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.

Image 4: 30mm thick larch sawn ‘through and 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 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