Series editors' preface - Wood Tools

82 Conservation of Furniture
environment of 70% RH (from Figure 2.14
assume an EMC = 13%). The board is then
moved to an indoor environment of 30% RH
(EMC = 6%). From Figure 2.14 assume an FSP
of 30%. Published data for mahogany indicate
a 5.1% tangential shrinkage (Hoadley, 1980).
Therefore, the estimated change in the width
of the board is calculated as:
14.5 in (0.13 – 0.06)
D =
0.
30
– 0.30 + 0.13
0.
051
D = 1.
015
= 0.178 in
5.
712
Thus the board would shrink by 0.178 inches.
Where direct measurement of moisture content
is impossible and where precise numerical
values are unrealistic, the above formula may
be of more academic than practical use. An
equally useful and reasonable approach is a
graphic method of estimating dimensional
change. Combining the oscillating curve of
Figure 2.14 with the idea of Figure 2.17, a
composite working graph can be devised, as
shown in Figure 2.18.
Based on the species and growth-ring orientation
of a piece of wood in question, the
appropriate shrinkage percentage (St, Sr, or
interpolated estimate) is taken from published
data. On the right-hand side of Figure 2.18,
choose the EMC/S line that most closely
matches the shrinkage percentage of the subject.
Estimates of changes in RH can now be
translated into percentage dimensional change
by following RH values up and over to corresponding
EMC values, then over and down to
corresponding S values.
The graphic relationship between relative
humidity, moisture content and shrinkage
draws attention to the important point that relative
humidity is the important controlling parameter
and dimensional change is the eventual
consequence. Too often, relative humidity is
not given the serious attention it deserves.
Although moisture content is usually not of
direct concern, it can be important indirectly, if
we remember that the weight of wood reflects
the moisture content. A furniture object probably
loses or gains weight primarily as a
response to changes of moisture content of its
wooden components. This change takes place
before any significant dimensional change
(where wood is restrained, it may not actually
be possible to observe dimensional change).
Placing of furniture on a weighing device such
as a load cell could be used as a quite accurate
and relatively inexpensive means of monitoring
the weight of an object. It could therefore be
an excellent way to detect and prevent changing
conditions which might eventually result in
dimensional change problems, especially when
wooden objects are being transported or relocated
in a new environment. Such a device
could actually be connected to humidification
equipment thereby allowing the object to control
its own environment.
Although dimensional change alone may be
a serious consequence of moisture variation,
shrinkage and swelling that is uneven, even
though in minor amounts, can cause distortion
of a piece from its desired or intended shape.
Various forms of distortion include cup (deviation
from flatness across the width of a board),
bow (deviation from lengthwise flatness of a
board), crook or spring (departure in end-toend
straightness along the edge of a board),
and twist (where four corners of a flat face do
not lie in the same plane).
A common source of uneven dimensional
change is simply the greater tangential than
radial shrinkage percentage, a routine cause of
cup in boards, as shown in Figure 2.16. Note
that flat sawn boards located closest to the pith
have most severe cup, concentrated near the
centre. Unequal radial and tangential shrinkage
causes round turnings to dry to ovals, squares
with diagonal ring placement to become diamond-shaped
(as a rule of thumb, growth ring
lines tend to straighten). In log sections, or timbers
containing the pith of the tree, the greater
tangential shrinkage develops circumferential
stresses which may exceed the strength of the
wood, resulting in radial cracks.
The fact that dimensional change is greater
at right-angles to the grain than parallel to it
also produces problems. The opening of mitre
joints and the loosening of mortise and tenon
joints are classic examples. Other causes of
uneven shrinkage are uneven drying, or material
with abnormal wood (such as reaction
wood, juvenile wood etc.) This topic is discussed
in more detail in Chapter 7.
With time, the dimensional response of
wood may lessen, in part because hygroscopicity
of the wood may decrease, or because of