The Structural Conservation of Panel Paintings

cracking of painting panels. The following discussion addresses the dimen-

sional change in wood due to changes in MC below the FSP.

Shrinkage percentage

The traditional approach to expressing the relative dimensional instability

of wood is to measure the total amount of linear shrinkage that takes

place in a given direction from its green^1 condition to its oven-dry condi-

tion, expressed as a percentage of the green dimension. Linear dimen-

sional change in wood is usually measured separately in the three principal

directions: longitudinal (Sl), radial (Sr), and tangential (St). Quantitatively,

the total shrinkage percentage is calculated as follows:

where: S 5 shrinkage, in percent, for a given direction (Sl, Sr,or St); Dg 5

green dimension; and Dod 5 oven-dry dimension.

Figure 8 illustrates the application of the formula in the determi-

nation of tangential shrinkage (St) based on green and oven-dry measure-

ments ofa tangentially sawn strip of wood.

Total longitudinal shrinkage of wood (Sl) is normallyin the range

of 0.1–0.2%. In practical situations involving typical moisture-content

changes over a moderate range, only aportion of this small quantity would

be affected, and the resulting dimensional change becomes insignificant. It

is therefore reasonable to assume that wood is stable along its grain direc-

tion, and for most purposes longitudinal shrinkage and swelling can be

ignored—in fact, longitudinal shrinkage data are not commonly available. It

should be cautioned, however, that abnormal wood tissue, such as juvenile

wood, reaction wood, or cross-grain pieces may exhibit longitudinal shrink-

age of up to ten to twenty times that of normal. In addition, it should be

expected that abnormal wood will occur unevenly in severity and in distri-

bution, and the resulting uneven longitudinal shrinkage will cause warp.

Radial shrinkage is quite significant, and tangential shrinkage is

always greater than radial. Tangential shrinkage varies among species

over the range of about 4–12%, with an overall average of about 8%.

Average radial shrinkage values range from about 2% to 8%, averaging

slightly over 4%. Values of average tangential and radial shrinkage are

given for woods commonly found in painting panels in Table 1.

Over the range of bound-water loss, shrinkage of wood is roughly

proportional to MC change, as shown by solid-line curves in Figure 9.

Careful measurement of changing dimension as wood is slowly dried will

show nonproportional behavior, especially at MCs near the FSP, because

of the moisture gradient inherent in drying. However, in theory, the effect

ofMC on shrinkage is essentially proportional, and the relationship is

assumed to be linear (see dashed-line curves, Fig. 9).

Estimating dimensional change

Based upon published percentages of shrinkage for individual species and

upon the assumption that shrinkage bears a linear relationship to moisture

content, the anticipated dimensional change in a given piece of wood

can be estimated. Because shrinkage percentages are averages and exact

S 5 Dg^2 DD od 3100

g

14 Hoadley

St 5 DDD

g

3100

Dg

Dod DD

Figure 8
Determination of the total shrinkage percent-
age for wood. Tangential shrinkage percentage
(St) is calculated as the change in dimension
after oven drying (DD 5 Dg 2 Dod), divided by
the original green dimension (Dg).