Series editors' preface - Wood Tools

128 Conservation of Furniture
elevated temperatures and its presence in certain
polymers (e.g. PVAC and PVAL) even at
room temperature is undesirable and renders
them unsuitable for applications involving
prolonged periods of stress. The closer a thermoplastic
material is to its Tg at room temperature,
the greater the likelihood of creep.
As with all categories, the clear distinction
between thermoplastic and thermosetting tends
to break down if examined closely enough.
Linear polymers can oxidize and crosslink as
they age, becoming in essence thermosetting,
while even a highly crosslinked polymer such
as oriental lacquer (urushi) is somewhat thermoplastic.
Although linear polymers are generally
more soluble in various solvents than
crosslinked polymers some, such as high density
polyethylene, are insoluble due to very
high molecular weight (long chain length)
and/or crystallinity due to the arrangement of
the chains. Properties of polymer materials relevant
to their use as adhesives, coatings and
consolidants are further discussed below.
Chain branches and cyclic polymer structures
also strongly influence solubility.
4.1.3 Polymer materials history and
technology
Many purely natural polymers were traditionally
used to manufacture small objects by
moulding and pressing. Horn and turtleshell
are quite thermoplastic and were die-pressed
into complex shapes such as decorative snuff
boxes. When used as thin veneers they could
be made to conform to mouldings or turned
pilasters by gluing under heated cauls.
Most natural resins are also thermoplastic.
Shellac was used to polish turned wood articles
in India and Europe by simply rubbing
solid sticks of the material on the spinning
stock until it melted by the heat of friction. This
use apparently pre-dates its use as a solvent
varnish. Some of the most extensively
employed early thermoplastic materials were
various compositions used to imitate carved
detail on picture frames in architectural interiors.
This craft saw its heyday during the late
eighteenth and early nineteenth centuries
when ‘composition’ ornament makers used
mixtures typically consisting of rosin, linseed
oil, animal glue and whiting to mould-press
elaborate sculptural detail.
Ornament made from ‘composition’ was popularized
in Britain by the Adam brothers in the
last quarter of the eighteenth century. Their fine
and repetitive style of decoration was difficult
and expensive to reproduce by the traditional
method of carving in wood so they introduced
a way of manufacturing their designs more
cheaply from composition pressed into reversecarved
moulds. This method of ornamentation
was used extensively throughout the nineteenth-century
for picture frames, furniture and
interior decoration.
Gutta-percha, a natural latex tapped from
trees native to Malaysia, was first brought to
European notice in 1843. It was used to manufacture
picture frames, book covers and various
other decorative and useful objects during
the Victorian era. Although chemically identical
to natural ‘India’ rubber, it was rigid at room
temperature and mouldable at around 90 °C. In
fact, natural rubber and gutta percha are the cis
and trans isomers respectively of polyisoprene.
Imitation woods made from ground coconut
shell or other wood dusts mixed with gutta
percha could be mould formed or worked by
standard woodworking techniques.
In 1839, Charles Goodyear patented a
process by which the much more ubiquitous
natural rubber could be made hard by
crosslinking the polymer with sulphur. This
ebony-like material was variously called
ebonite, vulcanite and hard rubber. Linoleum,
patented in 1860, combined the toughness of a
crosslinked polymer (partially oxidized linseed
oil) with the thermoplasticity of a natural resin
(pine rosin). The mixture of these polymers
and various fillers produced a tough, attractive
and wear-resistant floor covering.
Bois-durci, an imitation ebony, was patented
in 1855 and used to produce small decorative
plaques for the furniture trade until the 1880s.
Made from sawdust and albumen derived from
blood or eggs, the protein binder was denatured
and crosslinked during the hot moulding
operation and so could be described as an
early thermosetting resin. Likewise casein, the
protein present in milk and cheese, was used
as the basis for a mouldable composition
patented in Germany in the late 1890s. Casein
continues to be used world-wide by button
manufacturers.
Because many natural polymers were highly
modified to achieve different properties, the