Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 149
layers of the ground. Normally the ground is a
highly prepared surface of gesso and bole.
Water gilding can be left matte or it can be burnished
to a very high lustre. Oil gilding cannot
be burnished in this way but can be lightly polished
with cotton wool to achieve a sheen.
Examples of the different types of gilding, with
their corresponding layer structures, are discussed
in Chapter 14.
4.4.6 Oriental lacquer (urushi)
The term ‘lacquer’ is sometimes rather indiscriminately
used to represent various different
materials including natural resin varnishes and
synthetic coatings based on polyesters, acrylics
and cellulose derivatives. Oriental lacquer,
which the Japanese call urushi, is, however, a
unique material. The main raw material used in
the lacquer process, that is the lacquer itself, is
made from the sap of several species of trees
of the family Anacardiaceae, the most important
of which is Rhus vernicifera (also called
Rhus verniciflua and sometimes considered to
belong to the genus Toxicodendron). Other
species of importance include Rhus succedana
from Taiwan and Vietnam and Melanorrhoea
usitata found in Thailand and Burma. Other
members of the same family growing in South
East Asia and other tropical zones produce
similar materials. These include mango, and
cashew which is used by some commercial
restorers and craftsmen working in Japan.
In China where use of the sap originated,
archaeological evidence has continued to push
back the earliest known date for lacquer items
at least to about 700 BC (Kuwayama, 1988). In
1978 there were estimated to be 410 000 000
lacquer trees in China with an annual production
of some 2 000 tons of lacquer. About 250 g
of sap are obtained from each tree. This huge
output reflects the use of lacquer in China as
an industrial plastic used in the manufacture of
insulators and oil pipelines as well as domestic
wares and art objects. In Japan, where the
use of lacquer is confined to art objects and
domestic wares, production in the same year
was estimated at five tons.
Lacquer has been used in the production of
a wide range of decorative art objects, sculptures
and buildings. Variations in method are
found all over Asia but the technique was
brought to perfection by the Japanese. Rhus
species are relatives of the sumac and poison
ivy and are toxic, producing in most people an
allergic reaction with skin rashes and blistering
which can be severe. Once cured, however,
the risk of unfavourable reaction is very slight.
Cashew lacquer, a less toxic substitute for
urushi lacquer, has been marketed as Polycite
by the Mitsubishi Petrochemical Company.
Preparing the lacquer
The first stage in the production of lacquer is
obtaining the raw materials. Sap is collected
from mature trees through incisions made in
the bark (Quin, 1882). When first collected, the
sap is a double emulsion of water in oil in
water containing 27–50% water (Figure 4.4).
During maturation the sap converts to raw lacquer,
a water in oil emulsion. Raw lacquer contains
urushiol (60–65%) and glycoprotein
(2–5%) in the oil phase of the emulsion and
polysaccharide (5–7%), laccase enzyme (1–2%)
and water (20–25%) in the water phase.
Chemically, urushiol is a catechol – a benzene
ring with two adjacent hydroxyl groups –
which can have a variety of C15 or C17 carbon
chains attached to it (Du, 1988) (Figure 4.5).
Raw lacquer is used as an adhesive for the
priming layers in the manufacture of lacquer
objects. However, if sap is used for the upper
layers as well, the coating will be found to
have very changeable properties and is not at
all durable. The mechanical properties of a
coating made with refined urushi remain
unchanged over long periods of time
Water Oil Water
Figure 4.4 Diagrammatic representation of the double
water-in-oil-in-water emulsion nature of lacquer sap
when first collected