Series editors' preface - Wood Tools

150 Conservation of Furniture
Figure 4.5 The chemical structure of the main urushiol
constituents of Chinese and Japanese lacquer (Du,
1988). The zigzag line represents the C15 or C17 chain
attached to the primary urushiol molecule, and the
double lines indicate the position of double bonds on
these chains
(Kumanotani, 1983). The process of converting
sap to true lacquer is therefore very important.
Refining raw lacquer
The matured sap is filtered and then stirred,
first at room temperature then at 20–45 °C in a
specially designed open vessel. The conclusion
of the stirring is determined by the changing
colour and viscosity of the sap under treatment.
The refined urushi produced by this
process has a much reduced water content and
an appropriate viscosity for coating. During the
processing, several important things happen:
• Excess water is evaporated off, giving 2–4%
water content.
• The polysaccharides in the water droplets
are at first precipitated and then broken up
into fine particles which become dispersed
into the urushiol.
• There is partial polymerization of some of
the urushiol.
Urushiol polymer
Polysaccharide Polysaccharide associated
with glycoprotein
Figure 4.6 Diagrammatic representation of a proposed
structure for an urushi grain. The high molecular weight
urushiol polymer has outer polar hydroxyl groups.
Polysaccharides and glycoproteins are absorbed onto the
urushiol molecule to form an outer layer on each
urushiol grain (Kumanotani, 1988)
• The urushiol becomes grafted on to glycoproteins
and some of these urushiol glycoprotein
complexes join up with the finely
dispersed polysaccharides (Figure 4.6).
In practice different grades and qualities of
lacquer are prepared for use at different stages
in the application process.
Making a cured film
There is a great deal of variation in the technique
used. This partly reflects the requirements
of the final product and partly the
techniques used by individual artists. It is not
unusual for there to be 30–70 or more steps in
the production of a plain black lacquer article,
so only the main steps are reviewed here.
At each of the stages in the production of a
surface it is necessary to cause the lacquer to
harden. This is achieved in one of two ways.
In the first method, in which lacquer is baked
at 110–180 °C, oxygen reacts with unsaturated