panel paintings. It seems fair to say that the intention of the uniformback-
ing is to produce a panel with a minimum of elastic strain, so that the
wood structure is as relaxed as possible. Thus, if a panel is warped and the
backing simply supports this conformation, the previously developed plas-
tic change is retained. In the event that the backing has imposed a reduc-
tion in the warp, it is intended that the reduction of warp will have a high300 Horns
Figure 11
A warped wooden sample, with polycarbon-
ate glued to one edge over a layer of silver
paint, has been clamped against a flat surface
to eliminate the warp. When illuminated with
polarized light and viewed through a polariz-
ing filter, this photoelastic material shows the
variations in strain as colored fringes. The
darkest lines (one-third of the way down from
the top surface) represent the least strain.
Compression increases toward the top, ten-
sion toward the bottom.
Figure 12
Thesame sample shown in Figure 11, after
exposure to moisture at the concave surface
for several hours. The warp was slightly
reduced, and compression began to develop at
the top surface. Swelling produced tension in
the photoelastic material at the concave
lower surface.Figure 13
After twelve hours the sample was flat, and
substantial compression developed at the top
surface, along with tension from swelling at
the bottom. Strain gauge measurements at
the top surface indicated that the compression
produced by this flattening was somewhat less
than that produced by the clamping seen in
Figure 11.
Figure 14
After further exposure to moisture, the sam-
ple began to develop a slightly reversed warp,
and compression at the top surface began
to decrease.