268 PAINTING MATERIALS
Much the larger part of wood treatment as it has to do with the art of painting
is restorative. In the preparation of panels there are only the familiar processes
of joining members or of adding cleats, dowels, or splines to such joins. For pur-
poses of restoration, or to prevent or arrest the deterioration of wood, fumigation
is often employed. As a rule, the object is placed in an air-tight chamber and there
exposed to the fumes of a nature toxic to larvae and usually to fungi. Such fumi-
gants are carbon disulphide, carbon tetrachloride, formaldehyde, ethylene di-
chloride, and ethylene oxide, among others. Where wooden objects are stored or
exhibited in tight cases repellants like paradichlorobenzene are often kept with
them.
The treatment of weakened wood by impregnation is a method as yet little
used for panel paintings. Where it has been used, wax or a combination of waxes
and resins is made the impregnating material. This is forced into the wood by
heat or is carried in by absorption when dissolved in a solvent. In most instances,
such treatment is effective rather in sealing the wood structure and in making
the outside more firm than in actually strengthening it, for the depth of penetra-
tion is not great and the cavities in wood cells are rarely reached. Thorough
impregnation could only take place with immersion of the panel in the dissolved
material or with keeping it at a temperature above the melting point of the
impregnating material for somewhat prolonged periods. Either of these carries
with it a risk to the paint structure, which has kept it from being generally
adopted.
Various types of accessory supports have been attached to wood panels in an
effort to strengthen them or to keep them in an even plane. The most simple are
cleats or braces either glued to the reverse or mortised in. Occasionally a solid,
checked-out lattice of new wood has been glued on. This has the disadvantage of
putting the panel under great restraint and with the expansion and contraction
that is inevitable from changes of atmospheric humidity, the panel itself is apt
to check or crack. The attachment called a * cradle ' is designed to prevent this. It
is composed of longitudinal members set at intervals in the same direction as the
grain and glued in place. On the edge of these members, next the panel, notches of
uniform size are cut so that another member, transverse, can slide in them. These
transverse members are not attached and the cradle as a whole is expected to leave
the panel free to expand and contract without warping. Although effective in
many cases, the cradle must have favorable conditions or it, .too, becomes fixed
by cramping and endangers the panel as does a lattice.
For any accessory support to be effective, the wood of the panel must be strong
enough to carry it. When the wood is much weakened by worms or by dry rot,
another type of treatment is necessary. Removal of the weakened wood and re-
placement with a luting material is possible where deterioration is confined to
limited areas. If the panel is largely honeycombed by worm tunnels or desiccated
by fungi, the painting will probably have to be rebacked or transferred. In the