The Structural Conservation of Panel Paintings

Cracking in varnish and polyurethane coatings on wood has, in

fact, been recorded when the temperature has dropped from 24 °C to

2 20 °C. In the radial and tangential directions of the wood, the tempera-

ture must drop to well below 2 50 °C to produce similar strains in the oil

paint layers.

It is unlikely that cracks in oil paint layers could occur perpen-

dicular to the grain of the wood because of RH variations. However,

with regard to temperature, even moderate subfreezing temperatures can

crack oil paint in this direction. Low temperatures are less likely to cause

cracking of paint parallel to the grain, unless the wooden support panel is

fully restrained from thermal movement during the temperature drop. As

Figure 16 shows, oil paint layers applied to copper can survive a substantial

drop in temperature. Note that resultant embrittlement of the paint layer

is far more severe when it is exposed to low temperature at moderate RH

than when exposed to low RH at room temperature.

Other paint media suffer embrittlement similar to that suffered

by oil paint, but at higher temperatures. With alkyd paints, a Tgoccurs at

approximately 2 5 °C, while with acrylic paints, it occurs at approximately

5 °C. While unlikely, it is possible for the temperature inside packing cases

to drop to 5 °C in the cargo holds of aircraft, on the airport tarmac, or

inside an unheated truck. Tgshould be considered the lowest allowable

temperature for a safe environment, because embrittled materials are

more vulnerable to damage.

The effect of temperature on gesso applied to wooden panel

paintings is different from the effect of the same temperature on paint

applied to wooden panels. In general, gesso has a low thermal coefficient

of expansion that is higher than that ofthe longitudinal direction of

whiteoak and lower than the oak coefficients in the radial and tangential

directions. Figure 17 plots the calculated temperature-related mechanical

strains in the three different grain orientations for a gesso coating applied

to a white oak panel. First, the developed mechanical strains are minimal,

even at 2 40 °C. In the longitudinal direction the gesso strains are tensile,

and in the tangential and radial directions they are compressive. Thus, it

542 Richard, Mecklenburg, and Tumosa

240  220 0 20

Temperature (°C)

Me

ch

an

ica

ls

tra

in

in

the

ge

ss

o

Compression response

Oak—tangential

Tension response

Oak—longitudinal

0.0012

0.0010

0.0008

0.0006

0.0004

0.0002

0.0000

2 0.0002

2 0.0004

2 0.0006

2 0.0008

2 0.0010

2 0.0012

2 0.0014

Oak—radial

31 Richard fig 17 eps

Figure 17
Calculated temperature-related strains in
gesso when applied to white oak. The gesso
strains in the longitudinal (tensile) and cross-
gr ain (compressive) directions are never very
high, and failure is not likely to occur, even if
the temperature drops significantly.