Sliding crossbeams
Many techniques for linking the crossbeam to the panel while allowing for
some freedom of movement have been devised by the artisans who origi-
nally made these supports.^31
Some sliding crossbeams were linked to the panel by means
of metal bridges that were nailed or screwed on the back face, as in
Botticelli’s Primavera. Other sliding crossbeams featured wooden bridges
that were both nailed and glued to the panel, such as Beccafumi’s Madonna
e santi(Fig. 21).^32
An ingenious system based on iron pins fixed to the panel and
passing through slots made in the crossbeam may be found in Fra
Angelico’s Annunciazione(Fig. 22). The pinhead (along with many carefully
applied nails) has been embedded lower than the front surface of the panel
and protected by means of wood dowels. The distance between the cross-
beam and panel is adjustable at the opposite end of the pin by means of
small metal wedges.
Another system replaces bridges with a pair of beams appropri-
ately shaped and nailed to the panel to serve as a guide for a sliding cross-
beam with a trapezoidal cross section; an example of this system is the
support of Matteo di Giovanni’s Madonna e santi(Fig. 23).Dovetailed crossbeams
Although they may be considered capable of “sliding,” dovetailed cross-
beams are described here separately. Dovetailed crossbeams (Figs. 24, 25),
which may have been derived from the technique traditionally used in
icons, began to be widely used for panel supports starting in the early
sixteenth century.
The dovetail joint ensures a positive grip between the panel and
crossbeam, allowing the two elements to slide reciprocally but not to
warp.^33 In addition, the resulting constraining forces are distributed evenly
along the crossbeam, rather than being concentrated at specific points , as
happens with nails or similar devices. Hence, there is a smaller risk of rup-
tures generated by concentrated stresses.^34H O P-M T C I 125Figure20, above
Drawings showing typical methods for clinch-
ing the nail point and insulating the nail head.
Figure21, below
Beccafumi, Madonna e santi,reverse, detail.
Collection ofChigiSaracini, Siena. Wooden
bridge ofasliding crossbeam.
Figure 22, below right
Fra Angelico, Annunciazione.Convent of
Montecarlo, San Giovanni Valdarno. Drawings
ofmetal pins providing a sliding connection
for a crossbeam.