Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 175
secretions of a scale insect Laccifer lacca
which infests a variety of host trees in certain
regions of Southern Asia. In India the principal
host is Butea monosperma. The raw lac product
consists of resin encrusted sticks and twigs
(hence stick lac) intermixed with insect
remains, dyestuff and other unwanted contaminants.
The first stage of refining traditionally
involved crushing this raw material, sieving off
the larger sticks and twigs and washing in
water to remove the dye. The dye was valued
for colouring leather and imparting a beautiful
red to silk and was precipitated from solution
by the addition of lime. When dry, the resin
was winnowed to remove small fragments of
stick. This produced the semi-refined seed-lac
which had a grain like appearance and varied
in colour from yellow to reddish brown
depending on the district in which it was cultivated
and the type of tree from which it was
derived. Further purification involved heating
long sausage shaped bags about two inches in
diameter filled with seed lac and gently
squeezing the molten shellac through the cloth.
It was then stretched into a thin sheet. The
thinner central portion of the sheet was broken
up into shellac flakes while the thicker edges
were mixed with the subsequent batch. The
indigenous use of shellac buttons for applying
a decorative glaze by friction heating was
assisted by the addition of other natural resins
particularly cheap and brittle rosin. A small
quantity of orpiment could also be added to
lighten the appearance of both flakes and varnish
and this made it slightly opaque. These
adulterations were likely to have been undertaken
before the shellac was exported but it is
also entirely conceivable that unscrupulous
importers or retailers could have further adulterated
their supplies. At any rate shellac as
such was shunned in favour of seed lac by
Stalker and Parker and their contemporaries.
The chemistry of shellac is extremely complex
and only recently understood. Raw lac
contains 70–80% resin, 6–7% wax, 4–8%
coloured matter and some moisture. The resin
fraction divides into a hard (ether-insoluble)
fraction and a soft (ether-soluble) fraction.
Shellac resin consists largely of low molecular
weight polymers (oligomers) formed by esterification
of polyhydroxy carboxylic acids with
one another. It is possible by rigorous purification
under laboratory conditions to obtain a
material that has been designated ‘pure lac
resin’. This is a fairly low molecular weight
(2100) polyester formed by reaction of aleuritic
acid and a sesquiterpene acid. In this esterification
reaction, aleuritic acid, an aliphatic polycarboxylic
acid related to the common fatty
acids, functions as the acid and sesquiterpene
acid functions as the alcohol. It appears that
the hard and soft resin fractions are essentially
similar but differ in the number of polymer
units (oligomers) present in the molecule.
Shellac also contains aldehyde groups which
may be gradually converted by oxidation to
carboxylic acid groups. As there are plenty of
hydroxyl groups available in the molecular
structure of shellac, it is possible that the esterification
process could continue in a shellac
film and that this could explain the gradual
reduction in alcohol solubility of shellac that
occurs over time.
Historically, shellac has been used both
alone and in combination with other resins. An
early reference documenting the use of shellac
by itself is in the hand written notes of tradesman
Isaac Byington from Bristol, Connecticut
(Byington, 1795). It is produced in various
forms and grades as buttons, pellets, and
flakes, seedlac, garnet lac, orange, blonde and
white. These are all essentially soluble in
ethanol or blends of alcohols being usually
made in about a 25% solution (roughly a 2
pound cut). The less refined shellacs have a
fairly dark hue while the most thoroughly
processed, blonde, is quite clear. Commercially
available white shellac is bleached through a
chlorination process resulting in a less stable
material both before and after its use (Flexner,
1994). Copestake (1992) has shown that decoloration
using activated charcoal results in a
much more stable product.
Shellac is important in furniture conservation
where it is often used for finishing new wood
that is being matched to an old surface. It has
several qualities which make it particularly useful
in conservation. It has desirable colour and
refractive qualities for wood substrates, is consistent
with the visual qualities of traditional
coatings, and can saturate and adhere well to
them. It is relatively easy to formulate and use,
has a short drying time, is resistant to abrasion
and chipping and can easily be redissolved in
alcohols. Although shellac does change chemically
over time, practical experience suggests