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Dendrochronological investigations of timbers from the excavation Schleswig, Hafengang 11 (LA 153) (completed)

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Dr. Thorsten Westphal

Following the drawing, documentation and labelling of the timbers recovered from the excavation Schleswig, Hafengang 11, under the direction of Dr Joachim Schultze (ALM), sampling of the wood, by removal of slices, was carried out from April to December 2009. The samples were then taken to the dendrochronological laboratory in Frankfurt am Main for the purposes of dating. The material comprised exclusively building timber, derived from various structures. In addition to radially-split timber (mainly planks and boards), tangentially-sawn and -split boards and roundwood (including stakes) were also represented, together with squared timbers (scantlings) produced from either entire or cut logs. The state of preservation of the timber samples was generally good, only in a few instances was dendrochronological analysis no longer possible due to an advanced state of decay.

Schleswig Hafengang (Foto: ALSH)

The following investigations were carried out on the timber, with the emphasis on dating:

  1. Identification on the basis of wood anatomy using a binocular loupe (magnification: x3.6-40) and a transmitted-light microscope (magnification: max. x400)
  2. Tree-ring analysis:
    • Preparation of sample sections for measurement
    • Measurement of the width of the annual rings (accuracy: 1/100 mm)
    • Comparison and synchronisation of the curves resulting from the measurement series, visually and statistically, with the aid of special correlation programmes
    • Documentation of the wood from a dendrochronological point of view (number of annual rings, pith, sapwood rings, waney edge
  3. Recording of the parameters in a database
  4. Photographic documentation of the wood cross-sections

In 2009, 145 samples of wood were investigated. The species oak (Quercus sp., 141), alder (Alnus sp., 1), ash (Fraxinus sp., 2) and pine (Pinus sp., 1) could be identified. At 97.24%, oak was the most frequently-used building timber. The species common oak (Quercus robur) and durmast oak (Quercus petraea), together with various hybrids, all come into consideration as it is not possible to distinguish their wood reliably on the basis of anatomy. The technological properties of the wood of all of these are, however, very similar. Oak timber is very hard, workable, solid, elastic and of exceptional durability in both a wet and a dry condition. It has, at all times, been preferred as building timber.

Of the 141 dendrochronologically analysed timbers, 139 are of oak and there is one each of ash and pine. Selection was based on the number of annual rings. All samples with more than 15 rings were analysed, except when they were too poorly preserved or so irregular in growth that a sufficiently long continuous sequence of annual rings, unimpaired by distortions, could no longer could be ascertained. Of the timber samples analysed, 61% have less than 100 annual rings. These include the sample of ash with only 32 rings, which it was not possible to synchronise with either the corresponding oak curve or the ash chronology for Mecklenburg. Similarly, the pine sample with 86 rings gave no secure match with the corresponding reference curves.

The annual ring sequences for 90 oak samples could be synchronised. Of these, it was possible, due to their mutual similarity and correlation, to combine eight timbers into one group (no. 2). However, as yet, it has not proved possible to date these absolutely. Dates were achieved for the remaining 82 samples which could be synchronised. All the dates for waney edge lie between AD 1085 and 1100. Accordingly, the building activities date from the late 11th century.

Within this series of 82 datable timbers, it is possible, due to their great mutual similarities, to recognise six further groups (nos. 1, 2–6) encompassing a total of 76 samples, whereas six timbers could not be assigned to any of these groups. The individual groups match reference series from adjacent areas to varying degrees. Groups 1 and 4 show a strong affinity to Danish series and groups 5 and 7 match up predominantly with comparative series from (Western) Mecklenburg, whereas group 3 reveals a great similarity to chronologies from the Cologne region. Only a moderate degree of correlation is seen internally between the groups. The reasons for this could lie in the presence of imported timber; at the present stage of the investigations local differentiations are yet to be found. The analyses of further timbers recovered from this excavation will, in addition to increasing the number of dates/dating percentage, also help to illuminate such questions in more detail in 2010.

 

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