GEOLOGICA CARPATHICA, 49, 1, BRATISLAVA, FEBRUARY 1998
IRON AGE SLAGS AT SNORUP (DENMARK): MAGNETIC
PROSPECTING, MODELLING, RECONSTRUCTION AND DATING
, UWE KOPPELT
, BO HOLM JACOBSEN
and OLFERT VOSS
Department of Earth Sciences, Aarhus University, Denmark
Institute of Geophysics & Geology, University of Leipzig, Germany
National Museum, Copenhagen, Denmark
Physics Institute, St. Petersburg University, Russia
(Manuscript received March 18, 1997; accepted December 11, 1997)
Abstract: A description of the archaeological reconstruction of iron production (100 to 700 AD) based upon bog-iron
ore and charcoal in Denmark during the Iron Age is given, the total number of furnaces being of the order of 100,000.
Examples of magnetic prospection for slags in general and in more detail, including simple interactive interpretation
by inclined magnetic dipoles (spheres) are illustrated. Finally the mean magnetic remanent direction determined by a
paleomagnetic study of oriented cores from three slag pits, as well as by magnetic inversion of the surface magnetic
field from the same slag pits, is used to obtain a magnetic dating of the slag pits by comparison with the geomagnetic
secular variation, suggesting that the mean age of the slag pits is between the 2nd and 5th centuries AD.
Key words: Iron Age, Denmark, iron production, slags, magnetic prospecting, magnetic modelling, magnetic secular
variation, magnetic dating, archaeomagnetism.
Denmark is rather poor in natural resources such as black
coal and iron-ores for modern industrial production, but
bog-iron ore occurs widely just below the soil as thin, com-
pact layers typically 0.1 to 1 m thick. Mostly during the Ho-
locene the bog-iron ore has been segregated by geochemical
redox processes in the circulating groundwater, and occurs
especially in the sandy plains in Jutland west of the limit of
the last glaciation (Christensen 1966). Bog-iron ore has
been exported to some extent (about 1 mill. tons ore in the
period 19511960; Christensen 1966), but today bog-iron
ore is not smelted in Denmark.
However, local production of metallic iron has earlier oc-
cured in Denmark, mainly between about the 2nd century
BC and 1300 AD (Voss 1993a,b). The production was quite
intense in some periods (Nielsen 1924; Mortensen 1940),
being based upon the locally occurring bog-iron ore and lo-
cally produced charcoal. Especially in SW Jutland more
than 50 locations with slag-pit furnaces have now been lo-
calized, and the total number of furnaces is estimated to be
around 100,000. With a production of some 4050 kg of
metallic iron from each pit, the estimated total production of
metallic iron amounts to some 45000 tons, mostly from
prehistoric time (Voss 1993a).
The kilns were usually destroyed immediately or shortly
after the production, but the vestiges of the iron production
are still often well preserved as slag pits. Pits, which have
not been disturbed or broken up by the farmers during
ploughing, are situated just below the soil, the slag pit sur-
face being typically some 0.40.5 meter below the present-
day soil surface. The weight of the undisturbed slag pits are
typically 200±100 kg, and as the slags are strongly magnet-
ic due to the high content of iron-oxides (which were not
successfully reduced to metallic iron), it is an easy target to
locate by means of a detailed magnetic survey.
Magnetic surveying, a standard geophysical tool in geo-
logical mapping and prospection (Sharma 1974), has been
used in archaeology since about 1960 (e.g. Aitken 1961;
Linington 1964). In Denmark it has been in use since 1964
for mapping of iron-age slag pits, medieval brick-kilns and
other old constructions (Abrahamsen 1965, 1982; Abraham-
sen & Breiner 1993; Smekalova et al. 1993a,b, 1996; Bevan
& Smekalova 1996; Koppelt et al. 1996; Moller et al. 1996;
Abrahamsen et al. 1997).
The purpose of the present communication is to describe
some detailed magnetic surveys for iron-age slag pits in the
Snorup area in Jutland, to present a simple magnetic model-
ling tool, to date the slag pits magnetically, and to compare
the magnetic findings with the facts as interpreted by means
of archaeological excavations.
Iron production at Snorup: reconstruction
and smelting technique
The distribution of shaft furnaces known in Europe from
the Iron Age is illustrated in Fig. 1.
Snorup is one of these iron-producing areas, situated in
Denmark in SW Jutland (Fig. 2), comprising 23 or more
smelting sites with remains of slag-pit furnaces dating from
the period 100700 AD (Voss 1993b). The amount of iron
produced in prehistoric time may rank this area among the
more important prehistoric iron production areas in Europe.
Excavations over the years in SW Jutland, and most re-
cently especially in the Snorup area (Fig. 3), have revealed
62 ABRAHAMSEN, KOPPELT, JACOBSEN, SMEKALOVA and VOSS
details of the original construction of furnaces, as well as of
the smelting process itself.
The iron production process reconstructed is demonstrat-
ed in details in Fig. 4 (color photos of various details may
be found in Voss 1993b). Full scale experiments with a re-
construction of this type of furnace have been promising
and have contributed to the credibility of the reconstruction.
The slag production process by means of bog-iron ore and
charcoal are as follows (Fig. 4). A: The hole for the slag-pit
is first dug and stuffed with straw, preventing the charcoal
and ore in the 1.2 m high kiln from falling down. Fresh air is
supplied via small holes at the base. The glowing charcoal
develops CO-gasses, which reduce the iron-oxides to
spongy metallic iron, filling holes in the slag. B: When the
iron-sponge sinks down to the zone with a temperature of
C just above the air-holes, the slag smelts com-
pletely and runs down, being stopped at first by the straw.
C: Some times later the weight of the accumulating liquid
slag compresses the straw, and runs down in the hole, solid-
ifying immediately to a thin plate. D: Now the kiln has been
heated so much, that the liquid slag produced sequentially
does not crystallize in the upper part of the kiln in contrast
to the metallic iron, which (having a higher smelting tem-
perature) is typically caught at the sides just under the air-
holes. In the reconstruction experiment (Voss 1993b), con-
tinuous heating for 48 hours, during which sequentially
shifting layers of charcoal and bog-iron ore (280 kg of each)
were added into the kiln, ca. 60 kg of metallic spongy iron-
luppe was produced.
In Poland and the Ukraine so called organized smelting
sites including between 8 and 230 slag-pits are found. Because
of their uniformity it is assumed that these sites were result of
short-term efforts, for instance lasting just a few months in the
autumn. The 23 clusters in Snorup containing between 28 and
171 slag-pits must also be a result of such short-term produc-
tions. Variation in the number of slag-pits in each smelting site
in those areas therefore is likely to be dependent on the amount
of charcoal available at or near the site.
Furnaces of this type were only used once; when no more
charcoal could be produced within a reasonable distance,
the iron-smelting was moved to another site, selected for its
proximity to iron ore and charcoal. The charcoal was proba-
bly produced in an oak coppice which could be harvested
only once every 20 years. This coppicing technique is ac-
tually a way of producing charcoal, which was described
much later by Duhamel du Monceay (1761) in his book Art
du Charbonnier (the art of making charcoal). Because of
the 20-year cycle of the coppice, smelters were forced to
continuously move their craft to another site where the for-
est was matured for charcoal production.
Fig. 3. Local map of Snorup with magnetically explored areas
(modified from Voss 1993b). Field E11 is specifically shown.
Fig. 1. Areas of Iron Age shaft furnaces known in Europe (re-
drawn from Smekalova et al. 1993a).
Fig. 2. Index map of Denmark with slag-pit areas indicated by
dots. The biggest dot indicate the Snorup area in the SW of the
Jutland penninsula (from Smekalova et al. 1993a).
IRON AGE SLAGS AT SNORUP (DENMARK): MAGNETIC PROSPECTING 63
In Denmark the origin of coppicing is ancient. It dates
back to the Mesolithic period, in which willow and hazel
trees were coppiced to provide material for fishing traps and
weirs. This method produces long, straight and slender rods
which are also very useful as roof-rafters. Literary sources
from the 18th to the 20th century give very variable infor-
mation about the charcoal capacity of oak-coppices, ranging
from 5 to 20 tons per hectare. These differences arise partly
from the varying quality of the land and partly from the dif-
ferent uses of this kind of forest. Coppices can be used for
grazing around the year, since in wintertime the cattle can
eat the buds and also the fine twigs.
The mean weight of the well-preserved slag-blocks is
200 kg. According to an analysis of slag and iron ore from
the same slag pit, the theoretical output has been determined
as 60 kg of sponge iron, which could result in 40 kg of
smithing iron. An iron deposit of 20 kg has been found
within the Snorup area. It consisted of approximately 100
axe-shaped bars with an average weight of 130 grams, an-
other 100 smaller bars of the same type, weighing 30 to 40
grams, and 6 pieces of iron with a total weight of 3.7 kg.
This deposit has not yet been dated.
Air photos have revealed settlements of Iron Age charac-
ter within the iron smelting area of Snorup, as well as in the
area of Tirslund about 1 km further south. None of these set-
tlements have been fully excavated yet. The Snorup smelt-
ing area is certainly not unique in Jutland. At the present
more than 60 locations with slag-pit furnaces are known
there. It is believed that there must have been 50 or more
smelting areas in Jutland like that in Snorup; this means that
there may have been a total of some 100,000 furnaces in
Jutland, with a production of 45000 tons of iron. Such a
production may place the area at the same level as the major
Polish centers known from Kielce and Warsaw, which in-
clude slag-kilns from the same period.
Magnetic prospecting for slag pits
Because of the surface position, the high magnetic sus-
ceptibility, and the strong remanent magnetization of the
slags (as compared to the magnetically fairly neutral sur-
rounding sediments), a local magnetic survey is a very ef-
fective way of finding the exact locations, the areal extent
and even the numbers of the still remaining slag pits. In the
Danish localities the slag pits are typically buried at depths
of 4050 cm, i.e. at ploughing depths, just below the soil.
In Fig. 5 the total field magnetic anomaly of one of the slag
areas at Snorup, which has been magnetically surveyed, is
shown (a part of Field A in Fig. 3). Individual slag pits show
up as circular anomalies of typically between 200 and 500 nT.
When the slag pits are separated by more than ca. one slag di-
mension from each other, the number of individual slag pits
may easily be counted just from the number of almost circu-
lar magnetic anomalies. Elongated and more irregular shaped
anomalies indicate slag pit rows or clusters. Especially within
the clusters the number of individual slag pits are more diffi-
cult to evaluate. The measurements were made by a proton
magnetometer along north-south profiles in a 1
equidistance of the anomaly curves shown being 50 nT. The
magnetic daily variation was monitored by repeated measure-
ments at a local base station.
Following a more general survey of another Field E (c.f.
Fig. 3), a detailed gradient magnetic survey in the subfield
E11 showed a linear row of magnetic anomalies (measured
in a dense grid of 0.25
). The configuration of the
Fig. 4. Reconstruction of slag production from bog-iron ore and charcoal at Snorup. A: The slag-pit is first dug and stuffed with straw,
preventing the charcoal and ore in the 1.2 m high kiln from falling down. Fresh air is supplied via the holes at the base. The glowing char-
coal develops CO-gas, which reduces the iron-oxides to the metallic spongy iron, filling holes in the slag. B: When the iron-sponge sinks
down to the zone with a temperature of 12001300
C just above the air-holes, the slag smelts completely and runs down, being stopped
at first by the straw. C: After some time the weight of the accumulating liquid slag compresses the straw and runs down in the hole, solid-
ifying immediately to a thin plate. D: Now the kiln has been heated so much, that the liquid slag produced afterwards does not crystallize
in the upper part of the kiln in contrast to the metallic iron, which (having a somewhat higher smelting temperature) is typically caught at
the sides just under the air-holes. Continuing the heating for 48 hours, adding sequentially shifting layers of charcoal and bog-iron ore
(280 kg of each) in the kiln, produced ca. 60 kg of metallic spongy iron-luppe in this experiment (from Voss 1993b).
64 ABRAHAMSEN, KOPPELT, JACOBSEN, SMEKALOVA and VOSS
IRON AGE SLAGS AT SNORUP (DENMARK): MAGNETIC PROSPECTING 65
gradient magnetometer, as used in this survey, is sketched in
Fig. 6, and the vertical gradient map is shown in Fig. 7. The
magnetic map revealed a string of positive anomalies in the
range of 10170 nT/m.
As an initial interpretation, prior to detailed archaeologi-
cal excavation, the magnetic anomalies were grouped into
three categories: 1) solid slag pits, 2) disturbed or partly in-
complete slag pits, and 3) slag fragments (Fig. 7). After this,
the anomalies were interpreted by means of a number of
simple inclined dipoles (or magnetic spheres), resulting in
an indication of 10 solid slag pits, 4 disturbed/incomplete
slag pits, and one major fragment.
After the excavation the weights of the 14 slag pits found
in Field E11 were determined by weighting. In this case a
fairly linear relationship between slag weight and magnetic
anomaly of the bottom sensor was found (Fig. 8). The (rela-
tive) magnetic anomaly thus may be used as a first indicator
of the (relative) slag weight, and hence also of the volume
of the slag, the mean density being rather constant. Detailed
information from the excavations (Table 1) also showed,
Fig. 7. Following a more general survey of Field E, a detailed magnetic survey revealed a row of slags in Field E11. The vertical total
field magnetic gradient showed a string of positive anomalies in the range of 10170 nT/m. Before the excavation the magnetic anomalies
were initially grouped in three categories as 1) solid slags, 2) disturbed or partly incomplete slags, or 3) slag fragments. The anomalies
were interpreted as simple inclined dipoles (or magnetic spheres), indicating 10 solid slags, 4 disturbed/incomplete slags, and one major
fragment. (Proton magnetometer, aequidistance 5 nT, 0.25
Fig. 6. Sketch of proton magnetometer gradient configuration, as used in the detailed survey of Field E11 (c.f. Figs. 3 and 7).
that the initial magnetic interpretation (by sorting the slags
in solid and disturbed slag pits) was only partially correct.
Thus, the weight of the remaining slag appears to be the
most important parameter for the size of the anomaly ampli-
tude, whereas it is less simple from the anomaly amplitude
to estimate whether a slag has being slightly disturbed
(whether just after the iron production, or much later in re-
cent time); if, however, it has been strongly disturbed, ran-
domly orientated by fracturing and/or partially removed by
the recent farming, it will show up as a smaller anomaly or
as a dipole with an unusual dipole direction.
Magnetic properties of slags
Magnetic susceptibility was measured in situ with a hand-
held Czech kappametre, the susceptibility values being
around 17 ± 6
SI. Standard AF (alternating field) de-
magnetization experiments of two non-oriented slag speci-
mens from Field E11 are shown in Fig. 9, the intensity of
66 ABRAHAMSEN, KOPPELT, JACOBSEN, SMEKALOVA and VOSS
Fig. 9. Standard AF (alternating field) demagnetization experiments of two not-oriented slag specimens from Field E11. The stereograms as
well as the orthogonal plots (solid/open signature indicate horizontal/vertical projection, respectively) show, that the remanent magnetization
of the slags is a directionally stable primary TRM with median destructive fields around 25 mT (250 Oe).
Fig. 8. After being excavated the weights of the 14 slags found in
Field E11 were determined by weighting. A fairly linear relation-
ship between slag weight and magnetic anomaly of the bottom
sensor was found in this case. The (relative) magnetic anomaly
thus may be used as a first indication of the (relative) weight, and
hence the volume, of the slag. Detailed information by the excava-
tion later (Table 1) showed that the initial magnetic interpretation
(sorting the slags in solid and disturbed slags) was only partially
correct, the weight of the remaining slag apparently being the most
important parameter for the amplitude of the magnetic anomaly.
the NRM (natural remanent magnetization) being 28 and
19 A/m, respectively. The stereograms as well as the or-
thogonal plots (solid/open signature indicate horizontal/ver-
tical projection, respectively) show, that the remanent mag-
netization of the slags is a directionally stable primary TRM
with median destructive fields around 25 mT (250 Oe).
Thermomagnetic experiments (Lewandowski, pers. com.)
showed unblocking temperatures between 460 and 580
suggesting the dominant magnetic carrier to be titanomag-
netite. Chemical analysis (Grundvig, pers. com.) indicate
the dominant presence of Fayalite, with some Wustite and
small amounts of metallic iron.
A simple magnetic modelling (Jacobson & Abrahamsen
1997) by adjusting inclined dipoles programmed for MAT-
LAB (as applied in Fig. 7) is illustrated in more detail for
each of the two magnetic sensors in Fig. 10ab. For each of
the sensors the top figure shows the measured total magnetic
anomaly, the middle figure shows the magnetic response
from a number of simple dipoles (spheres), each dipole being
individually and interactively adjusted in coordinates (X, Y,
depth), declination, inclination, and dipole moment (propor-
IRON AGE SLAGS AT SNORUP (DENMARK): MAGNETIC PROSPECTING 67
tional to the area of the spheres shown), and the bottom figure
show the residual field. (The maximum number of spheres to
be handled was set at 10 in this case for practical reasons).
For the top sensor (a) the contour interval shown is 2 nT,
the magnetic anomalies being fairly smooth, and the dipole
approximation appears to be a fair assumption at this distance
between slag and sensor (ca. 2.0 ± 0.3 m). For the bottom
sensor (b) the contour interval is 10 nT, the magnetic anoma-
lies being stronger and more irregular, because the slag pits
are rather close to the bottom sensor (ca. 1.0 ± 0.3 m), the
slag shape and magnetic inhomogenity thus reducing the va-
lidity of a simple dipole assumption.
For the 10 dipoles modelled, the residual anomalies (mea-
sured field - model response) are between 10 and 30 nT, corre-
sponding to 515 % of the measured anomaly (the two small
anomalies in the right part of the field were not modelled).
Archaeologically the slag pits are difficult to date in de-
tail, the wood and straw usually being totally burned, and
tools very rarely being found closely associated with the
slags. To investigate in more details the remanent magneti-
zation properties, as a possible tool for magnetic dating of
the slags by the geomagnetic secular variation, three well
preserved and undisturbed slag pits (E16, E24 and E25)
from Field E therefore were detailed paleomagnetically
sampled in situ, using a portable, water-cooled drill. Al-
though the slag is often brittle and full of minor cracks, a
number of 78 individually oriented cores from each slag
were obtained. These cores were later cut into one-inch
Fig. 10. Simple magnetic modelling by adjusting inclined dipoles using MatlabÒ (as applied in Fig. 7) is illustrated in more detail for
each of the two magnetic sensors. For each sensor, the top figure shows the measured total magnetic anomaly, the middle figure shows the
magnetic response from a number of simple dipoles (spheres), each dipole being interactively adjusted individually (Jacobsen & Abraha-
msen 1997) in coordinates (X, Y, depth), declination, inclination, and dipole moment (proportional to the area of the spheres shown), and
the bottom figure show the residual field. (The maximum number of spheres to be handled were set to 10 in this case for practical rea-
sons). aTop sensor: The contour interval is 2 nT, the magnetic anomalies being fairly smooth. bBottom sensor: The contour interval
is 10 nT, the magnetic anomalies being more irregular than for the top sensor. For the 10 dipoles modelled, the residual anomalies are be-
tween 10 and 30 nT, corresponding to 10 ± 5 % of the measured anomaly.
Weight D ia-
m e te r
C om m e n t
Incom plete. Q uarter of slag
38 kg still in situ
C om plete block
C om plete, w ith roasted o re and
Incom plete, w ith red-burn ed
clay, TL -dating
Incom plete, TL-dating
C om plete; im prints of straw
D estroye d in recent tim e;
im prints of straw
C om plete; im prints of straw
Incom plete; im prints of straw
D estroye d in recent tim e;
bottom and w . side in situ
C om plete, but fragm ented ;
bottom slag up to 5 cm th ick
Incomplete, destr. in recent tim e;
only 13 cm of bottom plate left
Incom plete, rem oved in recent
tim e; only 8 cm of b. w as left
Incom ple te, rem oved in re ce nt
tim e; 10 cm of pit w as left
Of the 14 slag pits, 5 contained a complete block. Average weight of 5
slag blocks: 142 kg. From 2 slag pits the slag had been removed
immediately after the smelt. 7 were destroyed by recent farming.
Table 1: List of 14 slag pits excavated in Snorup, Field E11. The
slag weight, diameter and peak magnetic anomaly is indicated.
68 ABRAHAMSEN, KOPPELT, JACOBSEN, SMEKALOVA and VOSS
standard specimens and measured in the laboratory on a
Molspin Ltd. spinner magnetometer, using standard step-
wise AF demagnetization (c.f. Fig. 9). Characteristic mag-
netic directions of individual specimens as well as their
mean values with
circles of 95% confidence are shown
in the stereograms (Fig. 11).
Furthermore, after a detailed magnetic survey over each of
the three slag pits, magnetic modelling by inversion of the to-
tal magnetic field anomaly was used to produce a modelling
mean direction, the result of which is shown as a small dot in
each stereogram (Koppelt 1996). It is seen, that there is very
good correspondence between the modelled mean directions
and the paleomagnetically determined mean directions for
each slag. Numerical details are given in Table 2.
The directional mean results for the three slag pits E16,
E24 and E25 (Table 2), as obtained by the remanent magneti-
Fig. 11. Three slags (E16, E24 and E25) from Field E was paleo-
magnetically sampled in situ, using a portable, watercooled drill.
Up to 8 individually oriented cores from each slag was measured
in the laboratory on a Molspin Ltd. spinner magnetometer, using
standard stepwise AF demagnetization. Characteristic magnetic di-
rections of individual specimens as well as their mean values with
circles of 95 % confidence are shown in the stereograms. Fur-
thermore, magnetic modelling by inversion of the total magnetic
field anomaly was used to produce a modelling mean direction,
shown as small dots (Koppelt 1996). It is seen, that there is very
good correspondence between the modelled mean directions and
the rockmagnetically determined mean directions.
Table 2: Cleaned stable remanent mean directions with spherical
Fisher statistics on oriented cores from the three slags E16, E24
and E25 investigated paleomagnetically, as well as the mean direc-
tions determined from inversion of the magnetic surface anomaly.
Paleom agnetic results:
M agnetic inversion results:
m ean of all:
zation of oriented cores and by inversion, are shown in
Fig. 12, plotted onto the magnetic secular variation curve for
Denmark for the period 0 to 2000 AD. The curve is based
upon the British archaeomagnetic mastercurve (Clark et al.
IRON AGE SLAGS AT SNORUP (DENMARK): MAGNETIC PROSPECTING 69
1988), recalculated from Meriden in UK to Snorup in Den-
mark by a central dipole transformation (Abrahamsen 1996),
a distance of 800 km. The
-circle shown is the 95% signif-
icance circle of the combined mean direction. It appears, that
the paleomagnetically determined directions are rather scat-
tered (solid dots), and a dating based upon these alone is not
well constrained (
), the direction suggesting a paleo-
magnetic age of before the 6
The inverted mean directions are less scattered (open
dots) and the trend is in general agreement with the paleo-
magnetic mean directions (and well within the individual
-circles of the latter ones). Although scattered, the two
magnetic techniques thus appear to agree with each other,
thus supporting each other quite well.
The grand mean of all 6 mean directions gives a value of
(D, I) = (352.8
, k = 260. Considering the
uncertainties of the magnetic master-curve, which may well
be at least ±2
in inclination and ±5
in declination, the mag-
netic mean direction suggests a mean age of the three slag
pits in Field E between the 1
Century AD. Whether
the scatter (or systematic trend from E24 via E25 to E16) in
the magnetic mean directions is due to a real trend in the age
of the three slag pits, or it is due to scatter in the magnetic
data, is not known.
Our experience from the joint archaeological and geo-
physical investigations performed in the Snorup and nearby
areas over the last few years has confirmed, that the Snorup
area was an important iron production area in prehistoric
time. Magnetic surveying, in a free search mode, in a sys-
tematic mapping, and in a very detailed mapping sense, has
given valuable insight into the extent and amount of the old
slag pits still present and often well preserved below the
soil. Magnetic modelling has been useful to estimate the ex-
tent, amount and number of slag pits present, and the mag-
netic inversion and paleomagnetic methods may be useful
for dating the slag pits when these are undisturbed and geo-
metrically well behaved.
Acknowledgements: It is a pleasure to acknowledge the
thermomagnetic analysis on slag samples made by Marek
Fig. 12. Preliminary magnetic secular variation curve for Denmark for the period 0 to 2000 AD, based upon a central dipole transforma-
tion of the British mastercurve from UK to Denmark (Abrahamsen 1996). Small numbers indicate century AD. The directional results for
the slag pits E16, E24 and E25 obtained by magnetic cleaning of oriented cores (small solid dots) and by inversion (open dots) are also
shown. Solid dot with corresponding 95% significance circle indicate the overall mean direction.
70 ABRAHAMSEN, KOPPELT, JACOBSEN, SMEKALOVA and VOSS
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