measured with strain gauges and recorded. The investigation showed that
the movements of a new oak panel and a panel from the seventeenth cen-
tury were analogous (Klein and Bröker 1990).
Experiments with beech (hardwood) and Scotch pine (softwood)
demonstrated that the hardwood has a slightly higher moisture change
rate than the softwood, and that the movement of beech samples was
therefore larger than that of the Scotch pine samples (Stevens 1961).
The ratio of the area of exposed surface to the volume of the
wood also influences the reactivity of the wood. Thin pieces ofwood
respond more quickly than thick ones, while small pieces respond more
quickly than large pieces of equal thickness. When a panel is thinned, as
is often done during the cradling process, the ratio of exposed surface to
wood is sharply increased; therefore, the diffusion of moisture throughout
the bulk of the panel and the response to changes in the atmospheric envi-
ronment are accordingly accelerated.
It has also been demonstrated that the higher the temperature, the
more rapid the rate of moisture transfer. A piece of wood comes to equi-
librium about twice as fast at 24 °C as at 12 °C because the vapor pressure
ofwater at 24 °Cis twice as great as at 12 °C, if the RH is constant.
Finally, the greater the change in RH, the faster the rate of mois-
ture transfer (Buck 1961, 1979).
The preparation of a panel before the painting process must also
be considered (for a discussion of historical techniques, see Wadum,
“Historical Overview of Panel-Making Techniques,” herein). The size and
ground may contain hygroscopic materials, such as glue, that also react to
changes in RH and temperature.^4
The behavior of a number of materials found in traditional paint-
ings has been analyzed under the stress of temperature fluctuations and
varying RH (Buck 1972; Mecklenburg and Tumosa 1991). Another impor-
tant result of climatological fluctuations is the changing stiffness of paint-
ing materials and mediums in traditional paintings (Michalski 1991).
Changes in RH produce measurable changes in the dimensions of
a panel. Research has also shown that paintings change dimensionally as a
consequence oftemperature, independent ofa change in RH (Richard
1991). However, bearing in mind that the thermal expansion of a panel
enclosed in a case is small, the conservator should concentrate on keeping
the moisture content of the wood constant and thus ensure dimensional
stability of the panel.^5 The unanimous advice giv en by various authors498 Wadum
a Air
10080604020(^0010 20 30 35)
20
15
10
(^64)
1 2
Temperature (C°)
RH
(%
)
g/k
g g/kg
b Wood
100
80
60
40
20
(^0010203035)
(^2100
80)
160
140
120
100
80
60
Temperature (C°)
RH
(%
)
Figure 1a, b
Correlation between RH, temperature (°C),
and grams ofwater per kilogram (g/kg) in
(a) air, and (b) wood.