Series editors' preface - Wood Tools

174 Conservation of Furniture
the water evaporates. Casein glues were the
most water-resistant glues available until the
introduction of other thermosetting adhesives
in the 1930s and 40s but they are not waterproof.
Casein adhesives were popular before
emulsions were available and may be found in
small shop construction and do-it-yourself
repairs. Due to their high alkalinity they bond
very well to oily and resinous woods such as
teak, yew and resinous pine. They are hard
and creep resistant with good machinability.
Their open time is intermediate between hide
glues and other thermosets. Toxicity is low but
their strong alkalinity can cause staining on
some woods, such as oak and maple). Their
light tan colour may be an advantage or disadvantage
depending on the application. They
are insoluble in organic solvents and only
resoluble in strongly alkaline solutions.
Similarly, casein based paints tend to form very
durable coatings that are not easily redissolved.
4.7.5 Natural resins and lacquers
Natural resins form a chemically diverse group
of water-insoluble materials secreted or
excreted by plants. Most resins belong to the
class of chemicals known as terpenoids which
are formed from units of the compound isoprene.
Isoprene has the empirical formula C5H8 and the terpenoids are classified according to
the number of carbon atoms in the structure of
their compounds as follows: monoterpenoids
(C10), sesquiterpenoids (C15), diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30), carotenoids (C40). Higher molecular weight
compounds of isoprene form polymers (poly
isoprenoids) with the general formula (C5)n. Resins from living plants are collected by intentional
tapping or slashing. Copal resins, the
semi-fossilized products of long dead trees,
and amber, a fossil resin aged over geological
time scales, are collected by surface prospecting
or mining. Shellac, an insect resin, is
secreted by tropical scale insects that feed on
various trees found in India, Burma and
Thailand.
Mono and sesquiterpenoids form the main
components of the essential oils of plants and
are responsible for the initial fluidity of the otherwise
solid and involatile di- and triterpenoids
as they occur in nature. Such mixtures, classified
as oleo-resins and balsams are sometimes
used as they occur (e.g. Venice turpentine,
Canada balsam) but are more frequently
treated by distillation or other processes to separate
the oil component from the harder resin
component. Oleo-resins from pines are distilled,
yielding a hard resin known as rosin or
colophony and a volatile essential oil known as
spirits-of-turpentine or gum turpentine.
Natural resins have been relatively little used
as adhesives in recent times and, like waxes,
are now more commonly encountered as coatings
than as adhesives. However, mixtures of
waxes and resins such as beeswax and dammar
have been widely used as adhesives (e.g. for
lining paintings) and solid shellac resin has
been used as a thermoplastic gap-filling adhesive
in situations such as the joining of a
ceramic knob to a metal screw post for example.
Examples of primary natural resins used as
transparent coatings on wood include sandarac,
mastic and shellac. Oil soluble rosin could be
combined with a drying oil in a heated kettle
to make common varnish. Oleo resins have
also been added to harder resin compositions
to soften or plasticize them and increase toughness.
More expensive varnishes with greater
final hardness were often made with hard fossil
resins by first subjecting these insoluble
resins to a destructive distillation process called
‘cracking’ after which they could be combined
with oil. Oil-resin varnishes were normally
thinned for use with either turpentine or petroleum
solvents. While they are soluble in a variety
of polar and non-polar solvents, resins are
by definition insoluble in water. Gums, however,
are water soluble. Many resins, such as
‘gum mastic’, are misnamed since they are not
gums. The natural resins become less soluble
over time and some may be capable of
crosslinking. Solid shellac sticks called burn in
sticks have also been extensively employed by
furniture restorers to fill small areas of damage.
A summary of the main resins is given in Table
4.3 on page 176. For further information the
reader is referred to Gettens and Stout (1966),
Mills and White (1987), Horie (1987) and
Thorpe and Whiteley (1937). Natural lacquers,
film forming emulsions obtained from plants,
are discussed in section 4.4.6.
Shellac
Best known as the principal ingredient of
French polish, shellac is obtained from the