Natural resins
The use of natural resins as adhesives is not prevalent today; they are,
however, encountered in historic objects.^1 Probably the most extensively
used resin was shellac, a thermoplastic exudate of the bug Laccifer lacca,
indigenous to India and Indochina. The resinous exudate is refined by a
number ofprocesses, including heat, solvent, and aqueous extraction,
resulting in an amber-orange material of varying purity and composition,
depending on the specifics of the refining process. From the experience of
the author, shellac was primarily used to glue nonwood veneers to a
wooden substrate.
Shellac is normally used in solution with alcohol and dries by sol-
vent evaporation, although it can be used as a pure material liquefied by
heat only. It is relatively incapable of resisting thermal or chemical attack,
but under the proper conditions, it can remain stable indefinitely.
Contact adhesives
Contact adhesives form an immediate strong bond and therefore are also
called contact cements. Contact adhesives include both natural and synthetic
rubber in solution. They are thermoplastic and can be softened with heat
and/or organic solvents.
Due to their primary function as laminae adhesives, the most
common application of these adhesives is to glue wood or other veneers
to a substrate. They may also be encountered in an earlier, inept repair to
structural elements. These adhesives do not appear to be exceptionally
stable over a long period of time (Feller and Encke 1982). Deterioration
of the adhesive results in the delamination of the fabricated structure.
Synthetic resin adhesives
Emulsions
The most widely used general-purpose glues in the wood crafts today
are those based on aqueous emulsions of polyvinyl acetate (PVA) and are
commonly called “white” glues. Some closely related adhesives generically
called “yellow,” “aliphatic,” or “carpenter’s” glues may also be used. These
water-based emulsions are opaque white or yellow liquids that become
translucent when dry. Depending on their formulation and environmental
influences, these adhesives can remain stable for long periods of time, as
well as remain soluble and reversible to some degree.
PVAemulsion is the most common adhesive used for fabrication
in contemporary woodworking. It is also used with moderate frequency in
conservation. When the need arises, PVA and other emulsions bond well
to hide glues.
In many respects, acrylic emulsion adhesives are much like PVA
emulsion in appearance, use, and hardening mechanisms. While not
widely used in the nonindustrial fabrication of objects, acrylic emulsions
are used in conservation for the same applications as PVA emulsion. The
advantage of acrylic emulsions is that they can be obtained in a wide vari-
ety offormulations with specific properties, such as molecular weight
ranges and solubility characteristics for a hardened film.
A S A W C 81