structures, firing

temperature etc.

Methodological studies

Materials used for archaeomagnetic investigations

The main condition, determining the suitability of the

archaeological materials for archaeomagnetic investi-

gations, is that they must be all baked. This is necessary

because of the physical principles, creating the basis of

the archaeomagnetic determinations. It has been shown6

that ferrimagnetic minerals which are heated to high

temperatures in the presence of the Earth�s magnetic

field, �fossilize� its direction and intensity at the time of

the last firing by acquiring a so-called thermoremanent

magnetization (TRM). It has been proved7 that this TRM

is extremely stable with time, therefore the geomagnetic

field�s directional and intensity data can be �recovered�

by studying well baked archaeological remains of

different age.

There are many obstacles in solving the direct

problem in archaeomagnetism (tracing the palaeosecular

variation) which is of geophysical interest. One of these

problems is connected with prehistoric 14C dates. For

example during the Bulgarian Aeneolithic period (ca.

5000�4000 BC) there are many contradictions in the

calibrated radiocarbon dates, which made difficult the

juxtaposing of the archaeomagnetic determinations with

the absolute scale of time.3,4 The problem has been

solved using the well established relative chronology of

the prehistoric cultures in Bulgaria.3 Obviously the

problems in archaeology directly reflect the

completeness and exactness of archaomagnetic studies.

The second general problem is of methodological

character and arises from the difficulties in the

determination of the geomagnetic intensity. The main

purpose of this paper is to emphasize this second

Archaeological remains which are studied

archaeomagnetically consist of burnt soil layers, clay

plasters of ancient ovens, kilns, etc., bricks, pottery.

However, data for the direction of the ancient

geomagnetic field can be obtained only from those

remains found �in situ� since the last firing � these are

burnt soil, clay plasters, and in some cases bricks.8,9 The

other baked-clay finds can give information about the

palaeointensity, but not the direction. Although all burnt-

clay materials usually satisfy the main �requirement� to

carry a TRM, there are many factors which cause

differences in their properties and their suitability for an

archaeomagnetic study. We shall briefly discuss the most

important ones below.

* E-mail: [email protected]

** E-mail: [email protected]

0236�5731/2001/USD 17.00 Akadémiai Kiadó, Budapest

© 2001 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW

Degree of heating: There are significant differences

in the degree to which materials have been burnt in the

antiquity. Usually burnt soil layers carry only a partial

TRM and have not been heated to very high

temperatures. Clay plasters of ovens and kilns are burnt

to higher degree, as compared to soil, but this also

depends on the particular usage of each structure (e.g.,

multiple or single burning; the purpose of the structure �

domestic, religious, manufacturing center, etc.). In

contrast, bricks and potteries are baked to quite high

temperatures10,11 and almost always carry a full TRM.

As it will be discussed later, the degree of heating is one

of the most important factors in determining the

suitability of archaeological remains for archaeo-

magnetic study.

products of low-temperature oxidation could cause

significant difficulties in archaeomagnetic determinations, or

even make the material unsuitable for investigation.2 Our

experience suggests that archaeological materials, taken

from more southerly countries (Greece,16 Morocco),17,18

show in most cases magnetic properties which are

favorable for obtaining reliable archaeomagnetic results.

This is especially important in the experiments for

obtaining ancient geomagnetic field intensity.

Methodological problems in the determination

of the palaeointensity of the Earth�s magnetic field

Palaeointensity determination is the most difficult

and problematic branch of archaeomagnetism. The main

reason is that the experimental method, developed for

obtaining palaeointensity,19 requires particular magnetic

properties � single-domain assemblage of grains, which

carry the TRM; the mineralogy of these grains should

not have changed significantly during the burial time;

absence of chemical and/or phase changes during

laboratory heating. Moreover, the classical THELLIER

method is very time-consuming, while the success of the

experiment is not guaranteed and very often significant

part of the experimental results is rejected.

Composition of the initial unbaked material: Raw

materials used for initial preparation of the

archaeological structures are different clays, soils, loams.

All these materials contain mainly layered phyllosilicate

minerals, which are of extremely fine grain size. The

initial content of strongly magnetic Fe-bearing minerals

is usually very low, because they are present as

accessory phases. However, depending on the type of the

clay mineral (e.g., kaolinite, montmorillonite,...), there

are significant differences in the content of Fe-ions

present as substitutions in the structure of the clay

minerals.12 These ions are potential sources for the

creation of strongly magnetic minerals during heating to

high temperatures, when thermal breakdown of clay

minerals occur.10 As discussed in JORDANOVA et al.,13

existing differences in the dominant clay minerals are

one of the reasons for various magnetic enhancement of

burnt clays. Another factor, which is an �anthropogenic�

one, is the pre-selection of the raw material for pottery

and bricks manufacture, made by ancient people.2

Consequently bricks and potteries are more

homogeneous and fine-structured than materials from

ovens and kilns, while burnt soil layers contain the

widest grain-size and mineralogical spectra.

In order to have a complete as possible information

about the suitability of the material for carrying out

palaeointensity determination, a number of rock-

magnetic experiments were involved in the

archaeomagnetic practice in the Sofia palaeomagnetic

laboratory during last few years.

Determination of main magnetic minerals, carrying

the natural remanent magnetization (NRM): Several

methods are applied for this purpose, including thermal

and isothermal ones.20,2

Alternating field (AF) demagnetization of NRM

gives information about the coercivity of the mineral

magnetic phases present in the material, as well as about

the vector components of NRM.21 In most cases

magnetite (magnetically soft mineral)22 is the dominant

ferrimagnetic mineral in archaeomagnetic samples,5,20,2

so that AF demagnetization removes significant part of

NRM (Fig. 1) at 15�20 mT. Only in a few cases is

hematite (magnetically hard mineral) detected.

Influence of burial conditions: After their last use

different archaeological features have been buried until

uncovered by recent archaeological excavations. During

the burial period, the influence of environmental

conditions is inevitable. The most common process

which proceeds with time, is weathering. It causes

changes in the initial minerals (magnetic, as well as para-

and dia-magnetic), which can notably modify the

physical properties of the �primary� materials.14

Duration and temperature contrast between the seasons,

wet/dry cycles, the particular combination of the

landscape and the level of ground water at each site, all

influence the intensity of weathering processes in a

different way. As a result of weathering, low-temperature

oxidation of the initial ferrimagnetic minerals results in

formation of iron oxyhydroxides.15,5 The presence of

Thermal methods, based on heating samples to high

temperatures in field-free space or in the presence of

weak or strong magnetic field, are widely used for

determination of the kind of magnetic mineral using its

Curie temperature (Tc) (which is strictly specific for each

ferri/antiferrimagnetic mineral) or unblocking temperature

(Tb). In our practice we use continuous thermal

demagnetization of saturation remanence (Jrs or SIRM,

induced in a field of 2T), step-wise thermal demagnetization

of 3-component isothermal remanence (IRM)23 or high-

temperature behavior of magnetic susceptibility.

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The first two methods give the Tbs, while the last one

allows determination of Tc.

Wide variety of Jrs(T) curves is obtained for

different archaeomagnetic materials, some examples are

shown in Fig. 2. Low-temperature Tb (in the range

100�150 °C) is detected in many Bulgarian samples (one

example shown in Fig. 2a). It can be interpreted as a

presence of iron oxyhydroxides (e.g., goethite),24 but

also as a structural effect, reflecting unblocking of grains

of particular sizes. In most cases this low-temperature

phase disappears on the second heating curve, indicating

that it is converted to other phase or redistribution

of the grain-sizes occurred during heating. The main

Tbs reveal that in most cases magnetite/titanomagnetite

is the dominant ferrimagnetic mineral, since we

obtain Tb in the interval 400�580 °C (Fig. 2).

Fig. 1. Step-wise alternating field (AF) demagnetization of

archaeomagnetic samples of different kind of burnt-clay material

Fig. 2. Examples of continuous thermal demagnetization of saturation remanence (Jrs), induced twice in a pulse

magnetic field of 2T in one and the same specimen

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The ratio Jrs2/Jrs1, calculated from the Jrs values

measured after the initial saturation and after the second

saturation of the already heated sample, respectively, is

used to detect whether some changes occur in the sample

during heating. This parameter widely varies for the

samples studied and depends on the particular

mineralogy, and indirectly, on the degree of firing of the

corresponding material.

strongest one (Fig. 3), while the relative significance of

intermediate and hard components can vary among

samples. This confirms the dominant role of the

magnetically soft minerals, which determine the

remanent characteristics of the materials studied. In the

case of intensive weathering processes the hard

component shows low Tbs (~100�200 °C), indicating the

presence of iron oxyhydroxides (Fig. 3b). It is interesting

to note, that the products of low-temperature oxidation

are detected not only in samples, taken from ovens,

kilns, etc.2 but also in bricks.25

Step-wise thermal demagnetization of composite 3-

axis IRM23 carried out on archaeomagnetic samples,

demonstrate that in most cases the soft component is the

Fig. 3. Step-wise thermal demagnetization of composite isothermal remanence (IRM), carried out following LOWRIE�s method23

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Fig. 4. High temperature behavior of magnetic susceptibility for different archaeomagnetic samples. The thick line denotes heating run,

the thin one � cooling. Note that for brick sample the two branches (heating and cooling) almost coincide,

unlike samples from burnt soil and ovens

Thermomagnetic analysis of magnetic susceptibility

gives information about the Curie temperatures of the

magnetic minerals present. Some examples are shown in

Fig. 4. In all cases magnetite (Tc~580 °C) is detected, no

matter what kind of the material is studied. Commonly,

hematite (Tc~670 °C) is not observed, except in rare

cases. Tc of about 200�300 °C (Fig. 4) indicates most

probably the presence of maghemite, which is also a

product of the low-temperature changes in material26,22

and can be a serious obstacle for obtaining

palaeointensity result.

determination of domain state of the NRM carriers is a

very important stage in rock-magnetic investigations of

archaeomagnetic materials.

LOWRIE and FULLER28 proposed a test for the

determination of the domain state of materials with TRM

by comparing relative stability of NRM (presumably a

TRM) and the laboratory induced isothermal remanent

magnetization (IRM). It was shown that if the NRM is

carried by SD grains, it is more stable than IRM against

AF-demagnetization, and vice versa for MD grains. The

method seems to work for volcanic rocks,29 which also

possess remanent magnetization of thermal origin,

although there are also studies, demonstrating that it is

not valid.30 In spite of these contradictions, the Lowrie-

Fuller test is applied in the rock-magnetic practice,

combined with other methods. Figure 5 shows some

representative examples for archaeomagnetic samples.

Results, indicating SD or mixed behavior are the most

common (Fig. 5a and b), although MD characteristics

are also obtained (Fig. 5d). The example given in Fig. 5c

is also often encountered in archaeological baked clay.

Determination of domain state of the ferrimagnetic

grains: The most crucial point in the theory of

evaluation of the absolute value of palaeointensity is the

requirement for single-domain (SD) sizes of the

remanence carriers. The critical SD threshold is different

for the different minerals, because it depends on the

value of saturation magnetization (Js), which varies in a

wide range for ferri/antiferromagnetic minerals (e.g.,

Reference 22). For magnetite, the SD-grain size range

lies between 0.03�0.05 µm.27,22 Therefore, the

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Fig. 5. Examples of different results from the LOWRIE-FULLER method28 for determination of the domain state of rocks, carrying thermoremanent

magnetization. Classification according to that, proposed by DUNLOP29: (a) single-domain (SD) behavior; (b) mixed type behavior; (c) bi-modal

type; (d) multi-domain (MD) behavior

As it is explained in DUNLOP29 such kind of behavior is

typical for samples containing both the SD domain fine

grains and coarse ones. When the low field

magnetization (NRM) is governed by the fine grains, the

high field one is mostly influenced by the presence of a

coarse fraction and the AF demagnetization curve is

much softer and almost exponential. In these cases we

have a pronounced SD-type configuration named

�bi-modal�. It is important to bear in mind, that the

LOWRIE-FULLER test assumes that the only magnetic

mineral, which is NRM-carrier, is magnetite.28 So, if

other mineral phases (like hematite, goethite) are

present, the results will be biased.

Hysteresis measurements are also domain-state

diagnostic31,26,22 and are used in our laboratory practice.

However, very often the hysteresis parameters and ratios,

used for magnetic granulometry31 are influenced by the

presence of mineral and/or grain-size mixtures.32 Very

often this is the case in archaeomagnetic samples,25,20 so

that the hysteresis measurements are not useful for

reliable determination of the domain state. As it could be

seen (Fig. 6) on the plot Jrs/Js vs. Hcr/Hc, used for

identification of the domain state31 all the samples plot

in pseudo-single domain (PSD) range, which is most

probably a result of an admixture of very fine

[superparamagnetic (SP)] grains,32,33 or in some cases a

high-coercivity mineral (hematite, goethite).

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Fig. 6. Day plot (DAY et al.,31) for archaeomagnetic samples from various archaeological sites (see the legend) in Bulgaria

Detection of chemical/phase changes in the material

during heating: One of the main principles, on which the

THELLIER method19 relies, is that no chemical changes

occur in the material during successive heating cycles. If

this condition is not met, there is a big possibility that the

capacity for remanence acquisition changes in a certain

temperature interval and the laws of additivity of the

partial TRMs is not fulfilled anymore. This causes a

failure of the palaeointensity experiment and the result

should be rejected. Therefore, the question of monitoring

eventual chemical/phase changes during heating is of

crucial importance too.

susceptibility). The curve of SIRM-left(T) is compared

with the thermal demagnetization curve of SIRM of a

sister sample, in which SIRM (or the module of 3-axis

IRM, named 3IRM) is induced only at the beginning of

the demagnetization cycle. Usually for this SIRM we

take the result of previously applied thermal demagne-

tization of the composite 3-axis IRM23 on the sister

specimen. Two examples are shown in Fig. 7a and c.

Coincidence of the two SIRM(T) decay curves and

stable behavior of K(T) and SIRM-acqured(T) indicate

that during each successive heating no new ferrimagnetic

phase, capable of carrying remanence, has appeared

(Fig. 7a). In contrast, Fig. 7c shows a sample which

undergoes significant mineralogical changes, as

evidenced by the behavior of the monitored parameters

(continuous increase of SIRM-acquired; changes in K;

significant differences between the decay behavior of

SIRM-left and SIRM, monitored in a sister specimen).

For comparison, Fig. 7 b and d shows results from the

THELLIER experiment for obtaining palaeointensity,

carried out on sister specimens from the same samples.

Palaeointensity experiment for sample DG36z is

accepted, and the calculated FA is shown below the

graph. Result from sample DG44, which showed

chemical/phase changes during heating (Fig. 7c) was

rejected, based also on the acceptance-rejection criteria

established in the Sofia palaeomagnetic laboratory.5

Although during the THELLIER experiment carried

out in Sofia palaeomagnetic laboratory, monitoring of

behavior of room-temperature magnetic susceptibility

(K) after each temperature step is accepted as one of the

conditions for evaluating reliability of the result,2,5 we

are aware that in many cases this is not always a

sufficient indication for the absence of chemical

changes.20,2 That is why, we have applied an additional

test, based on the idea of VAN VELZEN and

ZIJDERVELD,34 but slightly modified.35,20 It consists of

step-wise thermal demagnetization of SIRM, induced in

a sample after each temperature step, and monitoring

behavior of several parameters before and after

saturation (SIRM-left after heating to a given

temperature, SIRM-acquired after that; magnetic

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The method for detection of mineralogical changes

which occur during heating, has the advantage, that it

relies also on remanence-carrying parameters (SIRM),

while magnetic susceptibility value reflects the

contribution of all grain-size and mineral assemblages

(including para- and dia- and ferrimagnetic phases).

The one �obstacle� for its application is that the

method is time-consuming, but on the other hand, it

gives much valuable information. The given examples

in Fig. 7 confirm the above stated importance

of the degree of firing in the antiquity.

Fig. 7. Two examples (a, c) of the experiment, applied to detect chemical/phase changes during heating in archaeomagnetic samples.

Results from the THELLIER experiment for determination of absolute palaeointensity value, carried out on sister specimens from

the same samples (b, d), is shown. See details in the text

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The samples numbers DG 36 and DG44 are from two

different structures of one and the same Thracian site.

Obviously the firing temperature of these structures was

not the same and the sample DG44 has been

insufficiently baked in the antiquity which is the cause of

the revealed chemical/phase changes during the

laboratory heating. Thus the firing heterogeneity which

we observe some time even in one and the same

structure can be an obstacle for palaeointensity

determination.

The presence of significant anisotropy of pottery

samples is well known39 and confirmed by our study of

Greek potteries,40 when the degree of susceptibility

anisotropy reaches 23%.

Anisotropy of remanent magnetization: The

remanent magnetization also exhibits anisotropic

properties, which are significantly enhanced in pottery39

and some bricks.41,42 It has been shown43 that the

laboratory-imparted remanences ARM (anhysteretic

remanent magnetization) and low-field IRM give good

approximation of the NRM anisotropy. The best

approximation is, of course, laboratory TRM but the

main obstacle to its application is common chemical

changes in the material in the course of heating.

Supplementary information obtained

from archaeomagnetic materials

Magnetic texture of the material influenced by

mechanical treatment: Magnetic susceptibility

measurements can give information not only about the

degree of magnetic enhancement of the material, but also

on some structural characteristics. Strictly speaking,

magnetic susceptibility is not a scalar quantity, but it can

be presented by a symmetrical second-rank tensor36�38

and shows directional anisotropy (i.e., there are �easy�

and �hard� directions). Geometrically it is visualized by

a rotational ellipsoid with maximum, intermediate and

minimum axes (Kmin, Kint, Kmax). It has been shown37,38

that for magnetite (with shape anisotropy dominant, e.g.,

Reference 22) the susceptibility tensor reflects the shape

of the magnetite particles. For minerals with strong

crystallographic anisotropy, the principal susceptibility

axes coincide with the corresponding crystallographic

directions.37,38 As it was discussed above, the most

common magnetic mineral in the archaeomagnetic

materials studied, is magnetite/titanomagnetite, which

means that the AMS would reflect the distribution of

magnetic grains in the baked clay.

In our practice, we apply laboratory induced IRM for

evaluation of the anisotropy effect on the remanence and

correction of the palaeointensity value.35,2,44 Our data

suggest that both methods give comparable correction

values in spite of the different origin of IRM, compared

to NRM. Correction for anisotropy of the palaeointensity

value, obtained for samples from clay plasters of ovens,

varies between 0 and 6�7%, which is quite small and its

influence on the final value is of the same order as other

errors arising from e.g., cutting technique, field

orientation, etc. Brick samples usually show a high

anisotropy (unpublished data) and in some cases highly

anisotropic samples are also known.41,42 Potsherd

samples show significant anisotropy and the correction

must be included to the final evaluation of the

palaeointensity.39,40 The correction factor reaches 27%

and significantly influences the result.

Frequency dependence of magnetic susceptibility of

archaeomagnetic materials: Magnetic susceptibility is a

characteristic which depends mainly on the

concentration of the ferrimagnetic minerals present,44

but where they are in only trace amounts then the para-

and diamagnetic contribution is significant.44 It has been

shown theoretically45,26 and experimentally45,46 that the

finest (SP) magnetite grains (sizes 0.010�0.020 µm)exhibit the highest values of magnetic susceptibility

when measured at low frequencies of the applied field.

Although in all cases of studied clay plasters of

ovens, the degree of anisotropy P=Kmax/Kmin36 is up to

7�8%,35,2 there is a very well defined magnetic fabric

(Fig. 8). The min-susceptibility axes are grouped in a

vertical plane and max- and int-axes are distributed in a

horizontal plane, which coincides with the oven�s

surface. This suggests that magnetic grains are aligned

with their long axes in this plane. Our conclusion is that

the observed �sedimentary� magnetic fabric reflects the

distribution of ferrimagnetic particles which mimic the

strong preferred orientation of the clay particles under

mechanical treatment.35,2 Investigation of AMS also

gives information as to whether the oven�s surface is

inclined (Fig. 8b) and different implications of this

discovery could be made, for example inferring an

ancient earthquake (unpublished data) or an attempt to

correct the obtained directional results. In case of

samples taken from destructions or burnt soil, the

magnetic fabric is completely random (Fig. 8c).

Frequency-dependent susceptibility measurements on

archaeomagnetic samples suggest that all materials of

baked clay contain significant amount of fine SP

grains.13 Percentage frequency-dependent susceptibility

χFD% (defined as: (χLF�χHF)×100%/χLF) varies

between 5�12%. This, according to the

phenomenological model of DEARING et al.46

corresponds to 20�50% and more percentage content of

fine SP grains in the material. The influence of the type

of parent clay material, degree of heating on the

magnetic susceptibility enhancement and magnetic

mineralogy of burnt clay materials is discussed in detail

elsewhere.13

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Archaeomagnetism as a dating tool

Provided that palaeosecular variations of the

geomagnetic field are well established for a given

territory, they can be used to resolve the inverse problem

in archaeomagnetism, that is dating. Since the Bulgarian

variation curves cover almost completely the last 8000

years, including numerous sites,1,5,2 they can be used for

archaeomagnetic dating. One significant advantage of

the existing data base is that it consists of not only the

direction of the ancient geomagnetic field (which is the

usual practice in many other archaeomagnetic

laboratories), but also palaeointensity determinations.

This fact ensures a higher reliability of the obtained

dating interval, because more parameters are involved.

Another aspect of the archaeomagnetic dating is the

possibility of independent synchronization amongst

different sites to be inferred on the basis of their

archaeomagnetic dates.

One example of archaeomagnetic dating is presented

in Fig. 9. The aim was to date the medieval site of

Lubimec, Southern Bulgaria. A collection of 28 oriented

samples has been studied and the directional

geomagnetic characteristics obtained are: I=59.71°,

D=14.85° with uncertainty7 α95=1.82°. The weighted

average value for the paleointensity5 is F=51.76±2.71 µT. From 12 experiments carried out for the

palaeointensity determination, the results of 11 were

accepted, according to the acceptance-rejection criteria

of the laboratory.5 In Fig. 9 only part of variation curves

of the ancient geomagnetic field characteristics is

depicted, including the interval, where the supposed date

of the site should be found. Variation curves for the

inclination (a), declination (b) and palaeointensity (c) are

shown with the corresponding uncertainty bands. The

obtained archaeomagnetic characteristics (also shown

with their deviations) for the studied site are marked with

horizontal lines, crossing the variation curves. The

shaded intervals on the time-axis correspond to the

intersection of the master curves for I, D and F with the

obtained data for the studied site. It is clear that the

solution for a given element is non-unique (Fig. 9). That

is why, the final dating is obtained by stacking all the

dating intervals, obtained for the three elements, I, D, F

(Fig. 10). Considering this example, it is evident that the

use of both directional and palaeointensity data, gives

the maximum possible dating accuracy in

archaeomagnetic dating.5 Therefore, the big advantage

of the secular variation curves of the ancient

geomagnetic field for Bulgaria is not only of geophysical

significance, but also of great help for archaeology.

Fig. 8. Examples from measurements of anisotropy of magnetic

susceptibility (AMS) for samples from horizontal oven�s floor (a);

slightly inclined oven�s floor (b); and destructions of burnt clay (c).

Stereographic projections on the lower hemisphere

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Fig. 9. Archaeomagnetic dating of medieval site Lubimec. The obtained results for D, I, F are compared

with the secular variation curves for Bulgaria. Details are in the text

Fig. 10. Final result from the archaeomagnetic dating. The obtained results for ancient direction (declination, inclination)

and intensity (F) allow more precise determination of the most probable dating interval

Conclusions important applied aspect of the archaeomagnetic studies

� dating � also benefits (although indirectly) from the

methodological progress. The rock-magnetic results can

also give additional information, which can be useful for

archaeology.

Archaeomagnetic studies in Bulgaria have a long

history of the accumulation of new data, but also of

improving the accuracy of the determinations.

Methodological improvements, involving a lot of rock-

magnetic experiments help significantly to give a better

understanding of the mineralogical background of the

burnt clay materials studied and hence a more reliable

determination of ancient palaeointensity. The most

The rich archaeological remains of the country and

the well established collaboration amongst the specialists

of two very different sciences (physics and archaeology)

are the basis of the results obtained.

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* 21. J. ZIJDERVELD, Methods in Palaeomagnetism, Developments in

Solid Earth Geophysics, Vol. 3, Elsevier, Amsterdam-London-

New York, 1967, p. 254.One of the authors (M.K.) is greatly indebted to Prof. I. LIRITZIS

for the financial support which made possible her participation in the

European Research Workshop �Archaeometry in Archaeology: New

Trends� in Rhodes (3-6. 11. 1999).

22. D. DUNLOP, O. OZDEMIR, Rock Magnetism: Fundamentals and

Frontiers, Cambridge Studies in Magnetism, Cambridge

University Press, 1997, p. 291.

23. W. LOWRIE, Geophys. Res. Lett., 17 (1990) 159.The authors are indebted to the reviewer Dr. Ian HEDLEY for the

valuable remarks making the text clearer and especially for improving

the English style.

24. M. DEKKERS, Geophys. J., 97 (1989) 323.

25. Y. CUI, K. VEROSUB, A. ROBERTS, M. KOVACHEVA, J. Geomagn.

Geoelectr., 49 (1997) 567.

References26. W. O�REILLY, Rock and Mineral Magnetism, Blackie, Chapman

& Hall, 1984.

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