Series editors' preface - Wood Tools

202 Conservation of Furniture
The shells vary greatly between species in
colour and pattern of markings and to a lesser
extent also within species (Richie, 1970). From
the Age of Discovery (the end of the fifteenth
century) onwards, marine turtles supplied a
beautiful and expensive alternative to horn
which was used for many of the same
purposes. The hawksbill turtle, with its striking
gold and brown mottled back plates up to
3.5 mm thick, was particularly sought after for
combs, jewellery and as a furniture veneer.
Turtleshell veneer was used extensively as an
overall covering for case furniture, boxes and
picture frames during the seventeenth century.
Marquetry panels of interspersed turtle shell
and brass are most closely associated with
André Charles Boulle, and the generic term
boullework came to describe the later reproductions
of this popular technique. Turtleshell
veneers were made from the back plates of
the turtle by softening them in hot water,
pressing them flat, and shaving them to final
thickness. Turtle shell veneers were commonly
adhered to carcass work with a mixture of
glue and a pigment such as vermilion that
would show through the translucent shell.
Hunting of the Hawksbill turtle for its shell has
brought it to the brink of extinction. Farmed
turtle shell is available from the Cayman
Islands (Cayman Islands Turtle Farm, PO Box
645, Grand Caiman, British West Indies,
www.turtle.ky/).
Horn
Horn, like ivory, is non-living, non-nucleated
tissue but unlike antler, ivory or bone, horn is
almost entirely composed of protein and has
only a small mineral content. Horn varies
widely in colour from black through reddish
brown to green and white and mixtures of
these colours. The natural surface usually
appears fibrous parallel to the long axis of the
horn. Horn has been readily available from
domestic animals and was adaptable to many
of the roles now filled by modern plastics.
Horn veneers required more steps in preparation
than tortoiseshell (Figure 5.10). The
conical horn sheath was removed from the
bony core, and the solid tips were cut off to
be carved or turned. The remaining tapered
cylinder was boiled, heated over charcoal to
soften it, then split up one side and opened
out into a plate. These plates could be worked
as they were or further modified by heating
and pressing, often with the addition of oil or
fat impregnants. This further processing
yielded a translucent material (green horn) that
could be readily split into thin sheets and
scraped and polished to a high shine (Poller,
1980). Horn could be tinted in a manner
similar to tortoiseshell, using a pigment mixed
into the glue used to lay it, for example the
smalt used to tint horn blue when interspersed
with pewter in boullework.
Properties
Horn and turtle shell are softer and more
translucent than bone or ivory, though cattle
horn can also sometimes be white and opaque.
Hydrogen bonding in the protein structure
makes both horn and tortoiseshell hygroscopic.
As horn grows from the inside, a characteristic
cone-within-cone structure develops. The
alignment of cells, and the keratin fibres of
which these are composed, in the long axis of
the cone gives horn anisotropic mechanical
properties (Fraser and Macrae, 1980). Small
parallel fissures are frequently seen on the
surface of horn, particularly as it ages, and
horn will also form small splits in plane with
the surface as human finger nails are also apt
to do. Tortoiseshell is much more homogeneous
than horn, much less anisotropic in its
properties and therefore much easier to work.
It is not fibrous and not so obviously laminated
though sinuous lines similar to the appearance
of burr-wood grain may be perceptible on the
surface of turtle shell.
Both horn and tortoiseshell are thermoplastic.
In the softened state they can be shaped
by bending or pressing into moulds, or
embossed using hot metal dies. On cooling
the new shape is retained. Tortoiseshell can
be softened in boiling water (which may be
salted to achieve a slightly higher temperature)
and can be welded to itself by clamping two
pieces together under pressure and boiling or
steaming. Horn requires higher temperatures
from superheated steam or hot metal cauls to
achieve the same result. Both materials in
powdered form can be fused in heated metal
moulds. Both materials can be sawn, carved,
and turned or decorated by piercing and
engraving and can be given a high shine by
polishing with a succession of finer and softer
abrasives. Horn veneers were often dyed to