Series editors' preface - Wood Tools

80 Conservation of Furniture
(a)
(b) (c)
Figure 2.16 Roughly speaking, tangential shrinkage is
double radial shrinkage. This difference in dimensional
change when wood is seasoned causes the distortion
seen in representative shapes cut from logs (a) and
commonly results in radial cracks in logs (b), or
squared timbers (c) containing the pith of the tree
are not commonly available, as are data for
perpendicular shrinkage (i.e. shrinkage across
the grain). It should be cautioned, however,
that abnormal wood tissue, such as juvenile
wood, reaction wood, or pieces with crossgrain,
may exhibit longitudinal shrinkage of up
to 10–20 times normal. In addition, it should be
expected that abnormal wood will occur
unevenly in severity and in distribution, and
that the resulting uneven longitudinal shrinkage
may produce warping.
Shrinkage across the grain (transverse
shrinkage), however, is significant, and tangential
shrinkage is always greater than radial
shrinkage. Average tangential shrinkage varies
between species over the range of about 4 to
12%, with an overall average of about 8%.
Average radial shrinkage values range from
about 2 to 8%, averaging slightly over 4%. The
result of differences in radial and tangential
shrinkage is illustrated in Figure 2.16.
2.4.4 Estimating dimensional change
There is a difference between shrinkage, which
occurs as the change in dimensions of wood
on initial drying from the green condition, and
movement, the dimensional change which
takes place when wood that has been dried is
subjected to changes in atmospheric conditions
below the fibre saturation point (Farmer, 1972).
It is possible that a wood may shrink quite
appreciably in drying from the green condition
yet it may undergo comparatively small dimensional
changes when subjected to a given
range of atmospheric conditions in service. The
reason is that the fibre saturation point, the
moisture content value at which appreciable
shrinkage begins to take place, varies between
different species. In addition, the moisture con-
Shrinkage (%)
8.6
8.0
4.0
Radial
Tangential
0.1
Longitudinal
0
(OD)
5 10 15
EMC (%)
20 25 28
(FSP)
Figure 2.17 Typical relationships of tangential, radial
and longitudinal shrinkage to equilibrium moisture
content, shown from oven dry (OD) to fibre saturation
point (FSP). Note that shrinkage along the grain
(longitudinal) is negligible. Tangential shrinkage is
roughly double radial shrinkage, thus the historical
preference for quartersawn boards when maximum
dimensional stability was required