Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 159
glue line thicknesses for wood are discussed in
Chapter 10.
Starved joints
Obviously sufficient adhesive must be present
in a glue line to fulfil its intended function. A
starved joint will result where insufficient adhesive
has been applied to fill the gap between
adherend surfaces, or where the porosity of the
adherends has drawn adhesive away from the
join by capillary action. Adhesive viscosity
plays a large role in this mode of failure.
Adhesives with low viscosities are generally
unsuitable for use on porous substrates and
have no gap filling capability where mating
surfaces are less than perfect.
Roughening surfaces
An early, and apparently naive, theoretical
model for the interaction of adhesive and
adherend stressed the mechanical interlocking
of a dried adhesive with voids and undercuts
on the substrate surface. Intuition suggested
that a roughened surface would produce a
stronger join and toothing planes were traditionally
used by cabinetmakers to key up
mating surfaces. Modern studies have demonstrated
that this actually decreases adhesion in
wood by damaging the fibres and creating
loose detritus. The increase in surface area due
to roughness should create a stronger join, but
air trapped in voids tends to offset this effect.
Modern adhesive science has found very few
practical instances where increased roughness
aids in adhesion. Although toothed surfaces
may result in weaker joints, it is conceivable
that a toothed surface could promote a more
even glue line thickness by allowing excess
glue to flow away (Hawkins, 1986). Also, it is
perhaps worth mentioning that roughening surfaces
removes contaminants and will increase
surface area. Therefore while intentional roughening
may be a poor strategy for wood, it may
be more effective for other materials, such as
gesso-type grounds or metals.
4.5.1 Factors governing the choice of an
adhesive
Most adhesives have a characteristic open time,
which is the time that may be allowed to elapse
after application before assembly. The closed
time is the length of time that may be allowed
to elapse after assembly before final positioning
and clamping, after which assembly is either
impossible or the strength of the final bond is
adversely affected. Adhesives with very short
open times can be chosen where the assembly
job is simple or where clamping is not possible
owing to shape, fragility or other factors.
Conversely, adhesives with long open times are
necessary where the assembly job is difficult or
where elaborate clamping systems must be
used. Open and closed times may be affected
by factors such as the ambient temperature, and
humidity or the age of the adhesive. Most adhesives
have useful shelf lives of a few months to
a few years. The problem of short shelf life can
be addressed by only purchasing in small quantities
and only as much as is required for a particular
job. The shelf life of most materials can
be extended by refrigeration or hermetically
sealed storage. Liquid hide glues degrade continuously
in the bottle and become less effective
with age. Hot hide glues begin to degrade
as soon as they are hydrated and heated, and
should be made fresh at regular intervals, and
not overheated while in use. Information on the
preparation and use of hot hide glues is given
in Chapter 10.
A great number of factors can affect the
choice of an adhesive. These include shelf-life,
surface preparation required, viscosity, bond
strength, size of loads and types of stresses it
can withstand, durability in service (effect of
relative humidity and temperature), whether
reversibility and/or re-treatment are possible,
open time, closed time, health hazards, characteristics
of cured material, colour, optical saturation,
machinability, chemical stability, water
resistance, gap-filling ability, reversibility, formulation,
substrates, price and the pressure
under which components need to be held
together while the adhesive sets.
An adhesive must do its job under the operating
conditions or it is unsuitable regardless of
ideal characteristics of chemical stability and
reversibility. Reversibility is a conservation principle
that must not be seen in absolute terms,
but as a concept that is influenced by service
conditions. As already mentioned, a museum
conservator can often make more conservative
decisions because re-treatment in the event of
failure is feasible, whereas a more intrusive
treatment and inherently less reversible adhesive
may be necessary if conservation is likely