Page 114 - The Building Conservation Directory 2013

1 1 4 t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 T w e n t i e t h a N N i v e r s a r y e d i t i o n
3.3
Structure & Fabric :
Metal,
Wood & Glass
to colour the glue line to match the object.
Quick-setting epoxy resins can prove useful in
a situation where effective clamping can only
be achieved by hand.
Fish glue and liquid protein glues applied
cold are reversible and convenient but are not
reliable unless environmental conditions are
good.
PVA is only semi-reversible in practice,
has poor gap-filling properties and should not
be used if a joint is under shearing pressure
as the glue line may creep. An aliphatic resin
(a common constituent of wood glue which is
more rigid than PVA) is a better alternative.
Typical defects and
causes of damage
Damp
The greatest enemy of carved joinery is damp
due to poor building maintenance. Damp can
lead to decay by wet rot, dry rot and insects
such as woodworm. It can cause the warping
of timber and the failure of protein glues.
Damp may still be a problem in some historic
buildings and should be addressed by a good
building maintenance programme, before
attempting other conservation work.
Low relative humidity
In many historic buildings today, better
maintenance and central heating mean that low
relative humidity is a very common problem.
This leads to cracking and warping, loose joints
and desiccation of glue. A relative humidity of
between 40 and 65 per cent is best, and ideally
the temperature should be no more than seven
degrees above external conditions. However,
this is likely to prove unacceptably cold at times
for the occupants of the building. There are
several possible solutions to the problem. For
further information consult English Heritage’s
Environmental Management Performance
Standards (see Recommended Reading).
Lighting
Unsuitable lighting can result in an unhelpful
increase in temperature near a carving.
Sunlight and strong artificial lighting
can cause bleaching to timber or cause
deterioration of surface finishes. Fitting
a window film that provides ultraviolet
protection can be an effective solution.
People
Woodcarving is subject to depressingly
regular damage and loss at the hands of
cleaners or contractors working in historic
buildings. People are often unaware of its
vulnerability. A short induction and written
guidelines for contractors stressing the
historic value and fragility of the fittings will
go some way to addressing the problem where
access cannot be restricted.
Insect attack
Damage by wood boring insects such as
furniture beetle, house longhorn beetle or
deathwatch beetle can cause considerable
damage. The drier conditions found in most
buildings today are less attractive to these
insects but if new flight holes or the insects
themselves are spotted, seek advice promptly.
Poor original design
Damage can also be caused by a failure of
short grain (where the cut ends of the grain
are exposed) on pieces of timber under stress,
or fixings that do not allow for movement in
the timber.
Treatment options
Cleaning
The surface finishes on carved joinery vary
widely and it is unwise to assume the piece
retains its original finish. Early carving
that was originally painted and gilded may
have been stripped and treated with wax,
linseed oil, French polish or oil varnish. Test
cleaning and inspection under ultraviolet
light for variations in fluorescence can help
to identify varnish layers and components
such as natural resins and drying oils. Once
the identification of the surface to be cleaned
has been made and the remains of any earlier
schemes noted, it should be possible to
formulate a suitable cleaning method. If a
piece retains its original finish this should be
preserved with great care.
Consolidation
If the timber has suffered extensive insect
or fungal attack the careful injection of a
consolidant may re-establish the structural
integrity of the carving or impart enough
strength to allow the piece to be re-backed
with timber. Two-part low viscosity epoxy
will impart considerably more strength to
the object than solvent-based consolidants
and will be easier to apply effectively. The risk
of spillage onto the surface of the piece can
be reduced by filling any insect flight holes
with wax of an appropriate colour before
consolidation. Paraloid B72 can be useful for
surface consolidation. Conservators should
be realistic about the practical reversibility
of anything other than surface consolidation
whichever consolidant is chosen.
Fillers
Wax can be a useful filler for small cracks
and gaps in situations with unpredictable
relative humidity, where there may be the
potential for excess movement. In more
stable situations a narrow crack or joint
injected with glue then filled with wax may
sometimes be less visually obtrusive than
a colour-matched wooden fillet. Wax is an
ideal filler for insect flight holes. Good results
can be obtained using microcrystalline wax,
which is less sticky than beeswax and gives a
harder finish. The wax should be melted and
tinted with good quality oil colours to match
the colour of the surface to be filled. It can
be scored into thin strips and these can be
worked with the fingers until soft enough to
apply. It then can be levelled to the surface of
the work with a plastic scraper.
Epoxy filled with microballoons can be
used in a situation where the surface of a badly
decayed carving is intact and re-backing the
piece with new timber is deemed impractical.
This will not be reversible but it may
strengthen the piece sufficiently to save it.
Epoxy microballoon fill and dowel, and (right) the two halves combined
Carving replacement after consolidating charred
area with Paraloid B72
The use of unglued wooden biscuits to hold a two-
piece spandrel panel in line while allowing movement