The Structural Conservation of Panel Paintings

compression yielding. As long as the RH remains between approximately

33% and 64%, the wood can respond dimensionally without its structure

being altered. However, if the RH increases above approximately 64%,

compression setmay occur, which is a permanent deformation of the wood.

Compression set also re-initializes the wood to a new, higher RH environ-

ment, causing the wood to behave like one acclimated to a higher RH.

The plots in Figure 4 wereobtained by recalculating Equation 1 for

the fully restrained white oak panel, now acclimated to 70% RH (the cir-

cumstances under which the panel acclimated to a higher ambient RH are

irrelevant—it does not matter whether the painting has always been main-

tained at 70% or whether it was temporarily stored in a damp location).

T C   T  P P 529

0 .0 050

0 .0 045

0 .0 040

0 .0 035

0 .0 030

0 .0 025

0.0 020

0.0 015

0 .0 010

0 .0 005

0

0 20 40 60 80 100

Ges so

Fl ak e whi te

RH (% ) oi l pai nt

Mo

ist

ure

co

eff

ici

en

to

fe

xp

an

sio

n White oak

ta ngential dire ction

Hide gl ue

0.040

0.035

0.300

0.025

0.020

0.015

0.010

0.005

0

0 20 40 60 80 100

RH (% )

Yield

Strai Fai lure

n

Figure 1
Moisture coefficients of expansion versus RH
for four materials: white oak in the tangential
direction, hide glue, gesso, and fifteen-year-
old flake white oil paint. The radial-direction
coefficient for white oak is approximately one-
halfof the tangential, and the longitudinal-
direction coefficient is about one-tenth of the
tangential. The swelling rate is the lowest in
the midrange RH levels.

Figure 2
Measured yield and breaking strains of
tangential-direction white oak versus RH
(axial tensile test).