crosspieces described above functioned adequately, they were substituted
with the type of secondary support first published by Ciro Castelli and
Marco Ciatti (1989). This system consists ofa strainer that follows the
perimeter of the panel exactly and, in this case, has two fixed crosspieces
and one fixed vertical member. The strainer is simply held in place by
springs, fixed to the strainer on one end and attached on the other end to
small blocks ofwood oriented in the grain direction; these blocks are, in
turn, spot-glued to the back of the panel (Figs. 31, 32). The spring is not
fixed rigidly to the small block but instead slides freely within a predrilled
hole, allowing for expansion and contraction of the panel, as well as con-
vex flexing or even straightening. The bottom edge of the strainer has a
small lip that protrudes to accommodate the thickness ofthe panel and
prevents the weight of the panel from fatiguing the springs over time.
This system offers several advantages over the traditional
Florentine-type crosspieces previously employed, especially in the case of
very thin panels. The strainer protects the fragile perimeter and offers
gr eater resistance to torquing and better overall stability, while more
closely approximating the original thickness of the panel and making it
easier and safer to handle. The system also reduces the surface area
adhered to the panel while distributing the support more regularly and
without adding any weight to be supported by the panel. It also allows
more localized, independent movement of any specific area of the panels,
and the spring tension can be calibrated to take into account the species of
wood, thickness, cut, degree of worm infestation, and past treatments.^4
Finally, ifthe screws that attach the springs to the strainer are
recessed, a lid or cover can be fitted over the back. Not only does this
cover offer protection, it also creates a microenvironment that can buffer
humidity fluctuations. Furthermore, silica gel tiles can be attached to
the inside ofthe lid between the various crosspieces. It should be noted,
however, that this solution makes no attempt to function as a climate-
controlled vitrine. There is no glass in front of the picture, and the sides
are not sealed. Prolonged exposure to low humidity will produce the same
effects as the absence of silica gel. However, the semicontrolled environ-
ment can constitute a substantial buffer to humidity fluctuations, even
eliminating movements of the panel that would be caused by daily humid-356 Bisacca
Figure 31, right
Reverse of the panel with the spring tension
strainer in place.
Figure 32, far right
Spring mechanism shown in place.