The Structural Conservation of Panel Paintings

540 Richard, Mecklenburg, and Tumosa

applied to a wooden panel, RH changes have very little effect in the longi-

tudinal direction ofthe wood. In the tangential direction, the movement

ofthe paint is almost totally dictated by the movement of an unrestrained

wooden panel. However, the RH change needed to develop yield in alkyd

or acrylic paints will be approximately 2–3% less than the change needed

for oil paint on wooden panels because the moisture coefficient of expan-

sion of the oil paint is higher.

Control of transport RH

RH levels may also vary during transport, but fortunately this problem

can be solved with proper packing. Since the RH levels in trucks depend

largely on weather conditions, the RH inside even an air-conditioned truck

may be very high on a hot, humid day. If the weather is very cold, the RH

in the truck may be low because of the drying effects of the cargo-area

heating system. At high altitudes, the RH in a heated and partially pressur-

ized aircraft cargo space is always low—often as low as 10–15%. Panel

paintings exposed to this extreme desiccation for the duration of an aver-

age flight could be damaged. This desiccation can be avoided if the paint-

ing is wrapped in a material that functions as a moisture barrier (wrapping

of panel paintings is discussed further below).

The dimensional response of wooden panels to temperature variations has

been largely ignored by many conservators, because temperature has been

considered to have a much smaller effect on wood than has RH. This pre-

cept holds true ifone considers only the relative dimensional response of

wood to temperature as compared to its response to moisture. It would

take a change of several hundred degrees in temperature to induce the

same dimensional change in wood that can be caused by a large change in

RH. Panel paintings are rarely exposed to such temperature extremes, and

they are usually exhibited or stored where temperature variations are rela-

tively small. The problem, however, is not so much the response of the

Temperature Effects

0 20 40 60 80 100

RH (% )

Mo

ist

ure

co

eff

ici

en

to

fe

xp

an

sio

n

Cad miu m yellow

alky d mediu m

Cad mi um yel low

sa ff lo we r oil

Co balt blue

acry lic emulsi on

0 .00 16

0 .00 15

0.00 14

0.00 13

0 .00 12

0 .00 11

0 .00 10

0 .00 09

0 .00 08

0 .00 07

0 .00 06

0 .00 05

0 .00 04

0.00 03

0.00 02

0 .00 01

0.00 00

Figure 15
Moisture coefficients of expansion versus RH
for oil, alkyd, and acrylic paints. The dimen-
sional responses of the alkyd and acrylic
paints are substantially lower than those of
the oil paint.