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Investigation of fire ash residues using mineral magnetism.

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Archaeological Prospection
Archaeol. Prospect. 8, 227–237 (2001)
DOI: 10.1002/arp.171
Investigation of Fire Ash Residues
Using Mineral Magnetism†
C. PETERS,1 ∗ M. J. CHURCH2 AND C. MITCHELL2
1
Department of Chemistry, University of Edinburgh, King’s Buildings, West Mains Road,
Edinburgh EH9 3JJ, UK
2
Department of Archaeology, University of Edinburgh, Old High School, Infirmary Street,
Edinburgh EH1 1LT, UK
ABSTRACT
As part of a wider research programme of experimental archaeology at Calanais Farm, Isle
of Lewis, Scotland, a number of experimental hearths were constructed, based on excavated
evidence from the Late Iron Age houses at Bostadh, Lewis. Controlled and repeated burning of
different fuel sources (well-humified peat, fibrous upper peat, peat turf and wood) was carried out
over a number of burning episodes, each of three days duration. A range of mineral magnetic
measurements, including remanences and the variation of susceptibility with high temperature,
were taken from the resulting ash samples. The high temperature susceptibility measurements
show that the fibrous upper peat and peat turf have a single magnetic component, with a drop
in magnetic susceptibility at ca. 600 ° C. In comparison the well-humified peat and wood have
one, sometimes two, distinct magnetic components characterized by drops in susceptibility at
ca. 330 and ca. 550 ° C. Stepwise discriminant analysis was performed on the room temperature
magnetic data. A biplot of the resulting two main variables distinguishes the well-humified peat
and wood. Some overlap is observed between the fibrous upper peat and peat turf. Magnetic
measurements also were carried out on Iron Age and Medieval hearth, floor and ash spread
samples from the multiperiod archaeological site of Guinnerso, on the Isle of Lewis. Comparison
was made with the modern ash samples in order to determine if fuel sources could be identified.
The high temperature susceptibility curves and the discriminant analysis biplot suggest that for the
selected archaeological samples the predominant fuel source was well-humified peat. Copyright
 2001 John Wiley & Sons, Ltd.
Key words: mineral magnetism; experimental archaeology; fuels; fire ash
Introduction
The use of fire in domestic and industrial capacities over past millennia has generated records
through the ash deposits left behind of how
people lived and worked. Burning produces
an enhanced magnetic signal and thus fire ash
is ideally suited to mineral magnetic studies.
Despite the suitability and widespread use of
ŁCorrespondence to: Dr Clare Peters, Department of Chemistry, University of Edinburgh, King’s Buildings, West Mains
Road, Edinburgh EH9 3JJ. E-mail: C.Peters@ed.ac.uk
† Paper presented at Third International Conference on Archaeological Prospection, Munich, 9–11 September 1999.
Copyright  2001 John Wiley & Sons, Ltd.
magnetism in archaeological prospection and
archaeomagnetic dating, only a few researchers
have utilized environmental magnetic techniques
to address questions of site formation, site function and ecofact and artefact taphonomy. Bellomo
(1993) used mineral magnetic measurements in
conjunction with other techniques to develop a
method for identifying human-controlled fires
from natural fires. Similarly, McClean and Kean
(1993) have studied the magnetic properties of
wood ash to determine the contribution of such
ash to the magnetic signature of hearths as
observed in magnetic prospection. Batt and Dockrill (1998) have integrated susceptibility, gradiometry and archaeomagnetic data with other
Received 17 October 2000
Accepted 4 April 2001
228
archaeological evidence from the multiperiod site
at Old Scatness, Shetland. This demonstrated the
potential of environmental magnetism for dating
purposes, site formation processes and modelling
anthropogenic amendment of soils. Recent studies by Morinaga et al. (1999) and Linford (2000)
have focused on how the magnetic properties
of different substrates below fires react to heating. Peters and Thompson (1999) have developed
a technique based on hysteresis loop data to
quantify magnetic components, including superparamagnetism (associated with burning), within
archaeological soils. In this paper we focus on
the mineral magnetic properties of modern fire
ash residues, produced under controlled burning conditions, with the aim of investigating the
use of mineral magnetism to distinguish different
burning regimes and application within archaeology. These applications include the investigation
of site formation processes (cf. Batt and Dockrill,
1998; Peters et al., 2000), the analysis of archaeobotanical taphonomy (Church, in preparation) and
the reconstruction of fuel procurement and selection strategies (Church et al., in preparation).
Methodological approach
Field basis of hearths
Three replica hearths were constructed based on
the Late Iron Age three-sided hearths commonly
uncovered in the Western Isles of Scotland.
Each hearth measured approximately 0.6 ð 0.4 m
and was designed on the basis of the hearths
excavated at the Late Iron Age site of Bostadh,
in Great Bernera, Lewis (Neighbour and Burgess,
1997). The hearth slabs consisted of Lewisian
gneiss, the common local rock, and they were
placed into approximately 0.1 m of magnetically
sterile beach sand from the beach at Bostadh.
Four basic fuel types were chosen; wood,
well-humified blanket bog peat, fibrous upper
peat and peat turf. These were chosen because
evidence for their use on prehistoric sites is well
attested in the archaeobotanical record across
the Western Isles. The fuel was taken from two
areas; the peat turf and fibrous upper peat from
near Gearrannan (NGR NB 205 445) and the wellhumified peat and wood from near Gearraidh na
Copyright  2001 John Wiley & Sons, Ltd.
C. Peters, M. J. Church and C. Mitchell
h-Aibhne (NGR NB 265 307). All the peat types
were cut in springtime and dried and stacked for
the summer. The wood came from dead pine trees
(Abies sp.) from a plantation recently blighted by
beetles.
Generally, a single fuel type was burnt in each
replica hearth for a 72 h period, which allowed
the construction, burning and sampling of a single
hearth in one week. Following the burning, the
hearths were allowed to cool before sampling.
The colour of the ash produced was first recorded
using a Munsell colour chart (Munsell, 1992).
Multiple samples were then taken for mineral
magnetic measurements, soil micromorphology
and archaeobotanical remains. Here we will
focus only on the mineral magnetic samples.
The total volume of ash produced from each
hearth was recorded before the remaining ash
from each fuel type was dumped onto specially
prepared areas covered by sterile beach sand.
These dumps were sampled in the summer
of 2000 and further sampling is planned for
2003 (five years after the initial dumping). The
sampling will assess issues of ash spread and
midden formation, exposure and erosion. It
will also assess any short-term modification in
magnetic properties of the material. Long-term
modification, i.e. on an archaeological timescale, is much harder to assess. However, the
common forms of post-depositional change, such
as compaction and bioturbation, would not affect
the magnetic properties, as they are mechanical
in nature. Forms of chemical weathering and
alteration present the greatest problem. However,
most of the sites were covered by a significant
overburden, sometimes metres in depth, which
acted as a protective blanket for many of the
archaeological layers. Also, the excavators at the
sites mentioned encountered no obvious signs of
chemical weathering and soil modification, such
as iron pan formation.
Sample preparation
All the modern ash samples and the archaeological samples were dried and sieved at 2 mm.
Subsequently, it was found that a better estimate
of fuel types for the archaeological samples could
be made if the ash component was isolated (see
‘Archaeological applications’ later) by sieving
Archaeol. Prospect. 8, 227–237 (2001)
Fire Ash Residues
at 63 µm, thus removing sand and other large
particles, which will contribute to the magnetic
properties of the bulk sample. All samples were
tightly packed into 2.5 cm cylindrical pots prior
to the magnetic measurements being carried out.
Laboratory-based magnetic measurements
Mineral magnetic measurements are rapid, easy
to measure and generally nondestructive. They
provide information on the concentration of magnetic grains, the size of magnetic grains and the
magnetic mineralogy (see Thompson and Oldfield, 1986). Observed differences in these three
factors can be applied to investigating the formation and make-up of natural and archaeological
materials. The following laboratory-based mineral magnetic measurements were carried out on
all the modern ash samples and also the archaeological samples.
(i) Susceptibility measurements were carried
out using a Bartington MS2 susceptibility
bridge. Room temperature measurements of
low and high frequency susceptibilities were
carried out in order to determine the initial mass specific susceptibility in and the
frequency dependent susceptibility fd for
each sample. The in value gives a rough
indication of the total magnetic concentration and fd provides an indication of the
concentration of very small superparamagnetic grains. In addition the variation of
susceptibility with temperature up to 700 ° C
was monitored for each sample. Information about the magnetic mineralogy and
domain state can be obtained from the heating curve. Comparison of the heating and
cooling curves provides information on the
thermal history of the samples.
(ii) Anhysteretic remanent magnetizations
(ARMs) were grown using an adapted
Molyneux AC demagnetizer and measured
using a Molspin fluxgate magnetometer.
Two measurements were made for each
sample; the saturation ARM (SARM) was
grown in a peak alternating field of 99 mT
superimposed on a direct field of 0.5 mT,
and subsequent demagnetization of SARM
in an alternating field of 40 mT. The SARM
Copyright  2001 John Wiley & Sons, Ltd.
229
value gives an indication of the concentration
of remanence-carrying grains and the ratio
ARMdemag40mT / SARM provides information
on domain state.
(iii) Isothermal remanent magnetizations (IRMs)
were grown using a Molyneux pulse magnetizer and also electromagnets, and were
measured using a Molspin fluxgate magnetometer. The IRMs were grown in fields of
60 mT and the saturation IRM (SIRM) in 1 T.
The SIRM value gives an indication of the
concentration of remanence-carrying grains
and the ratio IRM60mT / SIRM provides information on domain state and mineralogy.
Results
Hearths
In the first season of experimentation 18 hearth
cycles were run. The fire hearth numbers, fuel
types and magnetic sample numbers are listed
in Table 1. In general, one sample was taken
from each fire hearth for mineral magnetic
measurements. However, two samples were
taken from each of the fire hearths FH1, FH5
and FH11 corresponding to observed differences
in ash colour. Additionally profiles were sampled
vertically through sections of fire hearths FH16,
FH17 and FH18, resulting in multiple samples.
Magnetics
Figure 1 displays the biplot of in versus fd for
20 bulk and 23 sieved ash samples (excluding
samples S106 and S133, which are mixed fuel
types; data obtained only from sieved wood ash).
The sieved samples show a higher magnetic concentration in than the bulk samples, suggesting
that sieving has isolated the more magnetic ash
component from the sand and other large particles, which in general are only weakly magnetic.
In Figure 1 differences between the fuel sources is
beginning to emerge. Focusing on the envelopes
drawn around the sieved data for each fuel type
we observe complete discrimination between the
four fuel types. The wood ash samples are characterized by lower in values than the other fuel
types. We also observe that the well-humified
Archaeol. Prospect. 8, 227–237 (2001)
C. Peters, M. J. Church and C. Mitchell
230
Table 1. List of fire hearth numbers, fuel types and magnetic sample numbers: whp is well-humified peat, fup is
fibrous-upper peat, pt is peat turf and wd is wood
Fire
hearth number
Fuel
type
FH1
whp
FH2
FH3
FH4
FH5
fup
wd
whp
fup
FH6
FH7
FH8
FH9
FH10
FH11
wd
whp
fup
pt
whp
fup
FH12
FH13
FH14
FH15
FH16
wd
whp
fup
mixture
whp
FH17
pt
FH18
mixture
Magnetic
samples
S9
S10
S15
S21
S32
S37
S38
S44
S54
S58
S62
S70
S75
S76
S81
S95
S98
S106
S111
S116(2)
S116(3)
S125(4)
S125(5)
S125(6)
S133(2)
S133(3)
S133(4)
S133(5)
of 6.0%), which in turn have higher fd values
than the peat turf ash (average of 4.7%). Thus
the biplot of in versus fd has produced, for the
sieved data, a remarkable discrimination between
the four fuel types based on differences in total
magnetic concentration (in distinguishing the
wood ash) and the concentration of superparamagnetic grains (fd separating the three types of
peat fuel).
Discriminant analysis
peat ash samples have higher fd values (average
of 7.9%) than the fibrous upper peat (average
The biplot of in versus fd in Figure 1 is the
first step in attempting to discriminate the four
fuel types. In order to make use of all the room
temperature magnetic data, discriminant analysis was carried out. Discriminant analysis is a
multivariate statistical procedure that compares
variables from a number of groups and then
combines them linearly to produce discriminant
functions which show the greatest separation and
least dispersion between the groups. The statistical package BMDP, subprogram 7M (Dixon,
1985) was used to carry out the multivariate discriminant analysis. Linear combinations of in ,
fd , ARMs, IRMs and their ratios have produced
the two discriminant analysis variables shown
in Figure 2. The main contributors to discriminant analysis variable 1 are SARM, SIRM and
initial susceptibility (x10−6 m∧3/kg)
140
120
100
80
60
fup (b)
fup (s)
whp (b)
whp (s)
pt (b)
pt (s)
wd (s)
40
20
0
3
4
5
6
7
8
9
frequency dependent susceptibility (%)
Figure 1. Biplot of in (µm3 kg1 versus fd % for the 20 bulk (b) and 23 (s) sieved ash samples. Envelopes have been drawn
around the sieved data for each fuel type to emphasize the discrimination: fup is fibrous upper peat, whp is well-humified
peat, pt is peat turf and wd is wood.
Copyright  2001 John Wiley & Sons, Ltd.
Archaeol. Prospect. 8, 227–237 (2001)
Fire Ash Residues
231
and the well-humified peat ash. Some overlap
is observed between the peat turf ash and the
fibrous upper peat ash. It is appreciated that the
observed discrimination between the fuel types
may contain a component of variability stemming
from the area of peat extraction. However, at this
preliminary stage of the experimental research
project it is assumed that this variability is slight
as the solid geology of the study area generally is uniform, with basement rocks of Lewisian
Gneisses and few complex drift sequences in
West Lewis (Gribble, 1994). Future experimental research will address this issue of variability,
with regards to post-medieval township access to
specific peatbanks.
discriminant analysis variable 2
15
well-humified
10
peat turf
whp (b)
fup (b)
pt (b)
wd (s)
whp (s)
fup (s)
pt (s)
5
0
fibrous-upper
wood
−20 −15 −10
−5
0
5
10
15
20
discriminant analysis variable 1
Figure 2. Results of stepwise discriminant analysis carried
out on the room temperature magnetic data. The wood ash
(wd) and well-humified peat ash (whp) are both distinguished
from the fibrous upper peat ash (fup) and the peat turf ash
(pt), which show some overlap. Data for the bulk (b) and
sieved (s) ash samples are included.
ARMdemag40mT /SARM and to variable 2, in and
SARM/in . The biplot discriminates the wood ash
High temperature susceptibilities
Figure 3 displays the variation of susceptibility
with temperature for a selection of eight ash
samples spanning the range of fuel sources.
The peat turf ash and fibrous upper peat
ash display similar characteristic curves, which
well-humified peat
susceptibility (arb. units)
fibrous-upper peat
susceptibility (arb. units)
S9
S54
S37
S58
0
100
200
S95
100
200
300
400
500
600
500
700
100
200
300
400
500
Temperature (C)
600
700
S21
S44
S62
0
600
wood
peat turf
susceptibility (arb. units)
400
susceptibility (arb. units)
0
300
700
0
100
200
300
400
500
600
700
Temperature (C)
Figure 3. Variation of susceptibility with temperature for a selection of eight ash samples covering the range of fuel types.
Copyright  2001 John Wiley & Sons, Ltd.
Archaeol. Prospect. 8, 227–237 (2001)
C. Peters, M. J. Church and C. Mitchell
232
a single fuel type. Unknown quantities of the
four fuel types were burned in both hearth runs.
The fuel was burnt as single types rather than
mixes of for example well-humified and peaty
turf. Hence, layers of ash stemming from specific
fuel types were overlain one on the other. A single
ash sample was taken from FH15, S106, and a
profile of four ash samples from FH18, S133(2) to
S133(5). Values of the two discriminant analysis
variables used in Figure 2 were calculated from
the mineral magnetic data of the five bulk
ash samples. The resulting values are shown
on the discriminant analysis biplot in Figure 4.
The mixed fuel type nature of the ash samples
has indeed been highlighted by the magnetic
data plotted in the form of the discriminant
analysis biplot. The spread of the four samples
from the profile through FH18 (S133) shows that
within the build-up of the ash over a three day
period individual fuel types as well as mixtures
can be recognized. These results stem from the
burning and subsequent sampling of individual
fuel types within the three-day burning period of
various fuels. It is appreciated that archaeological
deposits from fires where fuels are mixed during
the burning may produce results that are difficult
to interpret. In that case, we assume that the fuel
that produces the most ash would be reflected
in the discriminant biplot and high temperature
readings. Hence, it may prove difficult to identify
burning episode of wood, interspersed with
15
discriminant analysis variable 2
increase in susceptibility with heating before
dropping sharply by ca. 600 ° C. In contrast, the
well-humified peat ash displays drops in susceptibility at lower temperatures. One, sometimes two drops are observed at ca. 330 and/or
ca. 550 ° C. The wood ash curves are similar to the
well-humified peat ash.
The three drops in susceptibility at ca. 330,
ca. 550 and ca. 600 ° C suggest three distinct magnetic mineralogies within the ash samples. At
present the actual mineralogy of these three magnetic components is uncertain, but may represent
modified magnetites or maghaemites. However,
the observed difference between the fibrous
upper peat/peat turf and the well-humified
peat/wood is useful for corroborating the room
temperature results displayed in the discriminant
analysis biplot of Figure 2.
The thermal history of samples can be investigated by comparing the heating and cooling susceptibility curves, in particular comparison of the
susceptibility at 40 ° C pre- and post-heating. Samples previously heated to above 700 ° C should
display no increase in susceptibility after heating/cooling. The susceptibility curves in Figure 3
all show that after heating and cooling the susceptibilities at 40 ° C are either lower or similar
to the pre-heating values, indicating that during
production of the ash temperatures above 700 ° C
were reached.
This information on thermal history is useful
when considering plant macrofossil preservation. Boardman and Jones (1990) demonstrated
through experimentation that the only plant
macrofossils to survive above this temperature, in
both reducing and oxidizing conditions, were the
more resilient elements such as the cereal caryopsis. This may explain the dominance of poorly
preserved cereal caryopses in many archaeobotany assemblages stemming from ash spread
and discarded from domestic hearths across
Atlantic Scotland (Church, in preparation). The
less resilient cereal components, such as the chaff
and other weed seeds, may have been totally
burnt and destroyed.
well-humified
10
peat turf
5
0
wood
−20 −15 −10 −5
Mixed fuel sources
For two of the fire hearth runs, FH15 and FH18,
mixtures of fuel sources were burned instead of
Copyright  2001 John Wiley & Sons, Ltd.
Mixed fuels
S106
S133
fibrous-upper
0
5
10
15
20
discriminant analysis variable 1
Figure 4. Discriminant analysis variables calculated for ash
residues from fire hearths FH15 (S106) and FH18 (S133),
which resulted from burning mixtures of fuel types.
Archaeol. Prospect. 8, 227–237 (2001)
233
S133 (3)
0
100 200 300 400 500 600 700
Temperature (C)
S133 (4)
0
100 200 300 400 500 600 700
Temperature (C)
Figure 5. High temperature susceptibility curves for samples
S133(3) and S133(4), from FH18 profile. The heating curves
are shown by solid lines and the cooling curves by dashed
lines.
burning of turf and peat, as the latter two fuels
produce significantly more ash than wood does.
Figure 5 displays the high temperature susceptibility curves for two of the S133 ash samples.
Sample S133(3) displays an increase in susceptibility before sharply dropping at 600 ° C, characteristic of fibrous upper peat/peat turf ash. The
heating curve for S133(4) shows a slight increase
in susceptibility before loosing its magnetization
by 560 ° C, which can be interpreted as a mixture of
fuel types dominated by well-humified peat ash.
On the biplot in Figure 4, S133(3) is the top lefthand sample and S133(4) the bottom right-hand
sample; thus the high temperature susceptibility
curves are consistent with and confirm the results
displayed on the discriminant analysis biplot in
Figure 2.
Archaeological applications
Case study: Guinnerso
Magnetic measurements were also carried out on
ash samples from the multiphase archaeological
Copyright  2001 John Wiley & Sons, Ltd.
site of Guinnerso on the Isle of Lewis, Scotland
(Church and Gilmour, 1999). A vertical profile,
sampled at 2-cm intervals, was taken from an Iron
Age hearth, resulting in nine samples. Initially,
bulk samples were measured. The discriminant
analysis variables calculated for the nine bulk
hearth samples are displayed on the biplot in
Figure 6. We observe that the bulk samples plot
outwith the range of the fuel sources. Subsequent
sieving of the samples at 63 µm, in an attempt
to isolate the ash component from larger sand
and other particles, and remeasuring of the
<63 µm particle size fractions has produced the
discriminant analysis variables labelled ‘sieved’
in Figure 6. The sieved samples plot within the
range of the modern ash residues. The biplot
suggests that the Guinnerso ash samples are
dominated by well-humified peat, with a wood
component in a couple of the samples. Fragments
of heather were found in bulk samples from the
hearth at a depth consistent with the wood ash
readings.
The high temperature susceptibility curves for
the profile are shown in Figure 7. The susceptibility is reduced to zero by ca. 550 ° C in all samples.
The heating curves are similar in nature to those of
well-humified peat ash and wood ash in Figure 3.
The cooling curves, however, are higher than
the heating curves, indicating that particles that
have not previously been heated to elevated temperatures are present within the archaeological
samples; as the particles were heated, alteration
15
discriminant analysis variable 2
susceptibility (arbitrary units)
susceptibility (arbitrary units)
Fire Ash Residues
well-humified
10
peat turf
5
0
−5
fibrous-upper
wood
Guinnerso
bulk
sieved
−10
−20 −15 −10 −5 0
5 10 15
discriminant analysis variable 1
20
Figure 6. Discriminant analysis biplot (Figure 2) with the bulk
and sieved Guinnerso hearth profile samples superimposed.
Archaeol. Prospect. 8, 227–237 (2001)
C. Peters, M. J. Church and C. Mitchell
234
A (0-2cm)
susceptibility (normalised)
B (2-4cm)
C (4-6cm)
D (6-8cm)
E (8-10cm)
F (10-12cm)
G (12-14cm)
H (14-16cm)
I (16-18cm)
0
100 200 300 400 500 600 700
Temperature (C)
Figure 7. High temperature susceptibility curves for the nine
samples from the Guinnerso Iron Age hearth profile. The
heating curves are shown by solid lines and the cooling
curves by dashed lines.
to a more magnetic phase occurred. Thus the
two newly developed methods for determining
fuel types, the discriminant analysis biplot and
the high temperature susceptibility curves, both
indicate that well-humified peat and wood, in the
form of heather, were burnt in the Guinnerso Iron
Age hearth.
Fire ash is of course not restricted to hearths
on archaeological sites. Thus, samples from floors
and ash spreads, in addition to hearth samples,
from both the Iron Age and Medieval periods of
occupation at Guinnerso have been analysed, to
investigate how effective the technique is in identifying ash components within the floors and ash
spreads. Figure 8 displays the discriminant analysis biplot for each of the four categories; Iron
Age hearths, medieval hearths, Iron Age floors
and ash spreads and Medieval floors and ash
spreads. The hearth samples from both periods
are clustered tightly. In comparison, the floor and
ash spread samples from both periods display a
much greater dispersion. Floors and ash spreads
are not necessarily composed entirely of fire ash
and therefore the magnetic grains within the fire
ash may not be their sole magnetic component.
One possible other magnetic component is bacterial magnetosomes. Biogenic precipitation of
Copyright  2001 John Wiley & Sons, Ltd.
magnetite by magnetotactic bacteria is feasible
within sedimentary and soil-forming environments at temperatures below 50 ° C (Maher and
Thompson, 1999). In order to investigate the
possible effect of bacterial magnetosomes on
the results of the discriminant analysis, Figure 9
was plotted. High values of SARM/SIRM versus
SARM/ were found by Barlow (1998) to indicate a bacterial magnetosome component within
sediments. Peters et al. (2000) have used the same
parameters to suggest a bacterial magnetosome
component within archaeological deposits at the
eroding Late Iron Age/Norse site at Galson on
the Isle of Lewis. In Figure 9, we observe that
four of the Iron Age floor and ash spread samples
and three of the Medieval samples for Guinnerso plot in the upper right-hand section of the
biplot, suggesting that bacterial magnetosomes
may contribute to the overall magnetic make up
of these samples. These seven samples have been
indicated by circles in Figure 8. With the exception of one of the Iron Age samples, the samples
without a high bacterial magnetosome component are more tightly clustered, and in the case
of the medieval samples no longer display such a
strong trend towards the peat turf. It is interesting
to note that the three medieval and one Iron Age
floor and ash spread samples with SARM/SIRM
values less than those of the hearth samples in
Figure 9, correspond to the samples with the
highest values of discriminant analysis variable 2
in Figure 8, possibly indicating another, as yet
unknown, magnetic component.
Use of the technique within archaeology
The technique has a number of possible applications to archaeological research. The identification of ash from different fuel types can aid
in the interpretation of site stratigraphy and
can complement those palaeoenvironmental techniques, such as soil micromorphology, that are
used more routinely on archaeological sites in
order to understand site formation processes.
The identification of different fuel types also aids
in the analysis of archaeobotanical assemblages
as it allows the separation of those macrofossils
that may have been introduced through the fuel
from plants relating to other human uses. For
example, past research has shown that different
Archaeol. Prospect. 8, 227–237 (2001)
Fire Ash Residues
235
Iron Age (hearths)
well-humified
10
peat turf
5
0
Medieval (hearths)
15
discriminant analysis variable 2
discriminant analysis variable 2
15
fibrous-upper
well-humified
10
peat turf
5
0
fibrous-upper
wood
−20
−15
−10
−5
wood
0
5
10
15
−20
20
discriminant analysis variable 1
peat turf
5
fibrous-upper
−10
−5
5
10
15
20
well-humified
10
peat turf
5
0
fibrous-upper
wood
−15
0
15
10
−20
−5
Medieval (floor, ash spread)
Iron Age (floor, ash spread)
well-humified
0
−10
discriminant analysis variable 1
discriminant analysis variable 2
discriminant analysis variable 2
15
−15
wood
0
5
10
15
20
discriminant analysis variable 1
−20
−15
−10
−5
0
5
10
15
20
discriminant analysis variable 1
Figure 8. Discriminant analysis biplots for Iron Age and medieval hearth samples and floor and ash spread samples. The
hearth samples cluster tightly in comparison with the floor and ash spread samples. The floor and ash spread samples
denoted by circles are samples that were found to have higher bacterial magnetosome components (see Figure 9).
fuel types produce varying numbers and proportions of plant parts and species (McLaughlin,
1980; Dickson, 1998). A complementary proxy
record, such as mineral magnetism, which can
highlight the probable fuel source in which the
plants were carbonized, therefore is an invaluable
tool for archaeobotanical taphonomy. The information on thermal history also is useful when
considering plant macrofossil preservation (see
above). Finally, the technique has considerable
Copyright  2001 John Wiley & Sons, Ltd.
interpretative value in terms of fuel procurement
and selection strategies. This can be approached
on an intra- and intersite basis over space and
time. Detailed analysis has been undertaken for a
number of archaeological sites in both the Western and Northern Isles, which demonstrates that
a pattern of varied fuel procurement and selection
existed across Atlantic Scotland in the later prehistory and early historic periods (Church et al.,
in preparation).
Archaeol. Prospect. 8, 227–237 (2001)
C. Peters, M. J. Church and C. Mitchell
236
SARM / susceptiblity (kA/m)
0.9
bacterial
0.8
magnetosome
0.7
component
0.6
0.5
I.A. hearths
Med. hearths
I.A. floors etc
Med. floors etc
0.4
0.3
0.03 0.04 0.05 0.06 0.07 0.08 0.09
SARM / SIRM
0.1
Figure 9. Biplot of the Iron Age and medieval hearths, floors and ash spreads. The combination of the parameters SARM/SIRM
versus SARM/ was found by Barlow (1998) to give an indication of a bacterial magnetosome component.
Conclusions
(i) Burning produces an enhanced magnetic
signal, thus fire ash is ideally suited to
mineral magnetic studies.
(ii) Discriminant analysis carried out on room
temperature magnetic measurements of experimental ash residues of known fuel type
has resulted in a biplot distinguishing wellhumified peat and wood, and showing some
overlap between fibrous upper peat and peat
turf.
(iii) Measurements of high temperature susceptibilities carried out on the experimental ash residues show differences between
well-humified peat/wood and fibrous upper
peat/peat turf.
(iv) Trial application of the two magnetic techniques in investigating fuel types from ash
residues excavated on archaeological sites
looks successful in identifying the main fuel
sources to be well-humified peat and wood
from Iron Age and medieval samples from
Guinnerso, Isle of Lewis.
Acknowledgements
CP held a British Petroleum/Royal Society of
Edinburgh Research Fellowship and MC held
a Caledonian Research Foundation Scholarship.
We would like to thank the following companies and individuals for help during the
Copyright  2001 John Wiley & Sons, Ltd.
experimentation at Calanais; Lewis Offshore for
the loan of a pyrometer and Mr ‘DR’ Morrison
of Gearrannan, Mrs Joan Chisholm and the Gearraidh na h-Aibhne Estate for access to the different
fuel sources.
References
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