98 Australian Wood Review
TIMBERconstrain the outside-face stretch. Consequently, the practical
procedures required to achieve successful bending without
failure are not trivial and few rules of thumb work^1.Other ways to bend wood
There are additional ways of softening wood other than
with steam by way of chemicals such as liquid ammonia
and urea, or by heating rapidly near-green wood with
microwave radiation². Most of these methods require more
complex handling and additional difficulties for re-drying
of the wood for use.Choice of material
Species type is an important consideration in terms of
appearance, degree of bend and end use. Even if the best
performing species is chosen however, its suitability may be
overshadowed by the selection of the individual pieces in
terms of their quality.As in design of building structures, defects and their
location are critically important and need to be avoided, or
if inevitable, they need to be small and located discretely
or close to the central zone where bending stresses are less
significant. Features to avoid include:- Knots. Pin knots are less important in backsawn
material but will give rise, along with species with wide
rays, to wrinkles with quartersawn material - Cross grain. (<1:15) wood near the pith (first 5–6
growth rings) in softwoods often contains spiral grain, this
is less of a problem in hardwoods - Interlocked grain. Many eucalypts and tropical woods
exhibit this feature - Decay, gum/kino veins discontinuities and shakes
- Drying checks. Surface and internal checks are common in
pre-steamed green wood and check prone ash eucalypt species. - Brash wood. Brash or brittle heart is usually found near
the tree pith and is either of natural origin or resulting from
the use of high temperatures in drying or felling damage.
From left: Myrtle
beech, southern
sassafras and
red tulip oak are
noted as having
excellent bending
qualities. Visually
it is not possible to
predict how well a
particular section
of different woods
might bend.- Timbers with wide rays. These should be backsawn^1 ,
for example silky oak, tulip oak and true oaks
Choice of species
Reportedly, density and rate of growth has little effect on
species bending performance¹. Interestingly, many of the higher
density species listed which bend poorly, have well developed
interlocked grain. Secondly, it seems that species with distinct
growth rings tend to bend better that those lacking them.Also, in a study of Australian plantation and regrowth
species including karri, jarrah, myrtle, beech, blackwood,
radiata pine, shining gum and Sydney blue gum it was
noted that when microwaves are used to heat near-green
wood, plantation and regrowth species performed better
than their mature growth counterparts, but reportedly
there were some discrepancies with previous reports
particularly for karri, and myrtle beech.The Trade Circular¹ list shows that the best species for bending
still in use today include myrtle beech and to a lesser extent
celery top pine with radiata pine, and Northern silky oak not
far behind. The ash eucalypts, blackbutt, spotted gum, karri
and blackwood are in about the mid-range position and many
of the high-density eucalypts and rose gum are under the poor
banner. Interestingly, the popular Sydney blue gum (Eucalyptus
saligna) is not listed, but its performance is expected to be little
different from rose gum on wood structure grounds. Generally
outside Australia, northern temperate oaks are among the best
with hickory and beech (Fagus) at midrange and the maples
and yellow poplar being poor.- CSIRO, Division of Forest Products 1948 Trade Circular
No. 22 – Timber Bending. Rev. Ed. Commonwealth of
Australia Council of Scientific and Industrial Research. - B. Ozarska and G.Daian (2010) Assessment of microwave
bending capabilities for Australian wood species. Forest Prod. J.
60(1):64-68
Dr Jugo Ilic worked as a wood scientist at CSIRO for
36 years. As part of his research activities he authored
the CSIRO Atlas of Hardwoods and curated the
Australian Wood Collection.