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Glas-hirfryn: When was the house built?

Tree-ring dating of Glas-hirfryn

Fortunately we now have a fairly precise date for when the house was built. Tree-ring dating, or dendrochronology, has shown that the main phases of construction of the house took place during 1559 or shortly afterwards - the year after Elizabeth I came to the throne. Dating was undertaken by Dan Miles of the Oxford Dendrochronological Laboratory (from whose website many of the following details are taken). Twelve different pieces of timber from the original building at Glas-hirfryn were sampled, including axial beams, rails, a wall-post, jetty plate and panelling. One piece of timber, clearly belonging to a later alteration, has been dated to 1826/27.

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Section of a newly-felled oak tree.

How tree-ring dating works

The way dendrochronology works is relatively simple. As a tree grows, it puts on a new growth or tree-ring every year, just under the bark. Trees grow, and put on tree-rings, at different rates according to the weather in any given year: a wider ring in a favourable year and a narrower ring in an unfavourable year. Thus, over a long period of time (say 60 years or more) there will be a corresponding sequence of tree-rings giving a pattern of wider and narrower rings which reflect droughts, cold summers, etc. In effect, the span of years during which a tree has lived will be represented by a unique fingerprint, which can be detected in other geographically-similar tree-ring chronologies. To obtain this fingerprint, a radial section of timber from the pith or centre of the tree out to the bark edge is required.

Hunting for suitable pieces for dendro dating.

Selecting suitable pieces of timber for dating

The aim is to make sure that the samples are representative of the building or building phase being studied and do not come from an older building or represent later repairs. Generally, between 6 and 8 suitable samples are needed from a single building phase to ensure a reliable result. This also helps to make sure that any growth irregularities in individual pieces of wood are averaged out. It is also important to select as many samples as possible with both a surviving bark edge and a high ring count. Usually, however, only a small proportion of samples have the complete sapwood surviving because of the way in which the original timber was worked or because of subsequent weathering.

Sampling one of the timbers from Glas-hirfryn.

Taking the samples

Samples are usually taken in situ from a standing historic building but in the case of Glas-hirfryn the building had been demolished so the samples could be selected in the workshop. Samples for dating are obtained by drilling a 16mm hollow corer which produces a core approximately 10mm in diameter (see photos below). These core bits are specially manufactured for the purpose and have hardened steel teeth in order to cut through timber hardened by centuries of seasoning. This is effected through the use of a 800w 240v drill with gear reduction and variable speeds so as to have sufficient torque and power to drill at the slow speeds required. The drill is run at sufficiently slow speeds so as to not be considered ‘hot work’. Where samples have been taken from visible timbers in the habitable areas of buildings, the resulting hole is plugged with a ramin dowel, stained and distressed to match the surrounding surface.

Record sheets for the Glas-hirfryn samples.

Keeping a record of where the samples are from

The best cores should have a sequence of tree-rings of at least 50, and ideally 100 years. Ideally, too, they should retain the final ring at the bark edge: that is, the last ring that was grown in the year in which the tree was felled. Therefore, the best samples are from large, slow-grown timbers, without distortions, and with bark edge remaining. The timber must be dry - oak timbers which have been exposed to rain will not core successfully. When timbers have been allowed to become soaked through exposure to weather, it can sometimes take a year for the oak to dry out. Roof spaces with no ventilation can also have high moisture build-up in the timbers.

It is vital to accurately record which timbers have been sampled and to understand where in the building the dated timbers are from and to which phase of the building they are likely to belong. These details are recorded on record sheets shown here.

Above, the hollow core drill bits used for taking the samples. Below, the cores from Glas-hirfryn used for dating.

Measuring the rings and interpreting the results

Once the samples are obtained, and are thoroughly dry, they are then sanded on a bench-mounted belt sander, using 60 to 1200 grit abrasive paper, and are cleaned with compressed air to allow the ring boundaries to be clearly distinguished.

Once prepared, the samples are then measured under a x10/x45 microscope to a precision of 0.01mm. Thus each ring or year is represented by its measurement which is arranged as a series of ring-width indices within a data set, with the earliest ring being placed at the beginning of the series, and the latest or outermost ring concluding the data set.

Whilst the principle behind tree-ring dating is a simple one, the determination of what is an actual match is much more involved. When an undated sample or site sequence is compared against a dated sequence, known as a reference chronology, an indication of how good the match is must be determined. Although it is almost impossible to define a visual match, computer comparisons can be accurately quantified. Whilst it may not be the best statistical indicator, Student's t-value is widely used amongst British dendrochronologists.

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