116 Uzielli
However, in many cases no specific ar rangement according to growth
rings can be observed. Boards with “inner” faces oriented one against the
other (one toward the front and one toward the back of the panel) have
also been noted (Fig. 5).
This question of board arrangement may be less critical than it
appears at first, since the effects of other distortion factors (i.e., the tem-
porary cupping caused by mechanical or hygroscopic asymmetry and the
permanent cupping caused by what Buck has termed “compression set”)
are superimposed and may even prevail over the cupping caused by the
wood’s transverse anisotropy (Thomson 1994; Uzielli 1994).Avoiding and repairing wood or board defects
Even the most carefully built fourteenth-century panels, which are charac-
terized by the great care that was taken in wood selection, contain some
defects, suggesting that the use of some boards with defects was consid-
ered acceptable.^10 The most frequent wood defects found in panels are
pith, knots, and grain deviations, whereas board defects relate mostly to
wane appearing on the back face.^11 The presence of wane shows that
boards have been used at the maximum of their available width and that
sapwood is present.^12
In addition to the gluing of cloth over the defective area before
the ground layer was applied, a number of other measures were often
taken to prevent or, at least, to reduce the negative consequences of
defects in selected boards. Knotholes and similar cavities were plugged
with a paste made ofglue and sawdust (as Cennino recommends) or with
tightly embedded wooden plugs placed with their grain parallel to that of
the board (Fig. 6). If wane or a relatively large decayed or defective area
were present on the front of the panel, a flat surface w as sometimes recon-
structed before the application of the ground layer. In the thirteenth and
fourteenth centuries, such reconstruction was usually accomplished by the
precise embedding of small boards (Fig. 7). Later, various materials were
used to plug the voids, including glue paste with sawdust or vegetable
fibers (Del Serra 1994).
With respect to widespread incipient decay in boards selected for
panel making (the decay of wood in the panel after painting is not dis-
cussed here), there is a possibility, which has not been unequivocally
confirmed, that boards affected by some early stage of fungal decay
(e.g.,boards recovered from other uses or boards left exposed to weather)
may have been purposely used for panel construction to take advantage of
their reduced shrinkage and swelling.^13Figure 5
Cross section, obtained by computer tomog-
raphy, of a panel painting made from two fir
(Abies alba Mill.) boards, probably cut from
the same log, with “inner” faces oriented one
against the other (one is placed toward the
front and one toward the back of the panel).
Madonna con Bambino,twelfth or thirteenth
century. 127 3 65 cm. Convento Suore
Agostiniane della Croce, Figline Valdarno.