another. This could not be achieved if the battens were individually shaped
to the surface irregularities of the panel, a process that would create areas
of rigidity and weakness in the battens. Initially, therefore, they were made
identical—of uniform thickness and with a flexibility calculated to provide
reinforcement. Calculations were made on the basis of using ten flexible
battens, and it was decided to use one retaining strip on each of the six
boards. Sitka spruce was again chosen as the most suitable timber from
which to make the lattice.
With the layout for the main elements of the lattice decided, the
panel was then laid facedown on a horizontal surface with support to
maintain its camber established at 55% RH. The prepared battens were
laid across it at the chosen spacing and weighted to deflect into contact
with the concave back surface of the panel. With the top surface of the
highest batten as a datum, the others were raised to the same level using
suitable packers.
When all of the battens conformed to a uniform curved plane,
the retaining strips were laid across the battens at the designated spacing.
The retaining blocks, which had been prepared oversized (in terms of
height), with slots already cut, were reduced in height and their bases
shaped to suit the position in which they would be glued to the panel,
with the slots aligned to engage on the retaining strips. This was a tedious
process involving 132 blocks, but it was important that it be done accu-
rately so as to ensure that the retaining strips would slide freely into place.
The packers supporting the battens were removed and replaced
with a balsa thicknessing layer glued cross-g rain to the underside of each
batten. This layer was shaped to the surface profile of the panel. When
completed, the addition ofthe balsa was found to have no measurable
effect on the comparativeflexibility of the battens. The battens were now
all engaged by the retaining strips with a reasonably consistent contact
over the irregularities of the panel surface.
To complete the support, the battens were linked together with
two supporting strips to form a lattice, and an angle section of timber was
produced to act as a support f or the weak bottom edge of the panel. This
angle was glued and doweled to the tips of the lattice, with bamboo pins
cut from swab sticks as dowels.Framing and retention
Now that there was an even surface alignment of the battens, the produc-
tion and mounting of back springs was quite straightforward. The springs
consisted offlexible bridging strips mounted centrally on each batten with
Plastazote-foam-cored timber pads. The space available gave the springs a
span of more than one-quarter of the batten length.^11 The use of pressure
pads was unnecessary, as it was proposed to use a retaining bar that could
bear directly against the bridging strips (Fig. 14).
With the auxiliary support engaged, the panel’s restrained warp
was monitored until stabilized at 55% RH, and the edge profiles of the
panel were then recorded. A slip addition for the frame rabbet was made
to followthe panel’s profiles. Alterations were also made at the back of
the frame to build up the rabbet. These alterations provided greater depth
to accommodate possible increased curvature in the panel and support
assembly of up to 30 mm.T D F A A S 395