Geophysical surveys in Priol’khon’e, Western Pribaikal’e.Part two: a geophysical discovery of an archaeometallurgical site in

the vicinity of Chernorud.

Lake Baikal and location of Chernorud

0 50 100 kmN

Chernorud

Lake

Bai

kal

Irkutsk

Map of Barun-Khal valley and Chernorud

Che

rnor

ud

N

0 200 400m

C am p

Barun-Khal valley

Ku

ch

ulg

a R

ive

r

Bul-Durun

boundary outlin ing the area w here surfaced iscre te s lag fragm ents occur

graves o f d iffe rent age

s lag accum ula tion

so il th row n out o f the gopher's burrowconta in ing s lags and charcoa l

excavation (not to sca le)

Barun-Khal valley, the university camp and Chernorud

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Electrode spacing AB /2 (m )

100

1000

a (o

hm. m

)a)

0 50 100 150 200 250

D epth (m )

1 0

1 0 0

1 0 0 0

(o

hm. m

)

b)

m easuredm odelled

Apparent resistivity curve of the Shlumbergergeoelectric sounding measured within the camparea (a), and corresponding 1D interpretedgeoelectric section (b).

1 0 1 0 0 1 0 0 0

T im e (s)

100

1000

U/I

(V

/A)

1 0 1 0 0 1 0 0 0

T im e (s)

1 0

1 0 0

a (

Ohm

. m)

a)

b)

t-0 .85

TEM response of 20m by 20m сoincident loops centred within the camp area: (a) measured, and (b) transformed to the apparent resistivity curve.

0.1 1.0

Tim e (m s)

1

10

100e(

t)/I

(V

/A)

e(t)/I1/t

TEM response of the small coil system in presence of a magnetic extractfrom the near-surface material sampled within the camp area

t

I, H

1

t

J, H

2

J0

J

t

e(t)

a)

b)

c)

TEM System

Transm itter R eceiver

I e(t)

J(t)

H 1 H 2

Tr. loopR . loop

m agnetica lly-viscous ob ject

M agnetisation of an assem bly of SPM partic les.

H

J

H=0 J=0

)/exp()( 0 tJtJ

Element composition of slags from the atomic-emission material analysis (in wt%)

Element Sample: met-2

(fine-vesicular slag)

Sample: met-4

(fine-vesicular slag)

Sample: met-6

(limonitezied "porousiron")

Si 1-4 3-5 3-8

Al 0.3-0.6 0.5-0.6 0.5-1.5

Na 0.15-0.6 0.1-0.15 0.3

K <1 Not found <1

Ca 0.3-1 1-1.5 0.8-1.5

Mg 0.6 0.2-0.3 0.3-1

Fe 60-80 25-30 40-60

Mn 1-3 20-30 0.4-0.6

Cr 0.3 0.02-0.2 0.015-0.03

Ni 0.1 0.02 0.002-0.006

Co 0.004 0.001-0.004 0.003-0.004

Ti 0.04-0.08 0.08 0.15

Ba 0.2 0.7-1 0.015-0.02

P 1-1.5 0.4-0.8 0.2-0.5

Chemical composition of the Chernorud site’s slags (in wt %)

Sample Si Ti Al Fe Mn Ca Mg Ni

met-8 7.8 0.34 2.09 53.20 0.22 1.09 0.37 Not found

met-9 10.86 0.34 1.97 49.00 0.21 1.25 0.37 Not found

met-10 5.31 0.40 2.29 55.65 0.08 2.68 0.79 Not found

met-11 4.04 0.48 3.45 55.65 0.79 1.76 0.17 0.18

met-12 8.23 0.46 2.98 48.65 0.80 2.66 0.79 Not found

Principal mineralogy of slags (in wt%) from the X-ray analysis results

Mineral Sample

met-1 met-2 met-4 met-6 met-8 met-9

Fayalite Fe2SiO4 57.99 - 19.05 41.65 64.89

Knebelite (Fe, Mn)2SiO4 - 30.14 - - -

Wustite FeO 17.78 51.81 20.83 40.62 21.39

Ferrous spinel Mg(Al, Fe)2O4 16.88 11.81 - 11.07 -

Quartz SiO2 - 2.22 23.57 6.66 6.36

Albite Na(Al.Si3O8) 7.35 - 21.81 - -

Dolomite CaMg(CO3)2 - - 8.19 - 7.36

BrugnateliteMg6FeCO3(OH)134H2O

- 4.03 - - -

Annite 1-M KFe3(Si,Al)4O10(OH)

- - 8.55 - -

Metallic iron Fe - - + - -

+ detected, but the content couldn’t be estimated- not detected

As may be inferred from their chemical composition and mineralogy, the Barun-Khal’s slags are closely similar tothose that are known to have formed during the bloomery iron reduction process.

This conclusion has motivated:(i) archaeogeophysical and archaeological surveys in theBarun-Khal valley that in turn have resulted in the discovery of an Iron Age metallurgical complex dated as early as 1750-2180 years BP;(ii) extension of slag-related studies primarily by application of magnetic methods normally used inrock magnetism.

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м

- раскоп

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T (nT)

Total magnetic intensity map for the mouth part of the Barun-Khal valley (1998); survey area is 150m х 200m, surveying grid is 2m x 4m, the height of the sensor is about 2m.

3D plot of the total magnetic field intensity: the mouthof the Barun-Khal valley.

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magnetic anomaly created byan archaeometallurgical target

330

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м

а)

- яма

- раскоп

Magnetic field intensity map for the mouth part of the Barun-Khal valley: blue contoured rectangle shows the area of the 1999 surveys, and that contoured by a red line shows the area of the 2000 surveys.

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3 0

4 0

5 0

6 0

7 0

8 0

9 0

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Y,

мDetailed magnetic surveys results) fom the mouth part of the Barun-Khal valley (2000. Surveying grid: 1m x 1m; the height of the sensor: 80 cm; contour lines interval: 5 nT. The rectangle indicates the archaeological excavation area, where no magnetic measurements were possible.

Rock magnetic techniques enable to identify mineralogy,size and concentration of magnetic grains and have severaladvantages over other techniques. Most measurements are: fast cheap highly sensitive non-destructive are performed on bulk-samples.

It allows for a large number of samples to be processed in arelatively short time and the possibility to construct slag-related data with high spatial resolutions.

Most of the samples can later be used for destructiveinvestigations such as chemical analyses or angle-dispersiveX-ray analysis.

Magnetic hysteresis curves of the met-1, met-2, met-4, and met-6 samples

m et-1

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

B (T)

-0.4

-0.2

0

0.2

0.4

-0.5

-0.3

-0.1

0.1

0.3

0.5

J (A

m2 /

kg)

-200 -100 0 100 200

B (m T)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

J (A

m2 /

kg)

m et-2

-200 -100 0 100 200

B (m T)

-0.8

-0.4

0.0

0.4

0.8

J (A

m2 /

kg)

m et-4 m et-6

-200 -100 0 100 200

B (m T)

- 8

- 4

0

4

8

J (A

m2 /

kg)

Magnetic hysteresis curve of the met-8 sample. Its wasp-waisted shape indicates the contribution from both stable SD and SP grains

m et-8

-200 -100 0 100 200

B (m T)

-1.5

-1

-0.5

0

0.5

1

1.5

J (A

m2 /

kg)

Magnetic hysteresis data of slag samples from the Chernorud site

Sample Js

[Am2/kg]

Jrs

[Am2/kg]

Hc

[mT]

Hcr

[mT]

Jrs/Js Hcr/Hc 0 Jrs /0

met-1 0.810-1 110-2 14.8 - 0.121 - 8.410-4 11.905

met-2 3.110-1 3.710-2 20.0 76.0 0.120 3.80 1.010-3 37

met-4 7.510-1 7.010-4 7.0 60.8 0.001 8.70 5.310-3 0.132

met-6 7.0 1.7 14.0 30.0 0.240 2.14 3.810-2 44.737

met-8 1,5 * 7.210-2 18.0 30.0 0.05 1.67 1.210-3 60

Js (saturation magnetization), Jrs (saturation remanence), Hc (coercive force), Hcr (coercivityof remanence), 0 (magnetic low-field susceptibility) and interparametric ratios.

* - Js was found by extrapolating the measured hysteresis curve beyond the maximuminducing field (250 mT)

Both hystersis curves and data listed in the above table in-dicate, that

slags vary significantly in both magnetic mineralogy andthe grain-size range of contributing magnetic minerals;

all samples contain magnetically "soft" fero- and/or ferri-magnetic minerals like magnetite, in the single domain(SD) or pseudo single domain (PSD) grain size range;

a contribution of para- and/or superparamagnetic compo-nents is also significant

Thermomagnetic curves of the met-1, met-2, met-4, and met-6 samples

m et-1 m et-2

m et-4 m et-6

0 200 400 600 800

T (OC )

0.0

0.2

0.4

0.6

0.8

1.0

J(T

) / J

max

0 100 200 300 400 500 600 700

T (OC )

0.0

0.2

0.4

0.6

0.8

1.0

J(T

) / J

max

0 100 200 300 400 500 600 700

T (OC )

0.0

0.2

0.4

0.6

0.8

1.0

J(T

) /J

max

0 100 200 300 400 500 600 700

T (OC )

0.0

0.2

0.4

0.6

0.8

1.0

J(T

) /J

max

Thermomagnetic curve of the met-8 sample.

m et-8

0 100 200 300 400 500 600 700

T (O С )

0.0

0.2

0.4

0.6

0.8

1.0

J(T

) / J

max

Estimating percent content of ferro- and ferrimagnetic minerals in slag

1 . W h e n i t i s e x p e c t e d t h a t s l a g ' s m a g n e t i s m i s d u e t o o n l y o n e m a g n e t i cp h a s e ,

100min

s

ss

J

JС

,

w h e r e J s s i s t h e s a t u r a t i o n m a g n e t i z a t i o n o f a b u l k s a m p l e , J s m i n i s s p e c i f i cs a t u r a t i o n m a g n e t i z a t i o n o f a m i n e r a l .2 . I n t h e c a s e t h a t t h e s l a g ' s m a g n e t i s m i s e x p e c t e d t o b e d u e t o s e v e r a l f e r -r i m a g n e t i c m i n e r a l s ,

100min

min s

ss

J

JС

,

w h e r e J s s m i n i s t h e c o n t r i b u t i o n o f t h e m i n e r a l ' s s a t u r a t i o n m a g n e t i z a t i o n t ot h e b u l k m a g n e t i z a t i o n o f a s a m p l e . J s s m i n c a n b e e s t i m a t e d f r o m t h e t h e r -m o m a g n e t i c c u r v e s i n d i c a t i n g t h e r e l a t i o n b e t w e e n m a g n e t i z a t i o n o f a m i n -e r a l a n d t h e b u l k m a g n e t i z a t i o n o f a s a m p l e .

Mineralogical composition of slags (in wt %) from the magnetic analysis results

Mineral Sample

met-1 met-2 met-4 met-6 met-8

Magnetite FeFe2O4 0.05 0.12 0.28 7.08 -

Manganiferous magnetite (Fe,Mn)Fe2O4

- 0.18 - - -

Metallic iron Fe 0.006 0.03 0.22 0.22 -

Cementite Fe3C 0.01 ? - ? 1.77

Ferro- and ferrimagneticminerals in aggregate

0.07 0.35 0.50 7.46 1.77

? there is some evidence for the presence of mineral

- mineral is not detected

Magnetic hysteresis curve of the met-1 sample

m et-1

-1 .5 -1 .0 -0.5 0.0 0.5 1.0 1.5

B, Т л

-0 .5

-0 .3

-0.1

0.1

0.3

0.5J,

Ам

2/

кг

w ith in-hysteresis-loopirreversib le m agnetization

out-of-hysteresis-loop reversib lem agnetization (due to SPM )

Parametrs used for estimating SPM features of slags

Sample Js

sp Jsp/Js Cff Cmg CFe Slag's features

met-1 0.810-1 510-4 0.80 0.07 0.05 0.006 Finely vesicular

met-2 3.110-1 910-4 0.73 0.35 0.30 0.03 Finely vesicular

met-4 7.510-1 210-3 0.64 0.50 0.28 0.22 Finely vesicular

met-6 7.0 1.210-1 0.36 7.46 7.08 0.22 Dense, from thelimonitized spongy iron

In this Table Js is saturation magnetization [Am2/kg]; sp is magnetic susceptibility[SI units] estimated from the linear, reversible part of the hysteresis curves; the ratioJsp/Js is approximately proportional to the contribution of SP material to Js atinducing fields corresponding to the within-hysteresis-loop magnetization; Cff, Cmg,Cfe are, respectively, the contents of ferro- and ferrimagnetic material, magnetiteand metallic iron in %, estimated from the magnetic analysis data.

Our measurements indicate, that slags contain in abundance ultrafine ferro- and ferrimagnetic grains. Such SP material represents the finest end of the magnetite or metallic iron grain size range present in the samples. If the melted iron-rich material was cooling and congealing rapidly, only very small grains of magnetic minerals had enough time to form during cooling period. So, it would appear reasonable that the content of SP is useful in estimating a rate of the slag's cooling. SP features of slags and their disintegrated remains can be used for surveying potential archaeometallurgical sites with the TEM method, using the rather short magnetic relaxation times in the order of milliseconds. Just in such a way, the magnetically viscous near-surface features caused by superparamagnetic material contained in the slags and their resulted in the discovery of the Barun-Khal archaeometallurgical monument.

Anomalously slowly decaying transientsare measured when carrying out TEM geophysical surveys in the vicinity of Chernorud.

Relaxation of magnetization of ultrafine (SP)particles abounding in the site's near-surface is supposed to be a likely cause.

Slag-like fragments are supposed to becandidates for the material containing SP particles where from they also were entering the soil.

Investigating chemical and mineralogicalcomposition of the slag-like fragmentsindicates they are nothing but slags formedduring in-bloomery iron reduction process.

Archaeogeophysical and archaeologicalsurveys result in discovery of an Iron Agemetallurgical complex in the Barun-Khalvalley. According to the radiocarbon datingmethod, the iron smelting started here asearly as 2180 years BP.

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Barun-Khal (1999): reconnaissance m agnetic surveyarea. S tation spacing is 4 m . M agnetic fie ld contourline in terva l is 10 nT . The rectangle in the upper le ftcorner de lin ia tes an area o f "prom ising" anom aly

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T (nT)

Barun-Khal (2001): deta iled m agnetic survey.M easurem ent grid is 2m x 1m . C rosses locatethe excavation centre positions.

Studying magnetic properties of slags is shownto be a valuable tool for investigating theirmagnetic mineralogy and evaluating magneticgrain size, which, in turn, provide new waysfor slags’ classification and elucidating theirsmelting-related features.

As in the case of naturally occurring rocks,magnetic features of slags’ ferro- andferrimagnetic minerals are as typomorphousas their form, color, hardness, luster and othermineralogically-related properties.

ACKNOLEDGEMENTS

We thank John Peck, Vadim Kravchinsky, Dirk Harmann, Irina Vasil’eva, Vitaliy Vanchugov and Oleg Kovalchuk for their important contribution to measuring magnetic properties and composition of slags. This research was supported by the Russian Foundation for Basic Research, Grant 01-05-65064 "The effect of natural and man-made geological materials magnetic viscosity on the EM fields measured in the geoelectric method".