BCD 2017

126 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 THE CONVERSION OF STRUCTURAL TIMBERS Recognising historical and modern techniques JOE THOMPSON I F ONLY historic timbers could talk – what questions you could ask, what stories they could tell. While they may remain stubbornly silent you can, however, ‘read the timbers’ and learn how they were converted and what size trees they came from. This is done by examining the surfaces of the timbers, often with a low angled light, for signs of both the method and the type of conversion. The information gained from these primary sources is invaluable for interpretation reports or conservation plans, helping to build up a picture of the chronology and development of the structure in question. Documentary evidence and scientific analysis can also be used but the initial appraisal is often based on a stylistic judgment which relies on understanding what you are looking at and putting it into context. It is worth remembering that nearly always the part of the tree (the log) that we are interested in derives from the main trunk or bole, not the branches or roots, although sometimes portions of branch or root stock are left attached to the main trunk for specific purposes, such as jowl posts. In the case of homegrown hardwoods – typically oak and elm from managed woodland, parks or hedgerows – the timber trees are often oval in section and crooked as well as tapering along their length. As a result, bark and sapwood are retained on the converted timber, and curved or cranked timbers nearly always come from trees with curved or cranked trunks rather than being cut out of a larger straight piece. Imported softwoods were often from virgin forest near to rivers and seas, and tended to be circular in section and straighter in length. CONVERSION METHODS There are three methods of conversion that have been and continue to be used to turn round logs into timbers for structural use by carpenters: cleaving, hewing and sawing. Cleaving, splitting or riving uses wedges which are struck with tools such as beetles (mauls) or mallets to split the log longitudinally, following the grain. If required, a variety of clamps or ‘brakes’ can be used to help hold the timber and stop it moving. Cleaving’s great advantages are the fact that it is the quickest of the three methods and that the maximum achievable strength is gained by following the grain. Its practical disadvantage is that it is relatively difficult to find a consistent supply of good quality cleaving logs. Our Anglo-Saxon and Norman forbears made extensive use of the natural wildwood (virgin forest), which contained many such trees. Based on the evidence of archaeological finds from waterlogged sites in England, they produced structures using cleft timbers, often ‘earthfast’ (built into the ground like a fencepost). The process of splitting tears the grain asunder and leaves a distinctive, slightly, ridged surface finish. Because the splits follow the grain, and the grain can both spiral and deviate around knots, however small, cleft timbers are often dressed with an edge tool like an axe or drawknife to produce greater regularity. The marks from where the edge tools have cut into the surface are sometimes visible. As a result, it is extremely rare, due to the difficulty of procuring the required amount of long, clean timbers, to come across any cleft structural timbers in buildings surviving from after 1200AD and those that do survive tend to be relatively short (less than 2m long). Splitting is used, however, to produce a wide variety and great quantity of associated elements that are quite short The author (top) demonstrating trestle sawing (Photo: Tim Walton)