Dendrochronology: Martin Bridge

Although it is possible to work on artefacts such as doors and chests, or archaeological timbers, much of the work I do is focussed on dating timber-framed buildings, or roofs in stone/brick buildings.

This usually involves extracting 15mm diameter cores using an electric drill. We are looking for timbers with a good number of rings and at least some sapwood on the outer edges, telling us that we have reached the outer rings of the tree. If the bark edge is present, the season of the year of felling of the tree may be determined.

The holes can be plugged and stained so as to be almost invisible to the casual observer. The cores are then sanded with progressively finer grit papers to give good resolution of the ring boundaries, and the sequences are measured on a specially constructed measuring machine, under a binocular microscope. The ring boundaries are automatically logged on a PC, and analysis can then be carried out. Essentially one is trying to match the patterns of wide and narrow rings, and so to have confidence in the results it is necessary to have enough overlapping rings. We generally like to see a minimum of 70-80 rings per sample, but sometimes fewer rings are acceptable. The eye is very good at matching plots of the varying ring-widths, but of course we need some objective measure of the goodness of fit - and statistics are used. Several possible methods can be used - but the t statistic has proved most useful for comparing oak sequences in Europe. 

The individual sample sequences from several timbers in the same phase are first matched to each other - and then combined to form a site master chronology. This has the effect of filtering out the individual growth characteristics of a single tree (which may be the result of disease, injury etc) - and enhancing the 'signal' which results from the weather in each season, and is common to the majority of trees growing at the same time over quite a wide geographical area.

This site master chronology is then compared with the large database that has been accumulated over the last few decades - and we also have access to data from abroad. This enables us to give a likely felling date range for timbers that have some trace of sapwood, or actual felling dates for those with complete sapwood.

It is often not possible to judge how useful a particular site might be before seeing the timbers - and therefore it is difficult to give an idea of costs for any given job. The usual practice is that if the timbers are judged unsuitable (usually because of insufficient numbers of rings being available) then the client is only charged for the day (or part there of) and travel. If the timbers are judged suitable, an idea of costs can be given on the spot - and will incorporate the time/travel above plus a cost per core. English Heritage Guidelines recommend a minimum of eight samples per phase - though this may not always be possible.  To give a 'ball park' idea, a typical single phase building is likely to cost in the region of £700-850 - depending on where it is in the country, how many samples are needed etc. Additional phases are obviously much less as the time/travel forms a large part of the cost for a single phase, and is unlikely to be much more for more than one phase.