rately. In the conservation of painting panels, the overall dimensions and
species of the panel wood are already determined; the change in MC is the
variable within our control.
It must be emphasized that the calculations given above are of
theoretical value in understanding potential dimensional change; however,
in practical terms they are approximations at best. The formula for pre-
dicting dimensional change in unrestrained wood has been found to be no
more accurate than 6 25%. It would therefore seem fitting to consider a
simple graphic method of approximating dimensional change.
Combining the oscillating curve of Figure 7 with the principle of
Figure 9, a composite working graph might be devised as shown in Figure- For the right-hand portion of the graph, the appropriate shrinkage
percentage (St, Sr,or interpolated estimate) is taken from published data
according to the panel at hand. Users ofthe graph may translate estimates
of changes in RH into percentage dimensional change by following initial
and final RH values up and over to corresponding EMC values, then over
and down to corresponding S values.
The graphic solution can be applied to the problem discussed
above. In the example already proposed, charted graphically as example 1
in Figure 10, a change in RH from 60% to 25% would result in a shrinkage
ofapproximately 1.6% for tangentially cut poplar.
As a numerical check for the calculation of dimensional change in
the poplar panel discussed above, ifDiwere considered simply as one unit
of dimension, the value ofDDwould have been calculated as:
Therefore, (76 cm)(1.65%) 5 (76 cm)(0.0165) 5 1.25 cm.
The graphic relationship among RH, MC, and shrinkage draws
attention to the point that RH is the important controlling parameter, and
dimensional change is the eventual consequence. Too often RH is not
given the serious attention it deserves. Although weight of wood is usually
not of direct concern, it can be important indirectly if we remember that
it reflects the MC. A painting probably loses or gains weight primarily
asa response to changes in the MC of its wooden panel. Therefore, the
simple monitoring of the weight of a painting, especially when it is beingC P P W 17EM
C
(%
)
RH (%) S (%)(^12)
0 20 40 60 80 100 0 2 4 6 8 10
32
24
16
8
0
32
24
16
8
0
Figure 10
Relationship between RH and EMC, and
between EMC and shrinkage (S),shown for
tangentially cut poplar (St 5 8.5%). As shown
by dashed and dotted lines, the two examples
discussed in the text illustrate the effect ofRH
change on potential shrinkage and swelling
(negative shrinkage).