Series editors' preface - Wood Tools

458 Conservation of Furniture
10.2.2 Cleaning joints after dismantling
If the joint has come apart at the glue line and
the components are intact, the old degraded
glue must be removed to provide a clean
surface for a new adhesive bond. The glue may
be scraped or chiselled away but care should
be taken to avoid removing wood because this
will result in a loose joint.
Old animal/hide glue may be softened by
the application of a water poultice using dampened
newspaper, cotton wool, methyl cellulose
or Laponite RD ® (see section 11.7). Application
of these materials should result in softened and
easily removable glue within 10 minutes to half
an hour. A 2% Laponite suspension has a pH
of around 9.8, which will aid in breaking down
the glue but will adversely affect many surface
finishes such as shellac or oil paint. Care
should be taken to prevent contact between
finished surfaces and the poultice.
Reversal of many synthetic woodworking
adhesives may present a hazard to the object
or the conservator. For example, although
epoxy resins can be softened by heating to
100–160 °C, by prolonged exposure to
dichloromethane or hot dimethyl formamide,
all these procedures entail a significant hazard
to the object and/or the conservator. Casein
adhesives that have not crosslinked by the
inclusion of formaldehyde in the original application
may be softened or dissolved by an
aqueous solution with an elevated pH. If the
pH required proves excessive or there is a risk
of darkening the wood, enzymatic treatment
with pepsin or trypsin may prove effective.
White proprietary PVAC-based wood adhesives
are often insoluble in organic solvents, though
they may be softened by exposure to water
and/or heat. Howells et al. (1984) has
suggested that crosslinking of some white
glues occurs after thermal ageing. Urea and
resorcinol formaldehyde adhesives are insoluble
in organic solvents and water and are not
softened by heat.
10.2.3 Repairs after insect infestation
A component or object that has been infested
by an insect, e.g. Anobium punctatum, will
suffer a loss of strength proportional to the
amount of wood destroyed by the larvae. If
severe damage has occurred, it may be neces-
sary to impart additional strength to structural
components. Structural repairs may include
consolidation (see section 12.2.3) and/or reinforcing
the structure by introducing dowels or
false tenons. Whilst a straight-grained wood
such as beech has been the traditional choice
for dowels, some have used more robust materials,
such as stainless steel studs (threaded
rods), glass fibre/epoxy dowels or Kevlar ® /
nylon dowels (Augerson, 1999). If such strong
materials are used, it is important to consider
how the object is used and where stress will
be concentrated. Stress from gravity or normal
use that runs parallel to the dowel (i.e.
compression stress) may be unlikely to cause
damage to the original in future. When stress
is directed at an angle that differs from the
direction of the dowel (i.e. shear stress), stress
may concentrate in an adjacent weak area. This
may eventually cause a second break that
significantly extends the original area of
damage. Where the material used is less
compressive than the wood and fluctuations in
RH are not controlled, the wood that encloses
reinforcing dowels will be subject to compression
set, weakening the adhesive bond in the
long term. Thus whilst structural repairs should
aim to combine durability with fitness for
purpose, conservators should consider the
potential for, and effects of, future failure of
their repair.
In cases of severe structural damage, it may
be necessary to replace the entire component
or, if the surface is valued, its core. The extent
of reinforcement or replacement may be determined
by balancing the loss of original material
against the risk of further damage to the
object or injury to people. Glue blocks in a
carcase or corner blocks in a chair seat may
need to be provided or renewed. Where
upholstery has been renewed many times in
the past it may be necessary to build up or
replace, in part or in total, some components
(e.g. rails) to ensure new fixings are secure. In
some cases fragile components may be reinforced
by the additional of new material
behind the original.
Where extensive support is required, this may
be provided by an independent structure capable
of being incorporated, and if necessary
subsequently removed, with minimum damage
to the object. The aesthetic impact of such structures
should be minimized as far as possible.