Journal of Radioanalytical and Nuclear Chemistry, Vol. 247, No. 3 (2001) 685�696
�������������� ���������������������� ����� � � ����
�� ���������������� ��������� � ��
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.
686
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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
687
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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
688
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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
689
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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).
690
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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
691
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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 ferri- magnetic 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
692
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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
693
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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
694
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
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.
695
M. KOVACHEVA, N. JORDANOVA: BULGARIANARCHAEOMAGNETICSTUDIES: A REVIEW
* 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.
27. D. DUNLOP, J. Geophys. Res., 78 (1973) 1780.
28. W. LOWRIE, M. FULLER, J. Geophys. Res., 76 (1971) 26 639.1. M. KOVACHEVA, Phys. Earth Planet. Inter., 102 (1997) 145.
29. D. DUNLOP, Earth Plan. Sci. Lett., 63 (1983) 353.2. M. KOVACHEVA, N. JORDANOVA, V. KARLOUKOVSKI, Surv.
Geophys., 19 (1998) 413. 30. S. XU, D. DUNLOP, J. Geophys. Res., 100, B11 (1995) 22 533.
31. R. DAY, M. FULLER, V. A. SCHMIDT, Phys. Earth Planet. Inter.,
13 (1977) 260.
3. Y. BOYADZIEV, Chronology of prehistoric cultures in Bulgaria,
in: Prehistoric Bulgaria, Monographs in World Archaeology, 22,
Prehistory Press, Madison Wisconsin, 1995, p. 149. 32. A. ROBERTS, Y. CUI, K. VEROSUB, J. Geophys. Res., 100 (1995)
17 909.4. J. GÖRSDORF, Y. BOJADZIEV, Eurasia Antiqua, 2 (1996) 105.
33. L. TAUXE, T. MULLENDER, T. PICK, J. Geophys. Res., 101
(1996) 571.
5. M. KOVACHEVA, A.TOSHKOV, Surv. Geophys., 15 (1994) 673.
6. M. AITKEN, Phys. Reports, 40C (1978) No. 5, 279.
34. A. J. VAN VELZEN, J. ZIJDERVELD, Geophys. J. Int., 110
(1992) 79.
7. D. TARLING, Palaeomagnetism, Chapman and Hall, London,
New York, 1983.
35. N. JORDANOVA, Rock magnetic studies in archaeomagnetism and
their contribution to the problem of reliable determination of the
ancient geomagnetic field intensity, PhD Thesis, Sofia University,
Sofia, 1996 (in Bulgarian).
8. M. KOVACHEVA, J. Geomagn. Geoelectr., 21 (1969) 563.
9. Ph. LANOS, Historie Mesure, IX (1994) 3�4, 287.
10. E. MURAD, U. WAGNER, Hyperfine Interact., 117 (1998) 337.
11. U. WAGNER, R. GEBHARD, G. GROSSE, T. HUTZELMANN,
E. MURAD, J. RIEDERER, I. SHIMADA, F. WAGNER, Hyperfine
Interact., 117 (1998) 323.
36. V. JELINEK, The statistical theory of measuring anisotropy of
magnetic susceptibility of rocks and its application, Geofizika,
Brno, 1977.12. R. CORNELL, U. SCHWERTMANN, The Iron Oxides: Structure,
Properties, Reactions, Occurrence and Uses, VCH, Weinheim,
1996.
37. F. HROUDA, Geophys. Survey, 5 (1982) 37.
38. D. TARLING, F. HROUDA, The Magnetic Anisotropy of Rocks,
Chapman & Hall, London, 1993.13. N. JORDANOVA, E. PETROVSKY, M. KOVACHEVA,
D. JORDANOVA, Factors determining magnetic enhancement in
archaeological artefacts of burnt clay, J. Archaeological Sci.
(submitted).
39. M. AITKEN, P. ALCOCK, G. BUSSELL, J. SHAW, Archaeometry,
23 (1981) 53.
40. N. JORDANOVA, V. KARLOUKOVSKI, V. SPATHARAS, Bulg.
Geophys. J., XXI (1995) No. 4, 49.14. C. E. WEAVER, Clays, Muds and Shales, Developments in
Sedimentology, 44, Elsevier, 1989, p. 103. 41. Ph. LANOS, Les nouvelles de l�archéologie, No. 64 (1996) 33.
42. Y. GARCIA, Variation de l�intensité du champ magnétique en
France durant les deux derniers millénaires, PhD Thesis, L�Univ.
de Rennes 1, Rennes, 1996.
15. M. BARBETTI, M. MCELHINNY, D. EDUARDS, P. SCHMIDT, Phys.
Earth Plan. Inter., 13 (1977) 346.
16. M. KOVACHEVA, V. SPATHARAS, I. LIRITZIS, Archaeometry,
(in press). 43. A. STEPHENSON, S. SADIKUN, D. POTTER, Geophys. J. R. Astr.
Soc., 84 (1986) 185.17. M. KOVACHEVA, Comp. Rendue Bulg. Acad. Sci., 37 (1984) 171.
44. R. THOMPSON, F. OLDFIELD, Environmental Magnetism, Allen
and Unwin, Winchester, Mass., 1986.
18. M. KOVACHEVA, Bull. d�Archéologie Marocaine, XVI
(1985�1986) 285.
45. A. STEPHENSON, Phys. Earth Plan. Inter., 4 (1971) 353.19. E. THELLIER, O. THELLIER, Annales Geophysique, 15 (1959) 285.
46. J. DEARING, V. HAY, S. BABAN, A. HUDDLESTON,
E. WELLINGTON, P. LOVELAND, Geophys. J. Int., 127 (1996) 728.
20. N. JORDANOVA, E. PETROVSKY, M. KOVACHEVA, J. Geomagn.
Geoelectr., 49 (1997) 543.
696
Top Related