Series editors' preface - Wood Tools

442 Conservation of Furniture
5%, the wood will rapidly absorb most of the
water from the adhesive. This may result in
glue line starvation and subsequent joint failure.
As a result of the anatomy of wood and its
fibrous nature, wood surfaces are inevitably
rough. This may be compounded by the type
of tooling used in timber preparation. A surface
that has been prepared by a sharp plane or
chisel is characterized by cleanly severed fibres
and minimal dust contamination, whereas
sawn or sanded surfaces result in torn or
crushed fibres and dusty surfaces. Since a
strong adhesive joint requires the least
damaged surface, a planed surface is preferable
to a sanded one. If sanding is unavoidable,
then a finely sanded surface is preferable
to a coarsely sanded surface. Deliberate roughening
the surface, for example by toothing or
scoring the wood, damages the fibres and
creates voids that the adhesive must then fill.
Adhesives traditionally used for veneering,
such as hydrogen bonded animal/hide glue,
have poor gap-filling properties, so this treatment
may result in a weaker adhesive interface
than would have resulted if the surfaces had
been planed smooth. Adhesive penetration in
hardwoods may be limited to one or two fibres
deep, and the resulting adhesive joint may be
compromised if these fibres have been
damaged in the process of preparing the
wood.
The gap-filling properties of an adhesive are
determined by its cohesive strength, which in
turn is dependent on the strength and type of
intermolecular bonding. Adhesives that form a
covalently crosslinked three-dimensional
network within their bulk, such as epoxides
and formaldehyde based glues, have sufficient
cohesive strength to act as gap fillers. All the
solids present are involved in bond formation,
which results in minimal shrinkage on curing.
The adhesive layer may be brittle as a result of
crosslinking holding the molecules rigidly in
place (Skeist, 1962). Gap-filling glues are
unsuitable for many applications in cabinetmaking.
In chair joints, for example, brittleness
caused by the fixed crosslinked bonds means
the adhesive cannot flex when the furniture is
in use, the glue is liable to fracture and the
adhesive fails. Adhesives that do not crosslink,
such as animal/hide glue, are held together by
secondary bonding only and are poor gap
fillers as a result. In order to form a strong
adhesive bond the glue line for such adhesives
must be as thin as possible – typically a maximum
of 0.1 mm for joints in cabinetmaking
(Tout, 1992). Thus the adhesive used in structural
joints in furniture will be most durable
when the joint is close-fitting and the adhesive
has a degree of flexibility in use.
10.1.8 Selecting an adhesive
The material properties of adhesives are
discussed in Chapter 4. The factors involved in
selecting a suitable adhesive may include the
following: the degree of surface preparation
required, viscosity, bond strength, the loads
and stresses that will be placed on the joint,
durability in service in relation to relative
humidity and temperature, retreatability or
reversibility, open time, closed time, toxicity or
other health hazards, characteristics of cured
material, colour, chemical stability, water resistance,
gap-filling ability, formulation, the
nature of the substrate and the cost of the
adhesive product. Adhesives that cure partially
or wholly by loss of a volatile solvent, such as
PVACs and animal/hide glue, are prone to
shrinkage during curing although thin glue
lines will minimize the resultant stress in the
cured glue line. The effect of adding fillers to
collagen glue has been examined by Von Endt
and Baker (1991).
Animal/hide glue has proven to be very
effective and continues to be used by the
majority of furniture conservators for structural
repairs. It should be freshly made and can be
kept for several weeks provided that it is refrigerated.
The protein chains slowly break down
each time the glue is heated, especially when
it is overheated, which reduces the strength of
the glue. Whilst large restoration workshops
may keep a continuous supply of hot
animal/hide glue to hand, if use is intermittent
it is better to make small fresh batches as
required.
Although animal/hide glue is a poor gap
filler, freshly prepared glue will provide an
adhesive bond comparable in strength to an
epoxy. It is usually sold in the form of pearls,
which should be covered with water and presoaked
overnight then warmed the following
day at 55–60 °C. Water is added as necessary
to produce the right working consistency,