Series editors' preface - Wood Tools

358 Conservation of Furniture
~CH 2 – CHCI – CH 2 – CHCI~
→ ~CH = CH – CH = CH~ + HCl
(a)
(b)
Figure 8.15 A chain unzipping reaction
(a) Degradation of polyvinyl chloride – the splitting off
of one hydrogen chloride (HCl) molecule from the
polymer backbone encourages adjacent (HCl) molecules
to split off, ‘unzipping’ the polymer chain. As the HCl
molecules break, a series of conjugated double bonds
form within the polymer chain. As the length of these
sequences increases, the colour of the polymer shifts
from white to yellow, red, brown and black (Shashoua,
1996)
(b) Detail of ‘Blow’, an inflatable chair designed by
Design Studio, Milan (1967) and manufactured by
Zanotta Poltrone from PVC. Yellowing is evidence of
the process of polymer degradation
(1996). PVC should be stored in stable conditions
of temperature, free from dust and as
cool as possible between 40–55% RH.
Cellulose nitrate
Cellulose nitrate has been used in upholstery
components but is also important to furniture
in the form of lacquer finishes and continues
to be used as a conservation material for the
coating of metals and as an adhesive used
with stone, metal, ceramics and glass despite
the fact that it is potentially a very unstable
material (Selwitz, 1988). It degrades at room
temperature by a combination of oxidation
and hydrolysis which are catalysed by the
presence of acid impurities and accelerated by
light. These reactions result in a reduction of
molecular weight and in the production of
nitric oxide and nitric acid. Progressive yellowing
to dark brown occurs and the changes are
accompanied by foul odours of nitrogen
oxides. Moisture causes hydrolysis which leads
to the emission of nitric acid (HNO 3), thus
accelerating degradation. Cellulose nitrate is
inherently brittle in the absence of a plasticizer
and camphor is commonly found to have been
used for this purpose. Loss of plasticizer,
frequently evident from the odour of camphor
which has migrated to the surface, is a
common form of deterioration for this material.
Degraded cellulose nitrate may be
discoloured, cracked, warped, crazed, sweating
or sticky with droplets of acid and can
disintegrate into a heap of crystalline material.
Objects which contain cellulose nitrate should
not be enclosed but should be stored in cool,
dark, generally dry, well-ventilated conditions
to allow air to circulate freely and to vent
gases formed by decomposition. Williams
(1988) and Reilly (1991) have both published
excellent guidelines for the storage of this
material. For further information on the degradation
of cellulose nitrate lacquers used on
furniture see Calhoun (1953), Hagar (1983),
Johnson (1976), Koob 1982, Selwitz (1988) and
Shashoua et al. (1992).
Trimmings
Textile trimmings are vulnerable to mechanical
abrasion when furniture is in use, particularly
along front seat rails and crest rails.
Textile trimmings which are part of window
furnishings, for example pelmets and curtains,
are particularly vulnerable to light damage, the
results of which can be seen in tendering and
weakness. The stresses between the different
materials constituting the trim may accelerate
deterioration, for example stress on the
threads from which a heavy tassel or bead
hangs. Threads such as silk floss wrapped
around thin strips of vellum or wood moulds
often wear away along protruding or sharp
edges. The rate of degradation of the thread
may be accelerated by the nature of the material
which it surrounds (the alkalinity of vellum,
the acidity of wood). Insect infestation can be
a problem as deep fringes provide an ideal
breeding and feeding ground. Additionally, the
wooden moulds which are a basis for many
trimmings are prone to furniture beetle attack.
The larval feeding activity may cause the
mould to collapse. The emerging adults cut
holes through textile coverings which may
lead to rapid unravelling. Adhesives holding