Journal of Archaeological Science 34 (2007) 1417e1433
http://www.elsevier.com/locate/jas
Charcoal analysis and dendrology: data from archaeological
sites in north-western France
Dominique Marguerie a,*, Jean-Yves Hunot b
a
CNRS, Archaeology and Archaeosciences (UMR C2A), University of Rennes 1, Beaulieu, 35042 Rennes Cedex, France
b
Archaeological Service of Maine-et-Loire (SDA), 114 rue de Frémur, 49000 Angers, France
Received 24 May 2006; received in revised form 14 September 2006; accepted 31 October 2006
Abstract
During the last 15 years, charcoal analysis of archaeological sites in north-western France has been carried out in conjunction with systematic
and detailed dendrological examination. By considering these extrinsic criteria in association with the analytical results, palaeo-ethnographic and
palaeo-environmental information can be obtained. The charcoals are classically identified under the microscope on the basis of their cellular
structure. This examination is associated with an observation of the ligneous structure on transverse sections using a binocular lens. When
charcoal fragments are large enough, the growth ring widths are measured. Tree ring curvature is also noted. Finally, alteration by fusion or
radial cracks, the presence of fungal hyphae, and insect degradation are also recorded. Results are thus obtained on the nature of fuel used
in domestic fireplaces and kilns. The selection of timbers and their catchment areas are also revealed. The average width of the growth rings
in oak charcoal from domestic hearths coming from about forty sites in north-western France shows a significant increase from 6000 to
2000 BP. There is a similar increase in the number of heliophilic taxa used from the Neolithic to the Iron Age. This implies that the environment
became more and more open because of use by society. The interpretation of the dendrology results applied to charcoal analyses is obtained
through a convergence of criteria. Thus, charcoal analysis can provide more than just an identification of the species used, and can yield
fundamental information on the interaction of society with the environment.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Charcoal; Dendrological analysis; Archaeology; Palaeo-environment; Palaeo-ecology; Palaeo-ethnography; North-western France
1. Introduction
During prehistoric times, societies established an economy
based on firewood. Therefore, charcoal analysis is an efficient
approach for studying the relationship between people and the
environment, reflecting the customs and techniques of management of ligneous vegetation. The collection of wood relates back
to the knowledge of its physical properties or to an attitude
motivated by traditions. However, the environmental conditions
are decisive in controlling the availability and quality of any exploitable ligneous biomass. Charcoals are excellent indicators
* Corresponding author. Tel.: þ33 2 23 23 56 77; fax: þ33 2 23 23 69 34.
E-mail addresses: [email protected] (D. Marguerie),
[email protected] (J.-Y. Hunot).
0305-4403/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jas.2006.10.032
of exploited environments and the vegetation that developed
within them.
1.1. Brief history of anthracology
The first identifications of charcoals were carried out using
samples from archaeological sites in Switzerland, Germany
and Hungary during the 19th century. In France, the forerunners of anthracology were Dangeard (1899) and Fiche (Breuil,
1903).
The application of episcopic microscopy to charcoal
analysis led to the analysis of large quantities of charcoals
(Dimbleby, 1967; Fietz, 1933; Stieber, 1957; Western, 1963).
The first global studies on prehistoric charcoals of anthropic origin were carried out in the south of France, and later
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D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
in Spain by Vernet (1972) and his students. More recently,
charcoal analysis has been used to study the relationship
between humans and forests through the economy of fuel.
Methods used for the study of firewood economy are in constant progress and many archaeological contexts are now studied. In this spirit, charcoal analysis of archaeological sites in
north-western France has been systematically carried out for
about 15 years using detailed dendrological examination
(Marguerie, 1992). Marguerie and Hunot supplemented the
taxonomic identification of charcoals by the study of rings
as well as the origin and state of the wood before carbonization, thus providing palaeo-ecological and palaeo-ethnological
information. This approach was referred to as a ‘‘dendrotypological’’ study by Billamboz (1987, 1992), who applied it to
waterlogged woods of the lake-dwelling settlements of the
Bodensee (Germany). He highlighted the first models of
Neolithic forestry economy. This kind of approach was taken
up again and developed in the Paris Basin (Bernard, 1998).
More recently, Bernard et al. (2006) worked on the effect of
trimming and pruning on ligneous productivity by studying
the radial growth of deciduous oaks.
However, an anecdotal application of dendrology to charcoals has existed for a long time. Around 1930, Guinier began
the analysis of charcoals from the Mesolithic site on Teviec
Island in Brittany (France) and, several years later, on the
nearby Hoedic Island (Péquart and Péquart, 1954; Péquart
et al., 1937). At these sites, Guinier recorded the presence of
twigs, branches and trunks of oaks and Pomoideae. Furthermore, he noticed that several oak charcoals had marks of
lopping. Salisbury and Jane (1940) studied a large number
of charcoals from Neolithic and Iron Age layers from Maiden
Castle in Dorset (U.K.). They measured nearly two thousand
growth rings of oak and hazel charcoals and compared the
results with the annual rings from recent specimens of the
same species to obtain ‘‘evidence regarding the climatic conditions of the past by means of the width of the annual rings’’.
In the cave of Lascaux (Dordogne, France), Jacquiot (1960)
systematically observed the aspect of rings of the charred
wood. In this way, he obtained information about climate, density of forests and diameters of wood used.
In her analysis of charcoal from the Neolithic sites of
Clairvaux-Les-Lacs (Jura, France), Lundström-Baudais
(1986) developed a method to evaluate the maximum diameter
of wood used based on the observation of tree ring curvature.
After Lundström-Baudais, Dufraisse (2002) analysed the
ring curvature of samples from the lake-dwelling sites of
Chalain and Clairvaux to estimate the calibre of woods converted into charcoals. In Cabrières (Southern France), using
construction wood (16th c.), observation and measurement
of ring curvature were used to calculate the minimum diameters, with oak samples having the greater dimensions. By contrast, the samples with bark came from small twigs of coppice
(Durand, 2002). In the study of the historic charcoal kiln sites
in the Southern Black Forest and Bavarian Forest (Germany),
Ludemann and Nelle (Ludemann and Nelle, 2002; Nelle,
2002) used a diameter stencil to estimate the original
minimum diameter of the charcoal pieces.
Over a few years, dendrological analysis applied to carbonized wood became increasingly employed and concerned not
only charcoals coming from archaeological sites but also those
found in natural soils (Bégin and Marguerie, 2002).
The aim of this paper is to present methods used for
dendrological studies of wood charcoal and give examples of
palaeo-ecological and palaeo-ethnographical results obtained
by dendro-anthracology in north-western France during the
last 15 years.
2. Methods
There are numerous origins of charcoals from archaeological sites (Fig. 1):
e
e
e
e
e
domestic fire places,
craft combustion structures such as kilns,
post-holes, pits or ditches,
archaeological layers,
burnt objects.
After sampling with the surrounding sediment, the charcoals are classically identified on the basis of their cellular
structure under a reflected light microscope. To this determination, we add a systematic examination of the ligneous structure
on transverse sections using a binocular lens (magnification 7
to 90). This dendrological approach gives valuable data for
aiding identification of the part of the woody plant the charcoals came from, recording the growth ring width, ascertaining
the state of the wood before carbonization and about visible
working marks where possible. A coding grid table is compiled for all transverse sections of charcoal samples of sufficient surface area having a readable record (Table 1) (Fig. 2).
2.1. Charcoal state
2.1.1. Presence of bark and pith
For fragments having both bark and pith, it is possible to
measure a complete radius and estimate the calibre of the
stem from which it came (Fig. 2A). Evidently, it is necessary
to take into account the reduction of size caused by combustion (see Section 2.3.1).
2.1.2. Presence of reaction wood
Reaction wood can be observed in a branch or leaning trunk
(Fig. 2B). To avoid drooping under their own weight, these
stems develop eccentric growth (Wilson and White, 1986;
Zobel and Van Buijtenen, 1989). In reaction wood, the walls
of the tracheids are thick and show radial grooves on their
inner surfaces. In longitudinal sections of the charcoal fragments, they appear as conspicuous parallel striations obliquely
aligned to the cell axis, commonly at an angle of about 40e45 ,
thus forming helices around the cell. These features are clearly
visible in the coniferous charcoals. The observation of reaction
wood in charcoals indicates the origin of the fragments within
the tree. Associated with strong ring curvature, this criterion
demonstrates that the charcoal originates from a branch.
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
1419
2
4
1
5
3
6
Fig. 1. Origins of charcoals at an archaeological site. (1) domestic fire places; (2) craft combustion structures; (3e5) post-holes, pits, ditches; (6) archaeological
layers.
2.1.4. Presence of fungal hyphae
In longitudinal sections of vessels, white filaments can
sometimes be observed (Fig. 2D). These correspond to fungal
hyphae that penetrate into the dead or dying wood under
aerobic conditions from fungus living on the surface. The
fungal attack takes place over several hours or days in the parts
of the wood not protected by bark. This process is all the more
rapid when temperatures are high (in summer) and when wood
humidity is about 70-90% (Schweingruber, 1982). This observation gives information about the state of wood before
carbonization. It is rare or impossible to find hyphae in the
heartwood of oak and some other species with numerous
tyloses.
2.1.3. Presence of tyloses
In the transition zone between sapwood and heartwood, living cells of axial or ray parenchyma adjacent to vessels may
grow out through the pits into the vessel cavity, forming tyloses (Fig. 2C). When this phenomenon becomes intense, the
vessel cavity may become filled with a close-packed cellular
structure, blocking it completely (Wilson and White, 1986).
These extensions of the parenchyma cells, mutually compressed into irregular prismatic shapes, are very clearly visible
under the light microscope. In charcoals, the tyloses are refractive. Such features can be readily observed in oak. This type of
systematic observation indicates that the charcoals were
derived from heartwood.
Table 1
Example of coding grid for dendro-anthracological analysis
Sample Code Species
no.
1
2
3
4
5
6
1
1
10
9
3
27
a
b
c
No. Total width Rings
Carboni- Hyphaec Insect
Pithc Barkc Tylosesc Reaction Working Comments
of
of rings
bendinga zationb
degradationc
woodc
marksc
rings (mm)
Quercus sp.
5
Quercus sp.
15
Juglans sp.
2
Fagus sylvatica
1
Corylus avellana 12
Ilex aquifolium
7
4.74
10.23
6.76
3.12
1
1
1
1
2
3
1
1
1
1
1
1
2
1 ¼ low curve rings; 2 ¼ intermediate curve rings; 3 ¼ strong curve rings.
1 ¼ radial cracks; 2 ¼ low brilliance; 3 ¼ strong brilliance.
1 ¼ ‘‘yes’’.
1
1
1
1420
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
Fig. 2. Scanning electron micrographs showing systematically observed dendrological criteria. (A) Stem of archaeological charred oak (Quercus sp.) with bark and
pith; (B) coniferous charcoal (Picea mariana) with reaction wood in tracheids; (C) tyloses in archaeological oak heartwood charcoal; (D) fungal hyphae in vessels
of charred oak sapwood; (E) tunnels of wood-boring insects or woodworms in Scots pine (Pinus sylvestris); (F) radial cracks in a white spruce charcoal (Picea
glauca); (GeI) different degrees of vitrification in archaeological charcoals; (J, K) different tree ring curves in archaeological charred oaks: (J) weakly curved
rings, (K) moderately curved rings; (L) large rings and narrow rings in archaeological oak charcoal; (M) working mark on a charcoal from an incense-vase.
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
2.1.5. Presence of insect degradation
In some charcoals, tunnels are sometimes observed
(Fig. 2E). Dimbleby (1967) mentioned that, in some Neolithic
charcoals, irregular patterns of large holes could result from
some wood-boring insect or woodworm degradation. Sometimes, it is possible to find beetles within these channels.
Even when the animal is not preserved inside, the aspect of
the channels can nevertheless indicate, not without some difficulty, the species of insect or worm that lived there (Serment
and Pruvost, 1991). The presence of many such channels in
charcoals is a good indicator of the combustion of drifted
dead wood. In their study of charcoals from Maiden Castle
(Dorset, U.K.), Salisbury and Jane (1940) mentioned that the
wood employed was ‘‘dead wood rests’’ because the charcoals
contained ‘‘bore-holes filled with carbonized frass’’, the result
of the activity of ‘‘beetle larvae of the type which attack dead
and not living wood’’.
2.1.6. Presence of radial cracks
Radial cracks are often common in charcoals (Fig. 2F).
Their frequency depends on the anatomy of the wood (more
frequent in the case of dense and large rays), the location in
the wood (less frequent when close to the pith), the level of
wood dampness (consequence of the discharge of ‘‘closed
water’’) and temperature (Théry-Parisot, 2001). According to
Prior and Alvin (1986), the carbonization of watterlogged
wood favours a substantial increase in the number of radial
cracks.
2.1.7. Presence of vitrification features
The vitrification of charcoals corresponds to a variable
fusion of anatomical constituents within the wood, leading
to homogenisation of the structure that makes identification
impossible when the process reaches its final stage. The
vitrified charcoals become very dense and refractive with
‘‘sub-conchoidal’’ fractures. Many stages of alteration by
fusion are known, ranging from a still- recognisable anatomical structure to a dense mass, completely molten and nondeterminable (Fig. 2GeI):
e low brilliance-refractiveness (degree 1),
e strong brilliance (degree 2),
e total fusionddense, non-recognisable mass (degree 3).
1421
The fusion may be associated with radial cracks. Vitrification often affects small pieces of wood such as twigs. Rapid
combustion at high temperatures frequently causes tissue deformation, fissures and fusion (Schweingruber, 1982). But, more
generally, this alteration of the anatomical structure by fusion
may result from specific conditions of combustion or taphonomy, and can reveal the state of the wood before combustion.
There appear to be numerous types of conditions affecting
fusion, and many anthracologists are currently attempting to
understand this phenomenon in all its aspects.
2.2. Evaluation of growth-ring curvature
The evaluation of tree-ring curvature (and the angle of the
rays) enables us to identify which part of the tree was used.
For example, a weak or smooth curve corresponds to the
tree trunk (Fig. 2J), while a strong or marked curve corresponds to the branches (Fig. 2K).
The ring curvature is estimated according to a standard
classification: with a constant magnification and using a transparent test card placed on top of or under the fragment (Fig. 3).
To apply this method, it would appear necessary to have
a minimal tangential width of charcoal of around 3 to 4 mm
to estimate the approximate parallelism of rays, or a radial
length of 5 mm to estimate the curvature of the rings. These
minimal conditions are not necessary when the fragments
have both pith and bark and therefore correspond to a branch
or twig.
Charcoals are divided into four groups, which exhibit:
e strongly curved rings,
e moderately curved rings,
e weakly curved rings (at this observation scale, the rings
seem ‘‘straight’’ and the rays parallel),
e indeterminate curvature (on fragments without minimal
conditions).
While such an approach reveals trends, it is not a measurement of the diameter of the wood, but merely a characterization.
In one case, it was possible to measure the wood section (where
pith and bark appear together in a charcoal; see Section 2.1.1).
More generally, abundant charcoals with strongly curved rings
in one sample indicate the use of small calibre wood or
Fig. 3. Test card for evaluation of tree-ring curvature.
1422
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
branches. On the other hand, the predominance of charcoals
with weakly curved rings suggests the use of large calibre
wood such as trunks or large branches. Finally, by combining
the criteria of ring- curvature and the presence of bark, pith
or reaction wood, it is possible to demonstrate the use of
branches.
2.3. Growth ring width
2.3.1. Measurement of growth ring width
When charcoal fragments were large enough, we systematically measured the growth ring widths (Fig. 2L). The average
growth ring width can provide information on the growing
conditions of the trees:
e narrow rings correspond to restrictive growing conditions,
e large rings indicate favourable growing conditions.
The average growth ring widths were only measured on
charcoal samples with a weak curvature of rings (derived
from trunks or large branches, far away from the pith), and
with regular width rings. This measurement was indeed
impossible on branches or small stems with eccentric growth
because of the development of reaction wood. The ring widths
are thus highly variable on two opposite rays. Furthermore, the
rings near the pith are always thicker than the outer rings. In
their response to Salisbury and Jane’s paper (1940), Godwin
and Tansley (1941) put forward serious criticisms about the
method of handling the tree-ring data obtained from specimens of young stems. They considered it was imprudent to ignore ‘‘the progressive diminution of tree-ring width with the
age of the tree’’. They suggested that comparison of ring
widths is ‘‘valid only for the outer rings of large trees’’.
2.3.2. Intra-site frequency distribution of the
charcoal ring-widths
In one sample of charcoals, an average ring width was
calculated for each charcoal sample of readable and measurable rings. Thus, we obtained as many average width values
as measurable charcoals. This procedure minimizes the
influence of extreme values and reveals trends in the growth
of woods.
A bar chart is used represent these measurements by classes
of 0.25 mm or 0.5 mm for each taxon and for each sample of
charcoal. Measurements were mainly performed on samples
with substantial numbers of charcoals (>50 fragments with
measurable rings). If not, there would be an excessive risk
of taking exceptional rings into account.
It is interesting to characterize the distribution of these
values for each taxon within a site and note their approximate
dispersion using descriptive statistics. A unimodal distribution
of these classes is interpreted as indicative of a homogeneous
stand or a single tree (Fig. 10a). On the contrary, a multimodal
distribution is interpreted as a heterogeneous community of
trees or several populations (Fig. 10c).
Finally, it is possible to produce growth curves for large
charcoals that contain dozens of rings. This method, for example, allows us to ensure that charcoals come from a single tree
and assemble fragments with the same ring-width patterns
coming from the same stand (Morgan, 2000).
2.3.3. Validity of the growth-ring width measurement
modified by carbonization
Combustion processes have some effects on the morphology of the wood. While the size and shape of wood constituents can vary, the qualitative anatomical characteristics remain
the same (Beall et al., 1974). The deformation of the charcoal
microstructure varies with wood species. The loss of 70e80%
of the substance of the wood causes a shrinkage of 7e13%
longitudinally and 12e20% radially/tangentially. The cell
wall is reduced to 1/5e1/4 of its original thickness (Schweingruber, 1982).
In 1940, Salisbury and Jane converted recent specimens of
hazel stems into charcoal and measured the width of the
annual rings both before and after carbonization using various
techniques (slow and rapid combustion). These authors concluded that the widths were ‘‘not significantly different whatever method was employed to produce the charcoal’’
(Salisbury and Jane, 1940). Based on these data, it would
appear possible to measure the radial growth that can be
observed on transverse sections of a charcoal and compare
these measurements from one charcoal to another. In any
case, the present study does not aim to compare the absolute
values measured on charcoals with data obtained from fresh
or waterlogged wood.
In practice, we were able to measure the average ring-width
with an electronic calliper or a dendrochronological digital
positioning table (accuracy: 0.01 mm) by examining the ligneous structure of charcoals on transverse sections with a binocular lens. To compare these two methods of measuring the
average ring width, we measured 161 fragments of oak charcoals (834 growth rings) from a Roman archaeological site
(Allonnes, Sarthe). The extremely close similarity of results
obtained shows that it is not possible to favour one of these
measuring systems above the other (Table 2).
To improve our knowledge of the growth conditions, it is becoming increasingly necessary to measure systematically
Table 2
Comparison of two dendrological measuring systems for growth ring width
Electronic calliper
Dendrochronological table
Mean
(mm)
Standard
deviation
Min.
(mm)
Max.
(mm)
N charcoals
N rings
1.64
1.65
1.14
1.29
0.2
0.17
6.53
6.31
161
147
834
521
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
1423
Atlantic
Ocean
Caen
Mediterranean
Sea
100 km
Brest
Saint-Brieuc
Rennes
Laval
Quimper
Vannes
0
50
100 Km
Angers
Sites with:
Domestic fireplaces
Nantes
Kilns
Other structures
Fig. 4. Location map of the study sites.
0
80%
0
40
90 0
0
80 0
30
10
0
90
le
x
U
In
S
de ali
te x s
rm p
in .
ab
le
sp
.
sp
ta
is
en
G
PO
M
O
ID
EA
E
us
Fa / C
gu as
s tan
sy e
C
ar U lva a
pi lm tic
C nu us a
or s
yl be sp
us t .
av ulu
el s
la
na
rc
Q
ue
Pi
nu
s
s
Q ylve
ue s
rc tris
us
sp
.
Loc EG C8
Loc TDM F5
Bilgroix
Rouick F1
Ernes Ee21
Ernes Ff26
Cairon St11
Condé St19
Condé St21
Condé Sub101
Mez Notariou
Kersigneau C4
Kersigneau C5
Ebihens C2
Pont-l’Abbé 438
Pont-l’Abbé 387
Tinténiac TF
.
Be
t
Ac Pr ula
er unu sp
Fr Fr ca s s .
ax an mp p.
in gu e
us la st
ex al ris
ce nu
ls s
io
r
Erdeven F1
Erdeven F2
Loc EG C5
Fig. 5. Percentage charcoal diagram for domestic fireplaces of north-western France. Upper diagram: Neolithic structures; lower diagram: Iron Age structures;
black dots correspond to sparsely occurring taxa (1%).
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
1424
%
100
Neolithic
Iron Age
A
90
80
70
Rings curvature
60
Weakly curved
50
Moderately curved
Strongly curved
40
30
20
10
100
Neolithic
Kersigneau
C4
Rouick F1
Bilgroix
Loc TDM F5
Loc EG C8
Loc EG C5
Erdeven F2
Erdeven F1
0
Iron Age
B
90
80
70
60
Rings curvature
50
Weakly curved
40
Moderately curved
Strongly curved
30
20
10
Kersigneau
C4
Mez Notariou
Rouick F1
Bilgroix
Loc TDM F5
Loc EG C8
Loc EG C5
Erdeven F2
Erdeven F1
0
Fig. 6. Calibre of wood in Neolithic and Iron Age domestic fireplaces of north-western France (A: oak; B: all woody plant species).
within the growth ring of a charcoal (i.e. the width of early and
late wood, vessel area, vessel density, frequency of rays, etc.).
Such measurements should be automated as much as possible
to facilitate the acquisition of results on many charcoals.
From this perspective, and following an initial study by Vernet
et al. (1983) on ring morphology in olive tree charcoals, Terral,
1999, 2002) used an eco-anatomical approach with morphometric analyses of wooden archaeological material to determine the origin of cultivation of olive trees and grapevines in
the western Mediterranean region.
3. Results
Examples of dendrology applied to anthracology are taken
from the studies of sites located in north-western France,
most of these being located in Brittany. Around seventy archaeological sites have now been studied in this area (Fig. 4).
3.1. Palaeo-ethnographic information
3.1.1. Fuel
3.1.1.1. Fuel from domestic fireplaces. We considered the contents of charcoals from twenty domestic fireplaces situated at
12 archaeological sites in Brittany and Normandy (Fig. 4).
They correspond to Neolithic burial sites or rural settlements
of the Iron Age (Marguerie, 2003).
The studied fireplaces contain wood representing from one
to nine taxa. Oak is found in 14 of the structures (Fig. 5). From
48% to 100% of oak charcoals in Neolithic fireplaces had
weakly curved rings and originated from wood of large calibre. On the other hand, in the only example from the Iron
Age where we could carry out this kind of examination, the
oak charcoals have strongly curved rings in 89% of cases
(Fig. 6A). Taking all the taxa together, the Neolithic structures
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
contain wood of large or medium calibre. On the contrary,
charcoals from the two Iron Age structures are of small
cross-section (Fig. 6B). During the Iron Age, the use of stems
with small cross-section was also the consequence of exploiting shrubs and small trees.
3.1.1.2. Fuel of kilns. The anthracological study of craft structures is based on material from 12 settlements (Fig. 4). The
archaeological sites dated chronologically from the second
Iron Age to the late Middle Ages. Twenty combustion structures were studied. They can be categorized into four groups
of kilns: ceramic or tile, glass, metal and lime (Marguerie,
2002).
A taxonomic richness factor from one to seven is observed
in the studied sites (Fig. 7). The tree ring curvature systematically observed indicates that the oak charcoals in craft structures originated from large-diameter timber (Fig. 8). In the
case of the beech charcoals, fragments in the pottery kilns
also came from tree trunks. Some oak and beech charcoals
from the pottery kiln of La Haute-Chapelle display radial
cracks (13.5%). On the contrary, such features are totally absent from the oak charcoals of the lime kilns of Jouars.
Shrubs or small trees such as hazel, Pomo€ıdeae, Prunus,
birch, maple, broom and alder provide high flames over a short
time when tied up into bundles. The ‘‘big fire’’ obtained in this
way produces a high temperature. Gallo-Roman potters used
some plant materials with fast combustion, such as straw,
reed, heather, bark or pine cones, to obtain a high and very
hot flame yielding a rapid rise in temperature. When added
to the ‘‘small fire’’, this additional plant-based fuel produced
0
90%
30
1425
the ‘‘big fire’’ needed for the main firing process (Brongniart,
1977; Le Ny, 1988; Pillet, 1982).
The cellular tissue of some oak (15%) and beech (23%) charcoals found in the metal kilns of La Bazoge had been molten
(vitrified). In the bell kiln of Ambon, more than 50% of oak
charcoals were ‘‘ vitrified ’’. The molten or vitrified features
of some oak and beech charcoals suggest that some metal kilns
could use burning charcoal and not exclusively wood. This hypothesis must be tested by identifying the origin of the process
involved the vitrification of charcoals. The vitrification process
is not yet well understood and its association with burning charcoal not clearly established. For the metalworking craft industry, the heat needed to be stable and intense.
In the ‘‘Ambroise Paré’’ site (Rennes), the excavation of
a Roman kiln has yielded oak charcoals produced from large
branches (Guitton et al., 2002). The majority of these have
weakly curved rings and an average age of 15 years. They are
assumed to be contemporaneous, representing the last phase
of the kiln activity. For some of the charcoals, it was possible
to measure the growth ring widths using a dendrochronological
table (Lintab Rinntech). Matching the average dendrochronology curve for Ambroise Paré (M1: 36 years long), which
groups together nine curves, was facilitated by the presence
of one charcoal of the same length (AP13: 36 rings) and
a fall in growth rate visible at the beginning of many sequences
(Fig. 9). The ring-width patterns of these carbonized woods are
so similar that they probably came from the same tree.
3.1.1.3. Fuel in incense-vases. Charcoals in incense-vases
from Anjou were derived from oak branches. This was
80
20
30
10
A
B
C
D
Fig. 7. Percentage charcoal diagram for kilns of north-western France (A: ceramic or tile kilns; B: glass kiln; C: metal kilns; D: lime kilns). Black dots correspond
to sparsely occurring taxa ( 1%).
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
1426
A
Pottery kilns
with a strong curvature of rings points to the use of small
twigs (Hunot, 1992, 1996) (Fig. 10). Moreover, the working
marks on several suggest the use of coppicing wood
(Fig. 2M).
100
90
80
70
60
probably a result of the over-exploitation of forests around
Angers over many centuries during Medieval times. The
wood of oak represented from 90% to 100% of the remains
contained in 70 vases dating from the Middle Ages to Modern times. The presence of bark and pith together, associated
3.1.3. Catchment areas
For La Hersonnais, we systematically analysed the
distribution of oak ring widths wherever possible. Owing to
this method, it was possible to assess the homogeneity or
heterogeneity of the sample as well as the supplier areas
(Fig. 13).
%
Fig. 8. Calibre of wood in Neolithic and Iron Age kilns of north-western
France (A: oak; B: beech).
3.1.2. Timber: example of the Neolithic settlements of La
Hersonnais
The site of La Hersonnais (Pléchatel, Ille-et-Vilaine) has
provided a great number of charcoals coming from the timbers
of different late Neolithic settlements (Tinevez, 2004). Each
building is surrounded by an enclosure. They belong to the
late Neolithic period, and it is possible to follow their architectural evolution from the 27th to the 26th century BC (Bernard
and Thibaudeau, 2002) (Fig. 11).
The studied charcoals come from post-holes and ditches
(Marguerie and Thibaudeau, 2004) (Fig. 12). The wooden superstructures were made from pieces taken from large
branches or trunks of oak. The majority (74.4%) of oak charcoals from all the studied samples show a weak ring curvature. The charcoals of others species came from pieces with
medium to small calibre. This is also the consequence of
the morphology of the smaller trees or shrubs from which
they came.
Very few charcoals of oaks (38/1959) from building A and
enclosures B and C have insect tunnels. Combined with the
frequent observation of tyloses in vessels, this result indicates
the preferential use of heartwood for construction.
Oak has excellent mechanical properties. In north-western
Europe, it is the building material par excellence. The density and the mechanical qualities vary greatly depending on
the proportion of latewood in the total width of a ring
(Zobel and Van Buijtenen, 1989). The thick annual growths
are of high density and are best suited to resist the static
and dynamic forces experienced. These woods are difficult
to work and have large shrinkages. In contrast, medium annual growths (less than 2 mm) correspond to mechanically
weaker and softer woods, which are easer to work and
have small shrinkages. Trees with such wood have even
and straight trunks that are ideal for obtaining long beams
(C.T.B.A., 1989; Rameau et al., 1989; Sell and Kropf,
1990).
At La Hersonnais, oaks with slow growth were used. This
was the case within any given architectural group, with a great
similarity between timbers from the building and the enclosures. There was no significant difference according to the Student t-test, except for group B where the average widths are
clearly greater in the fences (Sokal and Rholf, 1981). The trees
with narrow rings, and probably with long, straight and regular
trunks, were probably chosen owing to the large size of the
buildings.
50
40
30
20
10
Vannes
HauteChapelle
Roche-Mabile
Chartres
Tressé
Trémentines
0
A
%
Metal kilns
100
90
80
70
60
50
40
30
20
10
0
La Bazoge
Plélan
Ambon
Cesson
A
%
Lime kilns
100
90
80
70
60
50
40
30
20
10
0
Jouars
Touffreville
B
%
Kilns
100
90
80
70
60
50
40
30
20
10
0
Rings curvature
Weakly curved
Moderately curved
Strongly curved
Chartres
RocheMabile
HauteChapelle
Vannes
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
1427
Growth ring width
100%
A
90%
80%
70%
position 12
60%
Taxa contents
50%
Others 6 woody
species
Oak
40%
AP12
30%
20%
position 10
10%
AP8
0%
position 9
AP7
Weight
100%
90%
Fragments
B
80%
AP14
position 11
70%
60%
AP2
position 11
AP15
position 12
position 7
Rings curvature of oak
50%
Weakly curved
40%
Moderately curved
Strongly curved
30%
AP11
20%
10%
position 7
0%
AP13
position 1
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
AP5
Pith
1mm
Oak with strongly curved rings
No
Bark & Pith
Pith only
Bark only
Fig. 10. Anthracology and dendrology of charcoals from four incensevases from La Haye-aux-Bonshommes (Avrillé, Anjou), grave HB1/US60
(15th C.).
AP.M1
40
C
Years
Fig. 9. Ring-width patterns of nine large oak charcoals from ‘‘Ambroise Paré’’
site (Rennes).
The histogram of classes drawn up for building A shows
a very well classified normal distribution, which is unimodally
centred on the value 1.25 mm (M ¼ 1.30 mm, s ¼ 0.49,
N ¼ 486 charcoals and 5193 rings). The oaks used come
from a single homogeneous population (Fig. 13A).
At Saint-Symphorien (Paule, Côtes-d’Armor), the
sediments filling the underground passage (P1555) during
the second half of the 5th century BC contain charcoals
of oak with a large average ring width (M ¼ 2.78 mm,
standard deviation ¼ 1.41, N ¼ 59 charcoals and 363 rings),
showing a scattered and multimodal distribution from 1.01
to 7.25 mm (Fig. 13B). The woods of these mixed
fires came from different areas with different growing
conditions.
In the Carolingian settlement of Talvassais at Montours
(Ille-et-Vilaine) (Cattedu, 2001), oak charcoals corresponding
to the first stage of the use of the domestic kiln (St. 629) have
ring widths with a strongly scattered distribution from 1.71 to
7 mm (Fig. 13C). The average width is 3.32 mm with a standard deviation of 1.12 mm, calculated on 74 charcoals and
327 rings. This sample contains seven charcoals with very
high growth rate. In this case, charcoals seem to have come
from different areas or from the middle of a dense forest out
to its open edge.
1428
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
A
C
B
D
Fig. 11. Plan of buildings and enclosures at the Neolithic site of La Hersonnais (Pléchatel, Ille-et-Vilaine). (From Tinevez, 2004).
3.2. Palaeo-environmental information
In forest ecology, incremental variations in wood may be
interpreted using a complex series of parameters: adaptation
of the tree to the location, availability of water and nutrients,
tree spacing, attack by pathogenic organisms and climate
(Gassman, 1999; Otto, 1994; Schweingruber, 1988; Wilson
and White, 1986; Zobel and Van Buijtenen, 1989). In 1983,
Heinz carried out a palaeo-dendroclimatological analysis by
measuring the widths of tree rings on charcoals of Juniperus
(Heinz, 1983).
Along with environmentally induced variations in wood
formation, we consider the effect of spacing within communities on wood properties, as shown by the examples given
below. Differences in spacing have a considerably effect on
competition between trees, such as through the availability
of moisture and nutrients (Otto, 1994). Recently, Carrion
(2003) carried out a dendrological analysis of charred
material from the wooden support structure of the Neolithic
dolmen of Tres Montes (Navarra, Spain). This author
measured tree growth rings in charcoals of Juniperus, thus
highlighting the exploitation of two different stands as well
as the impact of climatic variations and human activities
around the site.
Following many studies in the north-western France covering the postglacial period, during which climatic variations
were too weak to modify clearly the average radial growth
of the trees, we conclude that the average annual growth
rate provided by the dendrological analysis of charcoals above
all reflects the density of the stands.
At the Neolithic site of La Hersonnais, the different settlement complexes were not contemporaneous. The stratigraphic
study carried out during the excavation, as well as the typological criteria and the relative dating of the structures, all suggest
that the oldest building was structure A, followed in order by
B, C and D (Fig. 11).
In this case, anthracology may provide information about
the environmental context and the structure of surrounding
woodlands. The increase of the average ring width over the
studied time interval reflects forest degradation and the
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
0
90%
1429
10
Building A
Enclosure A
Building B
Enclosure B
Building C
Enclosure C
m
pe
Be stri
PO t
s
M ula
O sp
ID .
C EA
or
nu E
Pr s s
un p.
us
Fr
ax Gen sp
.
in
us ista
ex sp
.
c
Al els
nu io
In S s s r
de a p
te lix .
rm sp
in .
ab
le
ca
er
Ac
Ile
x
ar aqu
pi
nu ifol
s ium
be
C U tul
or lm us
yl
us us
av sp.
el
la
na
C
Q
ue
rc
us
sp
.
Building D
Fig. 12. Percentage charcoal diagram for buildings and enclosures of La Hersonnais (Pléchatel, Ille-et-Vilaine). Black dots correspond to sparsely occurring taxa
(1%).
evolution of woodland potential around the site. It seems that
a single forest was exploited throughout the occupation of the
site. Following the same pattern, buildings became smaller and
the walls, previously made of big oak posts, were replaced by
thinner wattle walls.
The samples of oak charcoal have an average ring widthd
measured on 1501 fragmentsdthat varies between 1.26 and
1.94 mm. This average value increases from group AeB to
group D (Fig. 14). The Student t-test shows that the difference
between A and B is not significant and that oaks used for the
construction of these two groups probably came from the same
ecosystem. Between the groups A and C, and between B and
C, the average values are significantly different at a 99%
confidence level.
The narrowness of oak rings in groups A and B indicates
the existence of dense supplier forests. Then, the increase of
the average width observed during the development of occupation over the next century informs us about the opening up of
the forest, leading to a damaged formation of scattered and
isolated large trees.
From woods designated for the construction of buildings,
we can interpret the dendro-anthracological data in terms of
palaeo-ecology. The growing proportion of heliophilic species
(birch, hazel, maple, broom, etc.) used during the different
phases of occupation of the site follows the clearing of the environment and the greater availability of these species around
the settlement (Fig. 12).
In north-western France, the average width of the growth
rings in oak charcoal from domestic hearths, coming from about
40 sites and 55 samples, shows a significant increase from the
Neolithic to the end of the Iron Age. Linear regression analysis
gives a relationship between time (t years) and ring width
( y mm) that can be expressed as y ¼ 0.0028t þ 1.2682
(r ¼ 0.57) (Fig. 15A).
The graph shows the average width of growth rings observed, along with the standard deviation. Two sets of samples
are quite clearly contrasted:
e width of about 1.5 mm during the middle Neolithic period,
corresponding to narrow rings,
e width of about 2.4 mm during the Iron Age, with a large
dispersion of values, corresponding to wide rings in
general.
The increase in oak ring width and the greater dispersion of
the values suggests a clearing of the forest between these two
periods. This observation supports the increasing taxonomic
richness and abundance of recorded heliophilic taxa among
the charcoals from domestic fires, some of which are characteristic of heathland (Fig. 15B). This phenomenon is the result
of an increasing exploitation of the forest (Gaudin, 2004). At
the same time, there is evidence for the development of heliophilic vegetation over time. During the establishment of Neolithic peoples in north-western France, trees with low growth
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
1430
3
N charcoals
La Hersonnais A
100
A
80
60
40
20
0 - 0.25
0.25 - 0.5
0.5 - 0.75
0.75 - 1
1 - 1.25
1.25 - 1.5
1.5 - 1.75
1.75 - 2
2 - 2.25
2.25 - 2.5
2.5 - 2.75
2.75 - 3
3 - 3.25
3.25 - 3.5
3.5 - 3.75
3.75 - 4
4 - 4.25
4.25 - 4.5
4.5 - 4.75
4.75 - 5
5 - 5.25
5.25 - 5.5
5.5 - 5.75
5.75 - 6
6 - 6.25
6.25 - 6.5
6.5 - 6.75
6.75 - 7
7 - 7.25
0
Widths of oak charcoals rings (mm)
Buildings
120
4
2
Widths of oak charcoals rings (mm)
B
6
0
A
B
C1
C2
C3
D
2,5
2
1,5
1
0,5
0
0 - 0.25
0.25 - 0.5
0.5 - 0.75
0.75 - 1
1 - 1.25
1.25 - 1.5
1.5 - 1.75
1.75 - 2
2 - 2.25
2.25 - 2.5
2.5 - 2.75
2.75 - 3
3 - 3.25
3.25 - 3.5
3.5 - 3.75
3.75 - 4
4 - 4.25
4.25 - 4.5
4.5 - 4.75
4.75 - 5
5 - 5.25
5.25 - 5.5
5.5 - 5.75
5.75 - 6
6 - 6.25
6.25 - 6.5
6.5 - 6.75
6.75 - 7
7 - 7.25
A
B
C1
C2
D
3
C
16
14
12
10
8
6
4
Widths of oak charcoals rings (mm)
La Talvassais 629
0,5
Groups
mm
18
1
Enclosures
8
20
1,5
3
12
Saint-Symphorien 1555
2
0
mm
10
2,5
2,5
2
1,5
1
0,5
2
0
0 - 0.25
0.25 - 0.5
0.5 - 0.75
0.75 - 1
1 - 1.25
1.25 - 1.5
1.5 - 1.75
1.75 - 2
2 - 2.25
2.25 - 2.5
2.5 - 2.75
2.75 - 3
3 - 3.25
3.25 - 3.5
3.5 - 3.75
3.75 - 4
4 - 4.25
4.25 - 4.5
4.5 - 4.75
4.75 - 5
5 - 5.25
5.25 - 5.5
5.5 - 5.75
5.75 - 6
6 - 6.25
6.25 - 6.5
6.5 - 6.75
6.75 - 7
7 - 7.25
0
A
B
C
D
Fig. 14. Evolution of average ring widths of oak charcoals within architectural
groups of La Hersonnais.
mm
Fig. 13. Different bar charts of the ring-widths of charcoals A: unimodal
frequency distribution, La Hersonnais, building A. B: multimodal frequency
distribution, Saint-Symphorien, P 1555. C: bimodal frequency distribution,
Talvassais, St. 629.
were the first to be felled or collected as deadwood within
dense forest. With the considerable growth of human population in north-western France during the Late Iron Age
(Audouze and Buchsenschutz, 1989), wood collection led to
significant deforestation and people made use of numerous
different varieties including many heliophilic species. However, the Iron Age corresponds to a period of development
of regressive heathland in Brittany. Similarly, we can observe
more frequent use of oak firewood of small diameter from
branches or young trees.
The dendrological analysis of various samples of Medieval
and post-Medieval charcoals from the Anjou region, near Angers (Fig. 4), reveals large variations in the average ring width.
We were unable to detect any chronological trend. The charcoals
collected from the fireplaces of the Dark age sarcophagus quarry
of Doué-la-Fontaine yield an average ring width of 2.33 mm
(standard deviation ¼ 0.96, N charcoals ¼ 93). By contrast,
oak charcoals from the mortar of the abbey church of Fontevraud
(early 12th century) have an average width of 1.20 mm (standard
deviation ¼ 0.43, N charcoals ¼ 197). The various results in
space and time over this region point out a Medieval
landscape where dense forests coexisted with over-exploited
open zones.
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
mm
1431
A
5
4
3
ρ = 0.57
2
1
6000
5700
5400
5070
4770
4470
4170
3870
3570
3270
2970
2670
0
2370
2070
1770
BP
2
4
6
ρ = 0.49
8
10
12
14
16
18
B
N taxa
Fig. 15. Average widths of the growth rings of oak charcoals (A) and number of taxa (B) found in domestic fires from western France from the Neolithic to the Iron
Age (horizontal axis is time scale).
4. Conclusion
The application of dendrochronological methods supplements the analysis of charcoals. We can interpret the results
of dendrology applied to charcoal analysis through a convergence of criteria, without taking into account the small variations found in some of the parameters. This new tool in
charcoal analysis is more than just an identification of the
species or genus used, since it provides fundamental data on
gathering modes, fuel economy, types of supplier areas, timber
selection, catchments areas and, of course, the environment.
By considering these extrinsic criteria in association with the
analytical results, we can obtain palaeo-ethnographic and
palaeo-environmental information.
In north-western France, our work in this field over many
years has led to information of an ethnographic and environmental nature on a local and regional scale.
In the study of fuel used in domestic and craft combustion
structures, dendrology provides us with information about
the calibre and state of the wood chosen for combustion.
These studies of kilns show that craftsmen chose their
wood fuel according to the wood’s technical characteristics.
Similarly, detailed examination of woodwork from the Neolithic site of La Hersonnais allows us to assess its provenance, initial calibre and rate of growth. On wood with
porous zones, such as oak, growth rate confers the particular
technical properties that would be appreciated by Neolithic
builders.
From a palaeo-environmental point of view, the information
obtained at La Hersonnais, based on measurements of the
average radial growth rate of oaks used in the various construction phases, concerns the evolution of the forest environment exploited around the site and its progressive degradation.
From the systematic measurement of the average ring-widths
of oak charcoals on forty sites in north-western France, we
are able to distinguish two states of the forest environment:
tree cover remained dense during the Neolithic, but was
degraded and varied during the Late Iron Age.
Palaeo-environmental interpretations based on the measurement of growth-ring width in charred and fragmented material
are only valid only when applied to large charcoals (with weak
ring curvature) belonging to the same taxon in the same geographical area and ecological setting, while also coming from
the same archaeological context (i.e. domestic fire places) and
size of wood.
The next step in this application of dendrology to anthracology will be to increase the precision of the wood anatomy
observations. It is important to distinguish the phenomena responsible for a given width of ring. The morphometric analysis
provides valuable information on the conditions affecting
growth, such as climate, local conditions, cultural uses,
domestication, tree treatments, etc.
1432
D. Marguerie, J.-Y. Hunot / Journal of Archaeological Science 34 (2007) 1417e1433
Acknowledgements
The authors are grateful to Drs. V. Bernard, Y. Le Digol,
and N. Marcoux for their collaboration in the laboratory of
Anthracology and Dendrochronology in Rennes and to J. Le
Lannic for his assistance in the Scanning Electron Microscope
Laboratory of the University of Rennes 1 (C.M.E.B.A.).
M.S.N. Carpenter post-edited the English style.
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