ate, balsa wood is an inert material that is not subject to distortion over
time. Once the panel is backed, it is resistant but not much heavier. The
adhesive used, a mixture (by weight) of seven parts beeswax and two parts
damar resin, is relatively flexible, and its adhesive strength is moderate.
These particular qualities, while they contribute security to the panel, are
at the same time the technique’s weak point. Paintings treated this way
will require special precautions, especially with regard to mechanical
shocks and high temperatures.Panel paintings, which don’t have a paint layer on their reverse, are often
more or less convex. If the distortions are distributed evenly over the
whole of a panel, the viewer will not be troubled. But the presence of a
single, limited distortion can be so disturbing as to alter the look of the
painting completely. These distortions have internal causes—for example,
the wood’s nature, density, or method of conversion (the way it has
been sawn). They emerge as a result of poor conservation conditions,
such as serious fluctuations of relative humidity (RH) and the constraints
wrought by framing.
Balsa-wood backing glued with wax-resin has been used to main-
tain distorted panels after flattening. The method consists of increasing
the water content of the whole panel in an air-conditioned chamber and
locally applying damp compresses over extremely distorted areas. The aim
is to reach a point of balance at which the boards recover their inherent
flatness. Once this condition is achieved, the backing is applied to the
whole back ofthe panel. Balsa-wood backing with wax-resin acts as a
mechanically uniform maintenance device; moreover, it slows down
humidity exchanges. It forces the panel to remain flat during the drying
period, a process that completes the treatment and subjects the wood cells
to plastic distortion. The return to an RH of about 60% takes place gradu-
ally, in a matter of two to three months.
Balsa-wood backing, in this case, fulfills a provisional function. It
can be removed as soon as a panel is stabilized in an environment where
the RH is controlled. To date, however, as a precaution, such backings
have been left on. Balsa and wax-resin act as barriers against humidity.
The advantage of this technique lies in the fact that if the con-
straint on the drying panel is higher than the adhesive strength of the
wax-resin, the backing will come unglued. In such a case the panel will
reassume some curvature and will not be threatened with splitting.
Among the treated paintings, three thin (less than 2 cm) oak pan-
els that were formerly distorted are currently backed. This way, while
keeping them at a stable RH, we have succeeded in keeping them flat. If
the RH were not controlled, the backing would retard the emergence of
distortions, but it would not prevent them. The treatment can be applied
again if necessary.Balsa wood is commonly used in restoration. It is valued for its extremely
low shrinkage, its light weight, and its waterproof qualities. The kind used
at IRPA comes from Ecuador, and its weight varies from 80 kg m^23 to 290
kg m^23. The elements used were all of the same density, 170 kg m^23.
After experimenting with rectangular, square, and hexagonal
blocks, in radial and transverse conversion, we opted for 8 cm squares thatMaterials
Constraint and
Straightening of
Distorted Panels
B P P: R C 365