Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 151
side chains of the urushiol to produce peroxides.
These peroxides can then attack the ring
to produce free radicals which enter further
reactions or form crosslinks with other side
chains, both routes leading to a highly
crosslinked polymer. This method is used for
the first coats on metal articles but the method
normally used on organic substrates, including
wood, involves enzymatic polymerization. In
this method, lacquer is cured in a humidity
chamber via an enzyme catalysed reaction. At
20–30 °C, urushiol is oxidized in the presence
of laccase enzyme present in the sap to form
free radicals. These then react in a number of
ways to give a crosslinked polymer. High
humidity is required in this process for oxygen
transport. The relative humidity at which polymerization
occurs significantly influences the
qualities of the final product. During the ‘cure’,
which can take anything from a few hours to
several days, the net water content of the film
reduces to 1–3%. Part of this water is incorporated
in the molecular structure of the urushi
and cannot be lost without damage to the lacquer
coating.
The polymerization mechanism is complex
and not completely understood but involves
three important stages: first, chemical enzymatic
polymerization of the urushiol; secondly
some additional reaction of the polymeric
urushiol with the polysaccharides to form a
three-dimensional network and thirdly, physical
drying. The net result of the various reactions
is to produce a film which is densely
packed with grains of giant polymerized urushiol.
Each grain is surrounded with polysaccharide
and glycoprotein and ‘glued’ together by
polymerized urushiol and glycoprotein (Figure
4.7). The urushiol inside the grains is well protected
against degradation but material that
‘glues the grains together’ is more vulnerable.
At a macroscopic level, the final result of
applying a number of layers of lacquer is a
hard, transparent, brownish-black, lustrous,
durable coating of great beauty, the best of
which is highly resistant to water and organic
solvents and does not tend to dry up or crumble
with time. Many of the properties of the
cured film are comparable to modern plastics
such as melamine. However, the quality does
vary considerably depending on the quality of
the raw materials and the techniques used.
Lacquer articles incorporate other materials for
‘Glue’ consisting of
polymerized urushiol and
glycoprotein
Urushi grain
Irregularities are also present
in the cured film and may be
polysaccharides that were not
broken up when the raw sap
was processed into lacquer
Figure 4.7 Diagrammatic representation of the
morphology of a cured urushi film, characterized by
densely packed grains. The grains have a diameter of
around 0.1 μm and are bound together with a ‘glue’
composed of polymerized urushiol and glycoproteins
(Kumanotani, 1988)
strength and stability in the primary layers and
for the production of decorative effects in layers
close to the surface.
Applying lacquer to substrate
Metal, leather, ceramic, turtleshell, ivory,
horn and cloth have all been used as supports
for lacquer objects but the most common substrate
is wood. For flat objects, hinoki
(Japanese cypress) or Japanese cedar are typically
used. For turned objects a variety of timbers
(typically a ring-porous hardwood) is
used. Timber is meticulously selected to be
free of knots and other defects, with straight
grain and even texture, and is then carefully
prepared. Wood may be boiled to remove
resins, or smoke seasoned and is often left for
10 years or more after seasoning before use.
The final lacquer coating will add something