concentration for open joints and wider cracks. If the cracks are over a few
millimeters wide, rye flour can be added as a filler; if necessary, polyvinyl
acetate (PVA) dispersions can also be used to reduce the brittleness. A wet-
ting agent such as Oxgall can also improve adhesion when permeation is
not sufficient. Cascamite has a two-hour or longer working time, making
it very useful for working with the final alignment of deformations.
Cascamite is quite a brittle adhesive, although it is adequate to
cope with the natural movements of a panel if joined and used properly.
Subsequent applications are possible in inaccessible areas, so should
difficulties arise, it will rebond very well. Also, changes in RH should not
produce the same magnitude of dimensional change that proteinaceous
glues undergo, and it is not susceptible to attack by microorganisms.
Where possible, Cascamite is applied to both sides of the join. If
the join is partially sealed or only slightly open, the adhesive is applied
along the join back and front alternately, and the panel is flexed sideways
or up and down as much as the structure will allow without causing fur-
ther cracking. This action creates an absorption of the adhesive and expul-
sion of the air. A dabbing movement on the surface can also be effective.
In some cases it may be necessary to use another adhesive. PVA
dispersion emulsion (Resin W) is occasionally used; however, it is less easy
to work with than Cascamite, as it has a rather short drying time of ten to
fifteen minutes. The National Gallery’s Scientific Department frequently
re views new materials in search of alternatives. For larger cracks, wood
(preferably of similar age and type) may be inserted, with the grain run-
ning with the original.
Avariant of a widely practiced method used initially for trying
to correct warping and then for reinforcing cracks was used at the gallery
for a while in the late 1950s and early 1960s. A V-shaped router was made
and set to the desired depth to cut a groove along the line of the crack at
the back of the panel, removing the original wood. Another tool was then
used to produce a V-shaped wedge to fit into the newly cut channel,
eitherin long straight strips or in short strips if the cracks were irregular.
The idea usually behind this was to penetrate through to the back ofthe
ground and to produce two new side surfaces to bond to the V-shaped
wedge; this method is no longer used, however. In accordance with the
ideal of preserving as much of the original wood as possible, cracks are
joined edge to edge whenever feasible.
Cases involving insect attack or dry rot may require the removal
of the original wood to consolidate the panel; however, this procedure has
rarely been necessary on artworks in the National Gallery collection.Moisture treatments
After crack consolidation or release from previous restrictions (for example,
removal of battens or a cradle), a panel may adopt a greater concave or
conve x warp. It may be possible to reduce the warp by exposure to mois-
ture and relaxation under varying pressure over a period of days or weeks.
The low-pressure conservation table, using circulated moisture under a
controlled vacuum, is becoming an alternative for this treatment (Reeve
1984; Reeve, Ackroyd, and Wright 1988) (Fig. 3). This table and its use are
described in more detail below, in the account of the panel treatment for
Cosmè Tura’s Annunciation.406 Reeve
Figure 3
Multipurpose low-pressure conservation
table with small warped test panel during
moisture treatment.