Series editors' preface - Wood Tools

occur either as a result of the design of the
furniture itself (e.g. drawer interiors) or storage
(Thickett et al., 1998). Evidence of general
wood and metal reactions can be found at the
interface of unprotected metal parts and raw
wood. Tannin in oak (Quercus spp.) can
readily corrode metals, particularly iron and
other specific wood-metal interactions have
been described (Pinion, 1973). The relative
acidities of some woods and the risk of
damage from the evolution of organic acids
can be found in Table 10.1. Carbon and low
alloy steels, lead, zinc and magnesium and
their alloys are highly susceptible to corrosion
as a result of exposure to acetic acid in a
microclimate. Copper and brass are only
moderately susceptible to such corrosion,
whilst the corrosion of gold, silver and tin in
such conditions has been described as insignificant
(Corrosion of Metals by Woods, HMSO,
1985). Studies of the corrosion of metals
caused by organic acids present in various
woods are discussed by Umney (1992) and
Werner (1987).
Tanning materials used in leather and the
subsequent polishes can cause degradation
products on metals. Depending on the prevailing
conditions, a variety of corrosion products
may form on copper alloys including cuprous
and cupric oxides, cupric sulphide, cupric
sulphate and cupric nitrate. Corrosion products
that form on brass upholstery nails from
atmospheric pollution or contact with wood
textiles or leather may in turn stain other
materials with which they are in contact.
Corrosion of metals can also be caused, influenced,
or accelerated by bacteria (Korbin,
1993). For further information on corrosion see
Trethewey and Chamberlain (1988), Stambolov
(1985) and Jones (1996).
Mechanical damage
Mechanical damage will cause most metals to
bend or dent due to their plasticity. Many cast
alloys, however, are inherently brittle and
even metals that are flexible when new will
often become brittle with age, a phenomenon
known as precipitation hardening. Anyone
who has broken a wire by repeatedly flexing
it knows that metals will become harder and
eventually fail if bent repeatedly (cold working
leading to metal fatigue). Metals that have
been highly stressed by cold working during
Deterioration of other materials and structures 323
forming, such as stamped brass furniture,
hardware or lathe spun sheet metal elements,
may spontaneously crack with age or as a
result of chemical cleaning (stress cracking).
The effect of continual movement caused by
expansion and contraction due to changes in
temperature, or vibration caused by playing on
musical instruments, will eventually lead to
stress cracking.
8.5 Ceramics and glass
Because of the inherent fragility of glass and
ceramics, mechanical damage is the most
common cause of deterioration, or at least the
most commonly noticed. Ceramics also have
low resistance to thermal shock. The softness
of some glazes and decorative applications
makes them liable to scratching and wear.
Defects of manufacture may cause cracking of
the body or a tendency for glazes or enamels
to flake off. Ceramic bodies can be roughly
divided into two groups: low-fired bodies such
as earthen wares and high-fired bodies including
stoneware and hard paste porcelain.
Unlike high-fired bodies which become highly
vitrified during firing, low-fired bodies are
porous and are vulnerable to staining and to
the absorption of soluble substances. Both
ceramics and stone may contain various salts,
for example nitrates and chlorides, which they
have picked up from contact with other saltcontaining
material or from the air. Stone and
tiles which have been in contact with Portland
cement are examples. When the RH falls, the
salts may crystallize and this can lead to
powdering and cracking of the surface and
even complete disintegration of the outer
layers. Ceramics may have their glazes pushed
off and sculptures which have been made or
previously repaired with an iron dowel may
split because of internal pressure from corrosion
products. The extent to which ceramics
are affected by these problems depends on
their composition, design and firing conditions.
The higher porosity of low-fired materials
causes them to be weaker and they are
therefore potted more thickly. They are softer,
coarser and more crumbly than higher fired
bodies.
Glass is hard and brittle, causing it to be
susceptible to mechanical impact and stress.