Post on 21-Apr-2023
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
generalisations ‘juvenile’ and ‘mature’ heartwood are commonly used by foresters and wood
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
‘wriggling’ along a board’s length may cause various complex dimensional changes leading 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
from mature heartwood although the outer ‘falling’ boards with relatively high stiffness and 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 and through’ whereby logs are broken down 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 boards to give the appearance of a ‘reassembled’ log. This method 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 waney-
edged 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 grown Douglas fir cut ‘through & through’ then 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 ‘green’ in a cutting pattern which encourages distortion when different
wood types interact asymmetrically such as when juvenile corewood included on only one side of a
board. Image 4 below shows a stack of 30mm thick larch sawn ‘through and through’ and kiln dried
(under restraint) en boule, this timber has remained straight and is ready for edging and
dimensioning.
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 using the ‘MEM Teletwin’ saw
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