Solutions
Synthetic resin solution adhesives are not widely used for fabrication in
woodworking, but they remain a vital tool for the conservator.
Awide range of synthetic resins is available, and individual acrylic
resins (or blends) possess particular characteristics. Of these properties,
the two most important are solvent specificity and long-term stability.
Resin solutions are themoplastic solutions that dry through solvent evapo-
ration and, depending on the formulation, can remain resoluble for a
longer period of time. The stability of certain acrylics has been well docu-
mented in conservation literature.
Two synthetic resin adhesives—cellulose nitrate and cyanoacry-
late—are not used today for conservation but may be encountered in ear-
lier, inept repairs. Cellulose nitrate adhesive is a solution of nitrocellulose
and other film-forming materials in a mixture of organic solvents. This
adhesive dries solely through solvent evaporation. Cellulose nitrate is
not very effective as a bond-forming material with wood and therefore
is almost never used as a primary adhesive in wooden objects. It is an
unstable material unsuitable for use in the conservation treatment of any
wooden artifact (Koob 1982; Selwitz 1988). Cyanoacrylate hardens quickly
through an anaerobic chemical reaction with nitrogen in the atmosphere.
Briefworking time, poor adhesion to wood, and long-terminstability ren-
der it unsuitable for use by the wood conservator.Hot melts
While most thermoplastic materials could be broadly classified as hot-melt
adhesives (e.g., hide glue, which begins to harden by cooling, and shellac
and acrylics, which can be used as melted resins), this section will touch
on those materials specifically designed to be used in a molten state and
that harden solely by cooling. Hot-melt adhesives, as defined in this sec-
tion, come in a wide variety of compositions the formulations of which
can be very specific regarding the properties of the adhesive, not only in a
solid but also in a liquid state (Gutcho 1983). Because these adhesives must
often be heated to well above room temperature for them to flow, and
because they solidify by cooling, their use is limited to the penetration
possible in a very brief period of time.
Hot-melt adhesives are becoming increasingly important in the
industrial fabrication of wooden objects and are beginning to be used in
the conservation of historic wooden artifacts. However, knowledge of hot-
melt adhesives within the conservation field is limited, and little critical
study has been made oftheir long-term stability and other properties.Multiple-component reactive adhesives
Thermosetting, multiple-component adhesives are likely to be encoun-
tered by the conservator only in previous, ill-advised repairs. These materi-
als, which harden by the chemical reaction of the various components,
include urea-formaldehyde resin, epoxies, and phenolics.
Multiple-component reactive adhesives possess great strength
under a wide variety of conditions and can be virtually impervious to
thermal, physical, or chemical attack. Because of their hardening mecha-
nism, there are varying amounts of dimensional change from class to
class—that is, epoxies shrink very little, whereas ureas shrink considerably
more. As such, they may be good gap fillers, either in their raw state or
when modified with bulking agents.82 Williams