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Dendrochronologia 30 (2012) 23–34

Contents lists available at SciVerse ScienceDirect

Dendrochronologia

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . d e / d e n d r o

Original article

Precise tree-ring dating of building activities despite the absence of bark:A case-study on medieval church roofs in Damme, Belgium

Kristof Haneca∗, Vincent Debonne

Flanders Heritage Agency, Koning Albert II-laan 19, 1210 Brussels, Belgium

a r t i c l e i n f o

Article history:Received 8 February 2011

Accepted 7 June 2011

Keywords:

Wooden cultural heritage

Dendrochronology

Historical buildings

Roof constructions

Timber transport

Bayesian model

a b s t r a c t

A detailed dendrochronological survey was performed on the medieval roofs of the Church of Our Lady(CoOL) in Damme, Belgium. Seen its complex architectural history, special attention was paid to the

identification of consecutive building phases, based on combined architectural historical research and

tree-ring dating. In total 64 increment cores were taken throughout the roof structures of the CoOL. All

rooftimbersare made ofEuropeanoak (Quercus robur / petraea), ofwhichonly fewhavesurviving sapwood

or bark. Tree-ring dating confirms the late 13th/early 14th century construction date of the roofs. For all

chronologies that were composed, the highest correlation values are found with reference chronologies

covering the catchment area of the river Meuse. From the dating results of the timbers of the CoOL it

becomes clear that the same timber source was used for nearly a century. On several of the examined

roof timbers, rafting joints were observed, demonstrating that the timbers were indeed tied together as

a raft and floated down the river.

By implementing sapwood estimates in a Bayesian chronological model (OxCal), tree-ring series with

surviving sapwood from coeval roof structures were combined in order to narrow down the time range

for the felling date. Based on the refined interpretation of the felling dates, several consecutive building

phases can nowbe identified and dated,leading to a newinterpretation of the architectural history of the

CoOL. Intriguingly, a marked interruption in building activities is observed around 1300. Probably this is

related to the instable politicalsituation at thattime,causedby the armed conflict that emerged betweenthe Count of Flanders and the king of France. Since Damme served as the outport of the riotous city

of Bruges, it was alternately seized by the French and Flemish, both consuming considerable amounts of

timberand other building materialsfor military fortifications. Potentially this ledto a shortage in building

materials and provoked a stop in building activities.

This paper demonstrates the power of Bayesian models to refine the interpretation of dendrochrono-

logical dates in architectural analyses of medieval historical buildings.

© 2011 Published by Elsevier GmbH on behalf of Istituto Italiano di Dendrocronologia.

Introduction

Precise and accurate dating is the trademark of dendrochronol-

ogy when studying wooden cultural heritage (WCH). In the

case where sapwood survived on historical construction timbers,

archaeological wood, painted panels or wooden sculptures, tree-

ring analysis is able to provide a narrowtimewindow forthe felling

of the trees. In exceptional cases, where the outermost ring is still

present on a piece of historical timber, the felling date of a tree can

be determined up to the season (Eckstein et al., 1984).

However, it should be clear that the cutting date of trees not

necessarily corresponds to the finalisation of a roof construction,

∗ Corresponding author. Tel.: +32 02 553 18 67; fax: +32 02 553 16 55.

E-mail address: [email protected] (K. Haneca).

framework or art object. Some years may elapse between har-

vesting wood and the actual integration into a WCH-object or

-structure. Nevertheless, the process of cutting down a tree is the

start for a series of events. For instance, for the construction of a

roof the wood is fashioned, transported, (in some cases) seasoned,

shaped and eventually becomes part of a timber structure. It is

clear that one needs to acknowledge all these consecutive steps

in order to ‘translate’ the felling date of trees into the dating of a

construction phase as accurate as possible.

In essence, the dating of a historical building corresponds to

the dating of building activities. Those historical building activities

refer to the raising of a new building (-structure), the alteration

of an earlier construction in order to fit into a new design, or the

expansion of an existing building. In many cases, written histori-

cal sources are lacking whereas stylistic and constructive features

tend to offer crude datings. Consequently, one often has to rely on

1125-7865/$ – see front matter © 2011 Published by Elsevier GmbH on behalf of Istituto Italiano di Dendrocronologia.

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24 K. Haneca, V. Debonne / Dendrochronologia 30 (2012) 23–34

Fig.1. Locationof Damme. Theformercountyof Flanders(ca.1300) issuperimposedon the current political map.

scientific dating methods applied on construction materials in

order to unravel the structural history of a building. Luminescence

dating of quartz inclusions in fired clay brick (Bailiff, 2007) and

radiocarbon dating of lime mortar (e.g. Heinemeier et al., 2010)

both have proven to be able to provide dates that are in good

agreement with independent dating evidence. However, they are

not able to produce dating results at the same chronological res-

olution as expected by tree-ring analysis. Therefore, the roofs of a

historical building are often the most appropriate constructions to

position building activities in time (e.g. Hoffsummer, 2002, 2007;

Eißing, 2009; Bernard et al., 2007; Miles, 2006). Moreover, they

are usually closely related to the finalisation of the stone- and/orbrick masonry. Therefore,a detailed dendrochronological survey of

the roof structures of a historical building is the most appropriate

scientific method to date building activities.

Additionally, dendrochronology can provide more detailed

information aboutthe provenance of the timber (Bondeet al., 1997;

Eckstein and Wröbel, 2007; Hanecaet al., 2005; Wazny,2005). This

is equally important as the dating of the timbers, as it offers infor-

mationabout thedistancethe woodhad totravelbefore itarrivedat

the construction site. The region of the former county of Flanders

(Fig. 1), for instance, is known to have a long tradition of timber

import, something to be taken into account when studying roof

timbers from late medieval buildings in this region.

The Church of Our Lady (CoOL) (Fig. 2) is a prominent exam-

ple of late medieval architecture in the coastal region of Flanders.It is known to have a complex building history (Devliegher, 1971),

with the expansionandalterationof theoriginal choirof the church

duringseveral campaigns. In order to verify andsupport the devel-

oped theories on the construction of the CoOL (Devliegher, 1971),

a dendrochronological survey was performed on the roof struc-

tures. However, from previous dendrochronological campaigns in

this region, it is known that in many cases bark is no longerpresent

on the roof timbers. Thereforespecial attentionwas paid to the cor-

rect interpretation of dated tree-ring series that include sapwood

rings (SWR). It wasassessedhow to combine sapwoodestimatesfor

individual ring-width series including sapwood within one build-

ing phase. Furthermore, it was tested whether separated or coeval

construction phases can be identified, even when the ranges of the

dendrochronological dates are partly overlapping. And finally, it

Fig. 2. The Church of Our Lady in Damme, as seen from the south (postcard from

1911).

was evaluated whether the felling dates are indeed to be inter-

preted as construction dates.

Materials and methods

The Church of Our Lady

The Church of Our Lady in Damme is a key-monument in late

medieval brick architecture in Flanders. From the late 12th cen-

tury onwards,Damme becameone of themost importantharbours

in the county of Flanders. Its convenient location along the Zwin,

at that time a natural channel connecting Damme with the open

sea, was the basis for its economical significance. Especially dur-

ing the 13th and 14th centuries, this town served as the outport

for Bruges, at that time one of the most important trading cities

north of the Alps. Due to ongoing land reclamation the Zwin

started to silt up during the 14th century, making the harbour inDamme less accessible and initiating the economical decline of this

town.

Previous research had already revealed the long and complex

building history of the church (Devliegher, 1971). The oldest parts

are the 13th century western tower and the now ruinous nave of

the church, deprived of its side aisles and roof in the 18th cen-

tury. The choir of the church has since long been considered as

the oldest of the numerous hall-churches in coastal Flanders. As

opposed to churches of the basilica type, the side aisles of a hall-

church have more or less the same height and width as the middle

aisle. In Damme, the central choir is flanked by two slightly less

elevated and almost equally wide choirs to the north and south.

Clear differences in the finishing of architectural elements and the

choice of buildingmaterials canbe observed between thetwo mostwestward and the three eastern bays. Also, in each of the roof con-

structions of the three choirs, two series of carpenter marks can be

observed on the rafters and collar beams, with the scission located

between the 2nd and 3rd bay (Fig. 3). In a traditional interpreta-

tion, this is explained as the result of two distinct and consecutive

designs. The first phase, in the second half of the 13th century,

consisted of the addition of the northern and southern choir to the

older central choir. The second phase at the beginning of the 14th

century, comprised the extension of the entire choir towards the

east, thus giving the choir its definite size.

A detailed architecturalstudy of the different buildingmaterials

and typological observation indicated a close connection between

the roof constructions and the underlying masonry (Debonne

and Haneca, 2011). Therefore the roofs of the CoOL became the



K. Haneca, V. Debonne / Dendrochronologia 30 (2012) 23–34 25

Fig. 3. Simplified plan of the roofs of the Church of Our Lady in Damme, with the exact location of sampling points for the dendrochronological analyses, carpenter marksand rafting joints. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)

target for a combined dendrochronological and architectural his-

torical study in order to unravel and date the historical building

activities.

Sampling strategy

The sampling strategy was guided by the simultaneous study of

architectural features, such as the typology of the roof structures

and visible changes in the layout of the masonry. Furthermore,

special attention was paid to the recording of all carpenter marks

visible on the rooftimbers.The layout of thesecarpenter marks pro-

vides valuable information about the planning and construction of

the roofs. Ascending series of numbered rafters are an indication

that a roof was installed in one campaign, whereas a scission in the

carpenter marks could point towards an interruption in the build-

ing activities or a later replacement of timbers. In the case where

carpenter marks have a randomorder, the reuse of older (roof)tim-

bersshould be considered when interpreting dating results derived

thereof.

The entire roof structure was built with oak timbers (Quercus

robur L. or Q. petraea (Matt.) Liebl.). Timbers with bark or partially

preserved sapwood were targeted for the dendrochronological

analysis.For eachassumedconstruction phase or typological group,

we aimed at taking at least eight increment cores. All cores were




Fig. 4. Histogram representing the scanned and digitized sapwood data published

by Hollstein (1980). A lognormal function was fitted to the dataset.

extracted with a dry wood borer of 8 mm in diameter mounted on

a cordless power drill.

Measuring and dating protocol

Increment cores where glued on woodenholders and sanded in

a stepwise procedure using sandpapers with increasingly finer grit

size (P120, P240, P320, P400 and P800) (Sass and Eckstein, 1994).Afterpolishing withthe finest sandpaper all samples werebuffed to

remove dust particles from the earlywood vessels. This allows to

verify the presence of tyloses in the vessels and to identify sap-

wood. The number of SWR was meticulously recorded for each

core. All ring widths were measured to the nearest 0.01 mm using

a positioning table (LintabTM), stereomicroscope (10×–120×) and

measuring software (TSAPWinTM).

The raw ring-width series were grouped and synchronised

according to the findings of the simultaneous architectural histor-

ical research. For each assumed construction phase or typological

group a chronology was calculated. The chronologies were then

crossdated against absolutely dated, historical reference chronolo-

gies from Belgium (Haneca, unpublished data; Hoffsummer, 1995),

England (Bridge, 1988), Germany (Hollstein, 1980), and France(Bernard, 1998; Tegel, pers. comm.).

Sapwood estimates for individual series

From previous dating campaigns on historical buildings, it is

clear that construction timbers with bark edge are a rather rare

feature in medieval buildings in Bruges and Damme (e.g. Eeckhout

and Houbrechts, 2002; Van Eenhooge, 2009). The same could be

expected for the CoOL as well. In order to obtain precise den-

drochronological dates, estimates of the missing number of SWR

are needed. Sapwood estimates have proven to vary between geo-

graphical provenances and to be dependent of mean ring width,

tree age, mean sapwood width or a combination of these parame-

ters (Hillam et al., 1987; Hughes et al., 1981; Miles, 1997; Hanecaet al., 2009). For this particular case, the original sapwood data of

Hollstein (1980)was consulted. Thisdatasetreflectsthe variationin

sapwood numbers for 493 oak timbers fromstanding trees, archae-

ological contexts and historical buildings in southern Germany. It

has been widely used to produce sapwood estimates in southern

Germany, northern France, The Netherlands and Belgium, and it

is in good agreement with other sapwood estimates from these

regions (seee.g. the sapwoodestimates for Flandersin Hanecaetal.,

2009, Table 1).

In order to gain accessto the actualdata, the original graph from

the Hollstein (1980) publication was scanned anddigitized (Fig. 4).

However, only 490 points could be detected on the original graph

where it is claimed that 493 measurements were included. This

probably means that three pairs of records have exactly the same

value, making it impossible to discern these from each another on

the original graph. It was considered to have no significant influ-

ence on further analysis.

This dataset clearly demonstrates the dependency of the total

number of SWR on the age of the tree (see Hollstein, 1980, Fig. 21,

p. 34). When looking at the raw data, disregarding the number of

heartwood rings, it can be calculated that in 95% of all cases the

number of SWR ranges between 8 up to 38, with an average of 18. In order to use this dataset for further statistical modelling, a

lognormal distribution was fitted to the data.

The way sapwood estimates are produced and ranges for

felling dates are communicated varies between laboratories and

dendrochronologists. When only one tree-ring series has to be

interpreted, it is most common to produce a range for the felling

date by using the 95% confidence interval of the sapwood proba-

bility density function of a particular species, covering a specific

region (Haneca et al., 2009). Lower and upper limits of the desired

confidence interval provide boundaries for the felling date range.

When S sapwood rings survive on a piece of historical timber, and

S exceeds the lower limit of theconfidence interval of thesapwood

range, the felling date is estimated to lie between the date of the

last surviving sapwood ring and the upper limit of the 95% CI of

the sapwood range added to this date. However, in such cases the

narrowed range forthe estimatednumber of missing SWRdoes not

longerrepresentthe exact95% CI.Thiscan be overcome byincorpo-

rating the (prior) information on the number of actually measured

sapwood rings S into the model that describes the probability dis-

tribution of sapwood numbers.

Basically, this procedure is based on Bayes’ theorem (1763). In

fact, in a simplifiedform this theorem is perfectlyapplicable on the

estimation of the missing number of SWR, especially when only

a fraction of the sapwood survives on a piece of historical timber.

The theoretical background for thisapproach wasset out by Millard

(2002) andput into practicefor oakbuilding timbers inEnglandand

Wales by Miles (2006). When applied to calculate probabilities for

sapwood estimates on building timbers, the Bayes’ theorem canbe

re-formulated as follows:

p(n|S) ∝ p(S|n).p(n|a, br , bm) (1)

where p(n|S ) is the ‘posterior’ probability or the probability of hav-

ing a total number n sapwood rings when S sapwood rings were

measured on a timber; p(S |n) is the ‘likelihood’ or the probability

of the observed number of sapwood rings S , given a model describ-

ing the distribution of sapwood numbers; p(n|a, br , bm, ...) is the

‘prior’ or a mathematicalmodel chosen to describe thedistribution

of sapwood numbers n, which is defined by a set of parameters (a,

br , bm, . . .).

When a timber has S surviving sapwood rings, the likelihood

becomes:

p(S|n) ∝

1for S ≤ n

0for S > n (2)

This means that in the case of observing S sapwood rings on a

pieceof timber, the probability distribution function P forthe num-

ber of sapwood ring is represented by a distribution, truncated at

n = S . This distribution can be numerically normalized to provide

actual probabilities, and to calculate ranges for the expected num-

ber of sapwood rings, taking into account the observed number of

surviving sapwood rings S .

This procedure was implemented in OxCal, a software appli-

cation originally designed for the calibration and analysis of

radiocarbon dates (Bronk Ramsey, 1995). Furthermore, a sapwood



Table 1

Significant dating results for all chronologies from the CoOL. The agreement between the chronologies is expressed by t BP-values (Baillie and Pilcher, 1973) and the percenta

1969).

Object chronology No. of

series

Length Date

(AD)

FL.medieval

(Haneca, unpubl.

data)

BE.Meuse5

(Hoffsummer,

1995)

BE.Arden4

(Hoffsummer,

1995)

DE.Holl80

(Hollstein, 1980)

FR.B

(Ber

t BP %PV t BP %PV t BP %PV t BP %PV t BP

DAM.m1

(northern choir, bay 1–2)

5 156 1157–1312 6.8 71*** 7.4 72*** 8.0 68*** 5.6 70*** 5.2

DAM.m2

(northern choir, bay 3–5)

6 267 1042–1308 5.8 66*** 10.7 72*** 8.0 68*** 9.3 69*** 6.0

DAM.m3

(southern choir, bay 1–2)

5 169 1115–1283 5.3 69*** 6.2 70*** 6.9 72*** 3.3 65*** 4.2

DAM.m4

(southern choir, bay 3–5)

7 141 1153–1293 9.4 76*** 9.7 79*** 9.2 74*** 6.4 74*** 8.1

DAM.m5(central choir, bay 1–2, rectangular

rafters)

5 172 1140–1311 5.1 71*** 5.2 65*** 4.3 63*** 2.3 61** 4.4

DAM.m6

(central choir, bay 1–2, square

rafters)

7 187 1055–1241 8.1 70*** 11.0 74*** 8.3 78*** 8.1 69*** 6.1

DAM.m7

(central choir, bay 1–2, first collar

beams)

2 89 1213–1301 4.9 70*** 6.9 70*** 5.6 66** 4.2 69*** 4.2

DAM.m8

(central choir, bay 1–2, second

collar beams)

3 76 – – – – – – – – – –

DAM.m9

(central choir, bay 3–5)

2 156 1144–1299 7.9 68*** 8.5 74*** 5.6 66*** 6.4 73*** 6.4

Highest t BP-values for each chronology are in bold.*The level of significance p of the %PV is p≤0.05.

** The level of significance p of the %PV is p≤0.01.*** The level of significance p of the %PV is p≤0.001.




tool1 is implemented in OxCal that allows fitting a lognormal

distribution to any substantial sapwood dataset, and calculating

confidence intervals for sapwood estimates thereof.

Combining sapwood probabilities

When multiple tree-ring series with partly preserved sapwood

are available within one particular construction phase, one istempted to produce sapwood estimates and derive thereof the

interval for the felling date, based on the single tree-ring series

with the largest number of surviving SWR. However, this neglects

the (prior) information in the other tree-ring series with surviving

sapwood, each with their own probability of the felling date range.

To obtain the probability of a certain felling date F for a group of

timbers that have been felled in the same year, the information of

all dated timbers with at least some surviving sapwood rings can

be combined into one model. In a simplified way (Millard, 2002)

this model can be described as follows:

p(F |H, S)˛

i

p(H i, Si|F ) (3)

where H i is the date of thelast heartwoodringon a timber i, S i is thenumber of sapwood rings observed on timber i, and F is the felling

date for a group of i timbers.

In this case the likelihood is given by:

p(H i, Si|F )˛

p(F − H i|a, br , bm) for Si ≤ F − H i

0 for Si > F − H i(4)

This Eq. (4) attributes a probability to each potential felling date

F , for all measured and dated tree-ring series with surviving sap-

woodrings. The combination of all the resulting probability density

functions (3) usually narrows down the range for the actual felling

date.

In its current version, OxCal (v.4.1.3) allows to produce, both

graphically and statistically, felling date ranges of combined tree-

ring series originating from a coeval (construction) phase, based ontheactualnumberof measuredSWR oneachpiece oftimber and the

model fitted to thesapwood dataset. In this specific case-study,the

latter means the distribution fitted to the Hollstein (1980) dataset.

For each dated tree-ring series with surviving SWR included

in the model, an agreement index Ai is calculated. Ideally, these

index values approach 100% and do not fall below 60%. The latter

would indicate a significant inconsistency between the data and

the model. However, it can be expected that about 1 out of 20 Ai

values could drop below this threshold of 60% by chance (Bronk

Ramsey, 1995, 2009). Also for the model as a whole, an agreement

index Acomb is calculated. Equally, Acomb shouldnot drop below 60%.

This threshold is chosen so to be in the range of the 5% confidence

interval of a 2-test for simple combinations (Bronk Ramsey, 1995,

2009).

Results

Construction typology

Based on the typology of the roof framings (Debonne and

Haneca, 2011), only small differences could be observed between

the roofs coveringthe central choir and both side choirs. All belong

to thetypeof commonrafter roofs.Remarkable arethe two typesof

rafters encountered in the roof of the central choir. Above the two

most westward bays, a striking alternation between rafters with a

1

http://c14.arch.ox.ac.uk/oxcalhelp/Sapwood.html.

squared (ca. 16 cm×16 cm) and a rectangular (ca. 10 cm×18cm)

cross-sectionwas observed (Fig.5). Furthermore, openjoints on the

rafters with a squared section echo the former presence of collar

beams and ashlar pieces.

Although no apparent typological differences were observed

between the roofs, the layout of the carpenter marks displays

coherent series in all three choirs (Fig. 3). Each roof carries two

series of carpenters marks, engraved on every pair of rafters andcollar beams. The scission between the two series is located after

the two most westward bays. This scission in carpenter marks is

at the exact location where differences in the use of construction

materials are observed. Furthermore, a number of older carpen-

ter marks, in random order, were observed on the rafters with a

squared section in the roof of the central choir.

Dating results

In total 64 increment cores were taken throughout the roofs of

theCoOL.Ononlythreeraftersbarkwasstillattached.Furthermore,

30 increment cores included some surviving sapwood rings. Of all

measured tree-ring series, 42 (=65.6%) could be crossdated and

grouped into nine chronologies, representing each potential con-

struction phase or adjustment to an original roof.Five chronologieswere built for the central choir, and two for each of the side choirs,

based on the layout of the carpenter marks. Eight of them could

be dated using the consulted oak reference chronologies (Table 1)

and ‘traditional’ sapwood estimates for Central Europe (Hollstein,

1980) (Fig. 6). The shortest chronology (n = 76), representing the

upper collar beams in the central choir, could not be dated.

Thefellingdate forthe roof on topof thetwo most westerly bays

of the central choir, corresponding to the first series of carpenter

marks, was set between August 1241 and spring 1242. The final

ring under the cambium consists of earlywood and a considerable

amount of latewood. However, this dating result only applies to

the rafters with a squared cross-section. Most probably these tim-

bers originally belonged to the roof construction of the very first

choir of the CoOL, of whicharchaeological excavations have shownto be an aisleless construction with a three-sided apse. The roof

above the two most westerly bays above the southern choir was

dated between 1283 and 1298 AD. This means that at least 42 years

passed between the construction of a roof on the central choir and

the first phase of the roof of the southern choir. All other chronolo-

gies, representing the roofs of the northern choir, the three bays

of the southern choir and central choir, have been dated between

1299 and 1337 AD. Also included within this range is the felling

date for the rafters with a rectangular section in the central choir.

As the ranges of the sapwood estimates for the different roofs are

overlapping, it is impossible to arrange the felling events in chrono-

logical order. Nevertheless, it is clear that all these roofs were built

at least some years after the construction of the roof of the first

phase of the southern choir.From these results it is already clear that an alternative chrono-

logical order for the consecutive building activities has to be

developed.

Sapwood modelling

The original Hollstein (1980) dataset of sapwood numbers was

modelled in OxCal (v4.1.3, Bronk Ramsey, 2009) by fitting a log-

normal distribution to the digitized data (Fig. 4). The probability

distribution P of having S sapwood rings is defined by two param-

eters, a constant a and the residual standard deviation and is

proportional to:

P ∝1

S

· e(−(a−ln(S))2)/2 2 (5)

http://c14.arch.ox.ac.uk/oxcalhelp/Sapwood.html

http://c14.arch.ox.ac.uk/oxcalhelp/Sapwood.html




Fig. 5. 3D-representation of the roof above the two most westward bays of the central choir, with indication of reused, older rafters (brown). Scale bar represents 5m. (For

interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)

where a = 2.813579, and the residual standard deviation

= 0.416208.In order to obtaina confidence interval forthe probable number

of sapwood rings, the region with the highest probability density

is calculated. This is the shortest range that includes the desired

percentage of the probability in the probability density function

P (Bronk Ramsey, 2009). It should be clear that this method devi-ates from the approach to obtain the 95.4% confidence intervals

for a normal distribution where two times the standard deviation

fromthe meanprovides the boundaries for thisconfidence interval.

Fig. 6. Bar graph of the dating results for each chronology. The bars represent the length of chronologies, with their height being proportional to the number of tree-ring

series included in the chronology. The black parts represent the number of measured sapwood rings. The grey parts of the bars are an indication for the minimum number

of sapwood rings that are to be expected, when less than 8 sapwood rings could be observed. The horizontal lines delineate the interval for the felling date.




Fig.7. OxCal codefor presenting thedendrochronological dating resultsof theroofs

from the CoOL and the combination of sapwood estimates.

The 95.4% interval in the Bayesian approach is chosen merely for

comparability. For the model derived from the Hollstein data, the

boundaries for the 95.4% confidence are 5.7 and 34.4 (8.9 and 22

for the 68.2% CI).

When taking into account the age (R) of the trees observed by

Hollstein (1980) by including the number of heartwood rings into

the model, the probability density functiondescribing the sapwooddataset becomes:

P ∝1

S · e(−(a+br . ln(R)−ln(S))2)/2 2 (6)

with a = 1.063717, br = 0.399085 and = 0.297783.

Furthermore, the dependency of the number of sapwood rings

on the mean ring width of the heartwood H , or any other relevant

variable, could be taken into account when fitting a distribution

to the data. However, this variable cannot be deduced from the

original graph published by Hollstein (1980), therefore preventing

the calculation of the additional parameter (bm).

Combining felling date ranges

From the dating results (Fig. 6) it is possible to identify the tim-

bers that belong to the oldest roof constructionon thecentral choir

of the church. Next in line are the timbers from the roof of the first

phase of the southern choir. However, onecannot exclude an over-

lap in the felling date of this phase with the felling date range of

the northern choir and the further expansion to the east. Accord-

ing to the dating results of the roof of the northern choir and most

recentphase of the central choir and southernchoir(Fig. 6) there is

a potential overlap. Although not likely, this possibility cannot be

excluded.

In order to resolve this, a Bayesian model was built to combine

all series with surviving sapwood from one assumed phase (Fig. 7).

The sapwood model defined in Eq. (5) was used as the prior. It was

decided not to take the age of the trees (the number of heartwood

rings H ) into theprior (Eq. (6)), since most of theincrementcoresdo

not include or approach the pith. Therefore, estimating the miss-

ing number of heartwood rings would bring an additional error

term into the model. The outcome of this model provides a narrow

and statistically sound interpretation of the range for the felling

date for the timbers originating from the first phase of the south-

ern choir (Fig. 8). In Fig. 8, the posterior probability distribution for

the felling date, based on the combination of all samples with sur-viving sapwood for each assumed building phase is highlighted in

dark. When assumed that all timbers from the first two bays of the

southern choir have the same felling date, it has a 95.4% chance

to fall within the range 1283–1291 AD. Both the overall agree-

mentindex( Acomb = 117%) as the agreement index for theindividual

items (103.7%≤ Ai ≥128.6%) point towards a robust model, indi-

cating that all timbers indeed can have the same felling date and

therefore originate from one and the same building phase.

After theconstruction of theroof on the first phase of the south-

ern choir (1283–1291 AD), the dendrochronological evidence can

not provide more detailed information about the chronological

order of thebuildingactivitieson the remainingroofs (Fig.6). How-

ever, using the same Bayesian approach (Fig. 7) it can be tested

whether all timbers with surviving SWR belong to one, coeval

building phase. The statistical analysis demonstrates that the roofs

on the northern choir, the three most eastern bays of the central

choirand the southern choir, and the rafters with a rectangular sec-

tion in the roof of the two most westward bays of the central choir

can indeed be considered coeval (Fig. 8). The overall agreement

index Acomb for this model is 117%. In addition, a 2-test indicates

that all samples could belong to the same distribution (T = 8.514,

df =12, p < 0.05). When all dating results and sapwood estimates

are combined, the range for the felling date of the timbers can be

narrowed down to the interval 1312–1315 AD (95.4% CI). Including

the total number of observed heartwoodrings H into themodel and

using Eq. (6) as a prior in order to calculate a range for the felling

date, hardly changed the outcome of the model. The range for the

felling dated was only shifted by 2 years (1312–1317 AD).

Furthermore,including allseries that belongto thefirstphaseof the southern choir into the model, causes the Acomb value to drop

below 60% ( Acomb = 4.7%). Additionally the 2-test no longer sup-

ports the assumption that allseriesbelong to thesame distribution

(T = 38.5, df =16, p > 0.05). This makes it clear that a hiatus exists

between the felling dates of these two groups of roof timbers.

Discussion

The dendrochronological results confirm the late medieval con-

structiondateof the roofs.Besides, more importantly, the results of

thedendrochronological analysis forcedus to have a more detailed

look at the masonry and constructive layout of the church in

order to gain a better understanding of the successive construc-tion phases. Indeed, the traditional interpretation of the building

and expansion of the hall church, mainly based on the layout of

the carpenter marks, was not supported by the dendrochronolog-

ical dates. The combined efforts of the dendrochronological and

architectural historical research now allow us to split up the con-

struction of the hall church of the CoOL into three different phases

(Debonne and Haneca, 2011). It is clear now that the rafters with

a squared section in the roof of the central choir above the two

most westward bays represent the oldest phase of the hall-choir.

Probably they were part of the mid-13th century roof that cov-

ered the first choir of the church. Some older carpenter marks on

these rafters are no longer in mounting order and therefore sug-

gest that the rafters were reused duringthe construction of a more

recent roof that was erected on the elevated central choir in a later




Fig. 8. Graphicaloutput from OxCal (v.4.1.3), showing theprobability of each potentialfelling date forall serieswith survivingsapwood (grey), andthe posteriorprobability

of the combination of 4 series from the first phase of the southern choir and 13 series from the second phase of the southern choir, central choir and the entire northernchoir.

building campaign. During a second building campaign, at least 42

years later, a roof was constructed above the two western bays of

the southern choir (between 1283 and 1291 AD). After this point,

the dendrochronological evidence cannot provide more detailed

information aboutthe chronological order of the building activities

on the remaining roofs. The estimated range for the felling dates of

all chronologies are overlapping, and no logical chronological order

was suggestedby the rangesof thefellingdates. Theoutcomeof the

Bayesian analysis (Fig. 8) demonstrates that these roofs are indeed

coeval and that the felling date is situated between 1312 and 1315

AD.

The refined interpretation of the dating results of the roof

timbers from hall-choir of the CoOL now points towards an inter-

ruption in building activities between 1291 and 1312 AD. A

potential explanation for this is the armed conflict between the

Count of Flanders, Guy of Dampierre (◦1226–†1305 AD), and the

king of France, Philip the Fair (◦1268–†1314 AD), between 1297

and 1305 AD. As one of the main gateways to Flanders from over-

seas as well as the outport of the riotous city of Bruges, Damme

was alternately seized by the French and the Flemish, both forti-

fying the town with ditches, walls and gates. These military works

must have consumed a considerable amount of building materials,




Fig. 9. Traces on the timbers that are related to the trade in construction timber. (a) engraved trade mark and (b and c) rafting joints.

thus causing the construction of other buildings in Damme, such

as the hall-choir of the CoOL, to be interrupted.

From a methodological point of view, the research on the hall-

church of the CoOL shows that carpenters marks may indicate

differentdirections in the constructionof a roof,but not necessarily

chronologically different phases of construction. The same can bestated for differences in building materialsand architecturaldetail-

ing.In Damme, theseare notthe resultof twoseparatearchitectural

designs, but rather an illustration of the dynamics of a building

site subjected to sometimes unpredictable external events. Fur-

thermore, it demonstrates that not only precise dating canprovide

evidence for the exact order of building activities, but also allows

to link the building activities with specific political events.

Nonetheless, the potentially most accurate dendrochronolog-

ical dating failed due to the scarcity of timbers with bark edge.

In order to explain this phenomenon, the whole wood-processing

procedure should be considered. To start with, the dendrochrono-

logical analysis provides more information about the provenance

of the timbers. For all chronologies, the highest correlation val-

ues (Table 1) are found with reference chronologies covering thecatchment area of the river Meuse (Hoffsummer, 1995). This river

originates in north-eastern France and runs from south to north

through the eastern part of present-day Belgium. From the dating

results on the timbers of the CoOL it is apparent that the same tim-

ber source was used for nearly a century. Since transport over land

was economically not feasible (Rackham, 1982; Houbrechts, 2008),

the timbers were transported down the Meuse towards the coast,

passing several timber markets (de Vries, 1994; Houbrechts, 2008)

along the river. Several trading marks (Fig. 9a), probably left on the

timbers by merchants or buyers as a claim on their property, were

observed on the examined roof timbers.

From historical and iconographicsources it is known that wood

was tied together as a raft (Houbrechts, 2008). Some of the roof

timbers of the CoOL actually carry the physical remnants of this

practice. Various rafters and collar beams show a particular type

of perforation (Fig. 9b and c). These so-called rafting joints were

used to tie timbers together with ropes or twigs (Eißing, 2009)

while assembling a raft. However, freshly cut oak wood, with satu-

rated cell walls andcell cavities filled with ‘free’ water, is tooheavy

(according to Wagenführ (2007): 650–1160kg/m3

) to float overlong distance (Eißing, 2009, p. 23). Therefore, oak timbers must

have been accompanied by wood with lower density, probably

coniferous wood, in order to improve the floatability of the raft.

Related to this issue, van Prooie (1990) also mentions the use of

small barrels made of pine and rafters made of common beech to

float oak timber in the 18th century.

Nevertheless, from their appearance on the roof timbers it is

clear that the observed rafting joints are complete and intact. This

meansthatthe oakstems were already fashionedintobeams with a

squared or rectangular cross-section before theywere floated. Oth-

erwise these rafting joints would have vanished or only partly pre-

servedwhen they were shapedinto rafters during the construction

of a roof. Thereforeit demonstratesthatthe timbers were floatedas

cleaved or sawn half-products and not as a whole stem. Besides, inmedieval toll records from towns along the Meuse,the wood on the

rafts was often referred to as ‘kepershout ’ or ‘cantshouts’ (de Vries,

1994, p. 34), what might be interpreted as squared timbers. Proba-

bly, this process has a direct link with the rare appearance of bark

andsapwood on the rafters andcollar beams in the CoOL and other

medievalbuildings in andnear Bruges. While processing freshly cut

tree stems into beams a considerable amount of sapwood and bark

could be lost. So the absence of bark on roof timbers in medieval

buildings in and near Bruges and Damme is, at least partly, related

to the long-distance trade and import of timber.

From studies on toll records, only few documents allow to

reconstruct the time it took to float a timber raft down the Meuse.

In 1394, the transport of a raft between Mook and Ravenstein, two

towns located along the Meuse separated by approximately 18km,




took 1½ day on average (de Vries, 1994, p. 34). Comparable trans-

port times for timber rafts were recorded on the river Rhine (ca.

80 km in 6½ day, between Lobith and Zaltbommel). Extrapolating

these records suggests that transport of timber rafts from inland

areas towards the coast was only a matter of weeks. When assum-

ing that the timber was traded in Dordrecht, it still had to be travel

towards Damme. To date, travelling from Dordrecht to Damme

along natural waterways and channels would require transportalong coastal waters (Fig.1). However, during the LateMiddle Ages,

both cities were only separated by the large estuary of the rivers

Scheldt,Meuse andRhine, withnumerous scatteredislands. Timber

transport could easily have been done by small ships navigat-

ing through the estuary, avoiding the open sea. Consequently the

time needed to transport timber along the Meuse, via Dordrecht,

towards Damme is probably not an important factor to be taken

into account when interpreting dendrochronological felling dates.

One factor that was not taken into account so far is the time

neededfor seasoningfreshly cuttimbers. In England andWales it is

known from the combined efforts of dendrochronological research

and the examination of written records that most timbers were

used in a roof construction between 1 and 3 years after felling

(Miles, 2006). In the town of Lübeck (Germany) it was observed

that some roof structures of historical buildings contained timbers

from different felling campaigns, separated by up to 33 months,

whileothersonlyhad timbers with onesingle felling date (Eckstein,

2007). The occurrence of mixed felling dates within one roof con-

struction suggests that construction was dependent of the amount

of wood in stock at local timber markets. However, from the den-

drochronological dataset from Flanders’ WCH or written sources

no evidence emerges to support this for construction timbers. Also

during this research project on the CoOL, only few timbers with

attached bark were observed, what makes it hard to interpret the

‘seasoning factor’. It is assumed that ‘green’ wood was used as

building timbers. Therefore, felling dates are currently interpreted

as the starting date for the construction of a roof. Written his-

torical documents could potentially provide more details on this

topic. Nevertheless, including a ‘stockpile factor’ into the Bayesianmodel is possible (Miles, 2006) and would slightly shift the felling

date ranges. In this particular case however, it would not influence

the interpretation of the chronological construction history of the

CoOL.

From the case-study in the town of Damme it is clear that the

absence of bark on construction timbers impedes to take den-

drochronological dating to its limits and provide felling dates up

to the season. Therefore it is clear that a dendrochronological anal-

ysis on roof timbers benefits from taking into account all recorded

sapwood information. The results demonstrate the value of the

Bayesian modelling in OxCal for testing whether timbers belong

to separate building campaigns. Furthermore, this approach allows

to narrow down the range of potential felling dates when multiple

series with surviving sapwood are available from one coeval con-struction phase. However, it should be noted that in cases where

series with a considerable number of sapwood rings are included,

this approach tends to extend the outer range of the confidence

interval towards a younger age (Miles, 2006). The sapwood esti-

mates used in this study were deduced from the original Hollstein

(1980) data, but any substantial dataset on sapwood numbers can

provide parameters for a sapwood model dedicated to a particular

region (Haneca et al., 2009, Table 1).

Data accessibility

All tree-ring series that were recorded, analysed and presented

in this paper were submitted to the DCCD repository in TRiDaS

format( Jansma et al., 2010). The data and meta-data are accessible

following http://dendro.dans.knaw.nl.

Acknowledgements

We are indebted to Jan Van den Bulcke (Laboratory of Wood

Technology, Ghent University) for writing a Matlab® script to pro-

duce bar graphs representing dendrochronological dating results

(Fig. 6). Furthermore we would like to thank our colleagues Kris

Vandevorst for taking excellent photographs, and Nele van Gemert

for enhancing the quality of the illustrations.

References

Bailiff, I.K.,2007. Methodological developments in the luminescence dating of brickfrom English late-medieval and post-medieval buildings. Archaeometry 49,827–851.

Baillie, M.G.L., Pilcher, J.R., 1973. A simple crossdating program for tree-ringresearch. Tree-Ring Bulletin 33, 7–14.

Bayes, T., 1763. Essay towards solving a problem in the doctrine of chances. Philo-sophical Transactions of the Royal Society of London 53, 370–418.

Bernard, V., 1998. L’homme, le bois et la forêt dans la France du Nord entre le

Mésolithique et le Haut Moyen-Age. BAR International Series 733, 1–190.Bernard, V., Epaud, F., Le Digol, Y., 2007. Le bois: de la fôret au chantier. In: Epaud,F. (Ed.), De la charpente romane à la charpente gothique en Normandie. Evo-lution des techniques et des structures de charpenterie aux XIIe-XIIIe siècles.Publications du CRAHM, Caen, pp. 9–46.

Bonde, N., Tyers, I., Wazny, T., 1997. Where does the timber come from? Den-drochronological evidence of the timber trade in Northern Europe. In: Sinclair,A., Slater, E.,Gowlett,J. (Eds.), ArchaeologicalSciences1995. OxbowMonograph64, Oxbow books Ltd., Oxford, pp. 201–204.

Bridge,M.C., 1988.The dendrochronologicaldatingof buildingsin southernEngland.Medieval Archaeology 32, 166–174.

Bronk Ramsey, C., 1995. Radiocarbon calibration and analysis of stratigraphy: theOxCal program. Radiocarbon 37, 425–430.

Bronk Ramsey, C., 2009. Bayesian analysis of radiocarbon dates. Radiocarbon 51,337–360.

Debonne, V., Haneca, K., 2011. Baksteen en boomringen: een verfijndebouwchronologie van het hallenkoor van de Onze-Lieve-Vrouwkerk in Damme(prov. West-Vlaanderen). Relicta. Archeologie,Monumenten- en Landschapson-derzoek in Vlaanderen 7, 67–100.

deVries, D.J., 1994. Bouwen in de late Middeleeuwen: Stedelijke architectuurin hetvoormalige Over-en Nedersticht. Stichting Matrijs, Utrecht, 512 pp.

Devliegher, L., 1971. Damme. Kunstpatrimonium van West-Vlaanderen, no. 5,Bestendige deputatie van de Provincieraad van West-Vlaanderen, Brugge, 196pp.

Eckstein,D., Baillie, M.G.L., Egger, H., 1984.Dendrochronological dating, Handbooksfor Archaeologists, no. 2. European Science Foundation, Strasbourg, France, 55pp.

Eckstein, D., 2007. Human time in tree rings. Dendrochronologia 24, 53–60.Eckstein, D., Bauch, J., 1969. Beitrag zur Rationalisierung eines dendrochro-

nologischen Verfahrens und zur Analyse seiner Aussagesicherheit.Forstwissenschaftliches Centralblatt 88, 230–250.

Eckstein, D., Wröbel, S., 2007. Dendrochronological proof of origin of historic tim-ber - retrospect and perspectives, in: Haneca, K., Verheyden, A., Beeckman, H.,Gärtner, H., Helle, G., Schleser, G.H. (Eds.), TRACE–Tree Rings in Archaeology,Climatology and Ecology, vol. 5. Proceedings of the Dendrosymposium 2006,April 20th–22nd, Tervuren, Belgium. Schriften des Forschungszentrums Jülich,Reihe Umwelt 74, Forschungszentrum Jülich GmbH, Jülich, pp. 8–20.

Eeckhout, J., Houbrechts, D., 2002. Charpentes et caves médievales à Bruges. Cam-pagne d’analyses 2001–2002. Laboratoire de dendrochronologie, Université deLiège, Liège, 106 pp.

Eißing, T., 2009. Kirchendächer in Thüringen und dem südlichen Sachsen-Anhalt.Dendrochronologie– Flösserei – Konstruktion.ThüringischenLandesambtes fürDenkmalpflege und Archäologie, Erfurt, 230 pp.

Haneca, K.,Wazny, T.,Van Acker,J., Beeckman,H., 2005. ProvenancingBaltic timberfrom art historical objects: success and limitations. Journal of ArchaeologicalScience 32, 261–271.

Haneca, K., ˇ Cufar, K., Beeckman, H., 2009. Oaks, tree-rings and wooden culturalheritage: a review of the main characteristics and applications of oak den-drochronology in Europe. Journal of Archaeological Science 36, 1–11.

Heinemeier, J., Ringbom, A., Lindroos, A., Sveinbjornsdottir, A.E., 2010. SuccessfulAMS C-14 dating of non-hydraulic lime mortars from the medieval churches of the Aland Islands, Finland. Radiocarbon 52, 171–204.

Hillam, J., Morgan, R.A., Tyers, I., 1987. Sapwood estimates and the dating of shortring sequences. BAR International Series 333, 165–185.

Hoffsummer, P., 1995. Les charpentes de toiture en Wallonie, typologie et den-drochronologie(XIe-XIXe siècle). Etudes et documents, monumentset sites, no.

1, Ministère de le Région Wallonne. Division du patrimoine, Namur, 173 pp.

http://dendro.dans.knaw.nl/

http://dendro.dans.knaw.nl/




Hoffsummer, P., 2002. Les charpentes du XIe au XIX siècle. Typologie et évolutionen France du Nord et en Belgique. Cahiers du Patrimoine, no 62, Editions dupatrimoine. Ministère de la Culture et de la Communication, Paris, 376 pp.

Hoffsummer, P., 2007. The evolution of roofing in Northern France and Belgiumfrom the11thto the18thcenturyas revealed bydendrochronology,in: Haneca,K., Verheyden, A., Beeckman, H., Gärtner, H., Helle, G., Schleser, G.H. (Eds.),TRACE—TreeRings in Archaeology, Climatology and Ecology, vol.5. Proceedingsof the Dendrosymposium 2006, April 20th–22nd, Tervuren, Belgium. Schriftendes Forschungszentrum Jülich, Reihe Umwelt 74, Forschungszentrum JülichGmbH, Jülich, pp. 21–27.

Hollstein, E., 1980. Mitteleuropäische Eichenchronologie. Verlag Phillipp vonZabern, Mainz am Rhein, 273 pp.

Houbrechts, D., 2008. Le logis en pan-de-bois dans les villes du bassin de la Meusemoyenne. Dossier de la commission royale des monuments sites et fouilles, no.44, Commission Royale des Monuments sites et fouilles, Liège, 314 pp.

Hughes, M.K.,Milsom, S.J.,Leggett, P.A.,1981. Sapwood estimatesin the interpreta-tion of tree-ring dates. Journal of Archaeological Science 8, 381–390.

Jansma, E., Brewer, P.W., Zandhuis, I., 2010. TRiDaS 1.1: the tree-ring data standard.Dendrochronologia 28, 99–130.

Miles, D., 1997. The interpretation, presentation and use of tree-ring dates. Vernac-ular architecture 28, 40–56.

Miles, D., 2006.Refinements in the interpretationof tree-ringdates for oak buildingtimbers in England and Wales. Vernacular Architecture 37, 84–96.

Millard, A., 2002. A Bayesian approach to sapwood estimates and felling dates indendrochronology. Archaeometry 44, 137–143.

Rackham, O., 1982. The growing and transport of timber and underwood. In:McGrail, S. (Ed.), Woodworking Techniques Before A.D. 1500. BAR InternationalSeries, Oxford, pp. 199–217.

Sass, U., Eckstein, D., 1994. Preparation of large thin sections and surfaces of woodfor automatic image analysis. Holzforschung 48, 117–118.

Van Eenhooge, D., 2009. De middeleeuwse sporenkappen van de Onze-Lieve-Vrouwekerk in Brugge, M&L,Monumenten.Landschappen & Archeologie 28 (2),21–45.

van Prooie,L.A., 1990.De invoer vanRijns houtper vlot1650–1795.Economisch—ensociaal-historisch jaarboek 53, 30–79.

Wagenführ, R., 2007. Holzatlas, 6th ed. Fachbuchverlag Leipzig, Leipzig, 818 pp.Wazny, T., 2005. The origin, assortments and transport of Baltic timber: historic-

dendrochronological evidence,in: Vande Velde, C.,Beeckman, H.,Van Acker,J.,Verhaeghe, F. (Eds.), Constructing Wooden Images: proceedings of the sympo-sium on the organization of labour and working practices of late Gothic carvedaltarpieces in the Low Countries,Brussels 25–26 October 2002,VUB Press, Brus-sels, pp. 115–126.

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