This type of crossbeam typically featured a trapezoidal cross sec-
tion, inserted in grooves with a corresponding cross section forming a sort
ofdovetail joint (grooves are mortised across the grain into the planking,
as deep as approximately one-third of its thickness).^35 This crossbeam type
was also widely known to have a longitudinal taper, which made it pos-
sible to tighten the dovetail joint simply by displacing the crossbeam along
its axis; adjacent crossbeams were placed with the larger ends oriented
toward opposite edges of the support (Fig. 25).Glued crossbeams
Because glued connections are very stiff, tw o boards glued with their
grain directions perpendicular to each other develop very high stresses in
response to even small moisture changes. Therefore, glued crossbeams are
seldom found. There are some cases, however, in which complex structures
with cross-grain elements glued together behave fairly well over time.Interlocking crossbeams
In some cases, where distance, exceptional size, or other reasons would
make transportation of large polyptychs from the workshop to the church
too difficult, the painting would be made in sections for assembly in situ.
For instance, Bomford and coworkers (1989) describe Ugolino di Nerio’s
altarpiece from Santa Croce, whose surviving fragments are scattered in126 Uzielli
Figure 23
Matteo di Giovanni, Madonna e santi,reverse,
detail. Cathedral of Pienza. Two wooden bars
stabilize a rudimentary sliding crossbeam.