Drying higher grade softwoods D. Dauksta March 2015 High yield plantation-grown softwoods offer a range of challenges firstly in drying and then later in utilisation and service. Despite 150 years of study our understanding of typical radial patterns in the cross sections of conifer stems has been described as minimal. Even the terms used to describe the varying types of wood found within the stem of conifers have been debated although the ge eralisatio s ju e ile a d ature heart ood are o o l used foresters a d ood processors without necessarily understanding the complexities of typical radial patterns from pith to bark. All of the different wood types such as juvenile wood, spiral grain and compression wood can occur and change at different rates across a transition zone between the pith and mature zones. Drawing 1 (modified from the USDA Wood Handbook) below shows the shrinkage or distortion patterns of timbers cut from normal wood. Reaction wood and spiral grain can significantly add to these drying distortions. T shows cupping on a tangential board, R1 and R2 show how radially sawn boards and timbers shrink with little distortion, R3 shows how radially sawn timber shrinks more along growth rings. C, the centre board contains a high proportion of juvenile corewood, not only will it shrink as shown but it may also twist when spiral grain is present. Drawing 1: Characteristic shrinkage and distortion of normal wood (USDA Wood Handbook) Mature heartwood suffers little longitudinal shrinkage during drying (except when reaction wood is present). When both juvenile and mature heartwood occur in the same board, the juvenile wood will shrink more than the mature wood with consequent dimensional change causing problems such as bowing. Spiral grain in juvenile wood can cause boards to twist during drying. Boards cut from the centre of a sawlog (especially larch) containing a juvenile core are particularly prone to twisting, image 1 below shows a larch centreboard twisted through 20o. Image 1: larch centreboard twisted through 20o, juvenile core marked by green circle. Juvenile stems may take on helical or sinusoidal forms which are hidden beneath layers of mature heartwood until revealed during sawing; then the changing properties caused by the juvenile heart riggli g alo g a oard s le gth a ause arious o ple di e sio al ha ges leadi g to problems in machining and/or significant loss in yield. Image 2 below shows a board cut from a larch tree; the sinuous form of the juvenile tree has dominated the form of the mature tree. Image 2: the sinuous form of the juvenile tree has dominated the form of the mature tree In attempting to dry sawn softwoods to make them useable in construction the changes in wood anatomy across tree stems are only part of the range of challenges facing processors; moisture content also changes from pith to bark. For example in Sitka spruce heartwood moisture content may vary from 40% to 80%. However in the sapwood (the outer zone of wood closest to the bark), moisture content in excess of 120% is encountered and values close to 300% have even been found. Thus material properties can vary widely in one board. Modern sawmills convert material at such high rates there is little scope to change sawing patterns in order to separate juvenile heartwood fro ature heart ood although the outer falli g oards ith relati el high stiff ess a d high moisture content are normally separated but sold into low value markets. There may be potential for new scanning and selection procedures capable of classifying boards according to density, end grain imaging, distortion types (e.g. cupping or bow), stiffness and moisture content. However, at present sawmill timber selection and binning infrastructures in Britain may work too slowly for such complex grading routines. Researchers have suggested that boards be selected and grouped according to moisture content before kilning but in practice this does not happen generally. Therefore kiln charges may be composed of boards with many different material properties and a wide range of moisture contents. Risk aversion significantly influences kiln management as kilns grow in volume to accommodate the huge increases in sawmill production rates and so final mean moisture content has to be kept fairly high (around 18%) in order to take account of the varying timber types and distribution of final moisture contents found across a whole kiln charge. Cutting patterns can seriously influence drying distortion of timber. Across much of Europe from France to the Baltic region, sawmills have traditionally relied on sawing hardwoods en boule or through a d through here logs are roke do n by making parallel cuts across the transverse end face and down the length of logs. For drying, stickers are then placed between the resulting full width double waney-edged oards to gi e the appeara e of a reasse led log. This ethod of drying is still standard practice for hardwoods across the world. Image 3 below shows Welsh-grown Douglas fir logs which have been sawn, stacked en boule and successfully air dried. Even relatively large sawmills in southern Germany sometimes air dry some of their softwoods in this way and it may be one practical solution to the problem of drying British conifer timbers with their widely varying radial properties. Although the three centre Douglas fir boards in image 2 contain both juvenile and mature zones, the juvenile core is bound within mature zones along both edges thus balancing drying stresses and reducing distortion. Douglas fir spiral grain does not dominate behaviour of the juvenile core so that centreboards may be left within drying stacks. Double waneyedged boards can be processed through double band resaws or multirip saws for final dimensioning by taking off both waney edges simultaneously. This optimises width of each board and mature heartwood either side of juvenile material constrains the behaviour of the corewood. This traditional method has been studied in some depth and modified by American and Asian researchers with a view to obtaining better conversion yields from difficult hardwoods and fast grown softwoods; it is called Saw-Dry-Rip or SDR. Image 3: High grade Welsh grow Douglas fir cut through & through the successfully air dried SDR or en boule methods are unlikely to be taken up by high volume softwood sawmills in Britain for producing joinery or other high grade timber but may appeal to smaller processors who wish to differentiate their products and sell high grade softwoods into niche markets. When building drying stacks there may be scope to select out centre boards which include pith and much of the juvenile heartwood; these are the boards that are most likely to twist and induce distortion within stacks. Large drying stacks of softwood do not necessarily need to be assembled en boule; actually randomly distributed double waney-edged boards may dry more successfully within a stack which is randomly distributing drying stresses; this needs more study. The most important factor is that boards are not resawn whilst gree i a utti g patter which encourages distortion when different wood types interact asymmetrically such as when juvenile corewood included on only one side of a board. Image 4 elo sho s a sta k of 30 thi k lar h sa through a d through a d kil dried (under restraint) en boule, this timber has remained straight and is ready for edging and dimensioning. Image 4: 30 thick larch saw through a d through and then kiln dried under restraint Bespoke sawmillers may have some advantage over volume producers; they often use horizontal bandsaws which by their design allow through and through cutting. Traditional en boule drying of softwoods may offer relatively easy value adding opportunities for small sawmills seeking specialist markets. However there is at least one medium sized Welsh sawmill usi g the MEM Telet i sa which because of its between centres design is ideal for sawing full width boards from either side of the juvenile core. This topic is worthy of more study especially as softwood sawmilling becomes more polarised between high volume and bespoke processors. Drawing 2 below shows a pre-edged, centred-cant sawing pattern which also produces boards with roughly symmetrical properties which have the best chance of drying with low degrade. The MEM Teletwin can use this cutting pattern. More information about the MEM Teletwin here; http://www.memwood.com/gb/teletwin.html and here; https://www.youtube.com/watch?v=ryLGKcZq2Tg Drawing 2: Centred cant sawing pattern for SDR method, centre circle indicates juvenile zone