author’s studio. So far, the results of the structural conservation work look
very promising. The efforts made to ensure the long-term stability of this
panel painting will have likely been worthwhile (Fig. 18a, b).When a secondary support is attached to a weak, responsive panel, it
fulfills two functions. One is reinforcement, the other is restraint. Restraint
is the function that is potentially damaging and also the most difficult to
evaluate. It may be defined as the degree of rigidity required to resist the
bending force ofthe panel. If the resistance is too high, the panel may
be damaged.
A safe level of resistance could be calculated with basic engineer-
ing formulas if the panel’s bending force can be found, but this calculation
requires a figure for the modulus of elasticity (Evalue) across the grain of
the panel. Approximate Evalues perpendicular to the grain may be derived
from reference tables, but only for sound timber samples. For aged, stress-
weakened, or damaged timber, these figures are not relevant and cannot be
used. If the panel’s strength cannot be estimated, then it is virtually impos-
sible to calculate the rigidity of battens needed for tolerable restraint.
An alternative approach is to consider the problem from the point
ofview of reinforcement. This assessment can be made with the panel
lying horizontally over a central beam, with the battens providing the
rigidity necessary to support the panel’s weight without it deflecting too
far. Calculating reinforcement in this way is relatively easy, and the judg-
ments involved are not too demanding.
In practice, it has been found that a support with a flexibility cal-
culated for reinforcement also provides the safe level of restraint—a level
that was difficult to determine by other methods.
If battens, which have been made up to the calculated dimensions
with a uniform section, are now tapered in width from the center to the
ends, their rigidity will decrease progressively away from the center. The
bending force that the panel exerts on the battens also reduces progres-
sively from the center to the outer edges. Therefore, the resistance to
bending imposed by the battens on the panel will be balanced, producing
an even restraint across the width of the panel. As a result, when the sup-
port battens are attached to the panel, the tension on all of the retaining
blocks will be more equally distributed than if the battens were left as a
uniform section.
The deflection calculated for point loading at the end of the bat-
ten will increase by about 50% after the batten is tapered. This increase
does not constitute an error in the method of calculation, as it is compen-
sated for by the actual load imposed by the panel’s weight being uniformly
distributed, so that a corresponding reduction in deflection is produced.
With this method ofcalculation for batten dimension, the support
system has been applied to several panels that varied considerably in size,
weight, thickness, and timber type. In all cases, the measurable reduction
in curvature after the supports were engaged has been 30% or less. This
level ofrestraint is judged to be belowthe threshold where damage is
likely to be caused.
The support also provides a degree of reinforcement, enabling
thepanel to support its own weight and to be handled safely and with
more confidence.Summary of the Principles
of Calculating Batten
Flexibility398 Marchant