Series editors' preface - Wood Tools

trims to the frame may render fibres weak and
brittle or discolour on aging. Gimp pins, tacks
and nails damage trim physically when driven
through the trim and chemically when they
corrode. Brass nails are susceptible to corrosion
in the presence of various acidic gases
and vapours. This may weaken the nail, particularly
where the shank meets the head. The
nail shanks may become bent and twisted
when they are hammered into a wooden
frame. Both types of damage are problematic
only when removal and reapplication to the
frame is required. Nails may be finished with
lacquers or gilding. This may be abraded in
use or easily damaged in removal and replacement
during treatments. For further information
see Berkhouwer (1990) and Westerman
Bulgarella (1987).
Understructures
Upholstery understructure materials are
susceptible to biopredation and the same
physical, mechanical and chemical degradation
as outlined in the sections on textiles,
elastomerics and hardware. Light is less of an
issue with the inner-most concealed layers,
however, photochemical degradation has been
observed on exposed webbing, base cloth and
bottom linings as a result light reflecting up
from a polished floor onto the underside of
seat furniture. The fibres of loose fillings such
as horsehair and coir fibre often break as a
result of degradation. Denser fillings such as
straw, ulva marina and tow become embrittled
and also break down. The rubber in rubberized
fillings crumbles and polyurethane
becomes soft and sticky. In all cases the
fillings become less resilient, distorting the
profile of the upholstery under-structure.
There are three major contributing factors
which accelerate the degradation process of
these structures which relate broadly to
manufacture, usage and intervention. The first
is the interrelated stresses occurring between
the materials due to the way they are built up
on the frame. For example springs pushing
against the adjacent upholstery layers may
cause the webbing to break and consequently,
the springs to drop. Similarly, the twine
holding the springs under compression may
deteriorate, causing the springs to rip through
the overlying upholstery layers, resulting in a
deformed profile. Additional stresses occur
Deterioration of other materials and structures 359
between tensioned upholstery material and
frame as they respond to fluctuating humidity
levels. For example, tensioned vellum may
split, a leather panel may pull away from point
of attachment or a textile glued to a table top
may buckle, lift or tear as the wood substrate
responds to RH fluctuations. The second factor
is the mechanical damage created while the
object was a functional piece of seat furniture.
The weight load of the sitter imposes tension,
stress and mechanical abrasion upon the
upholstered frame. Sweat and accidentally
spilled drinks do affect top covers but are not
usually in contact with understructures. The
third factor is a result of later upholstery interventions.
As a result part of the original understructure
may be replaced with new materials
causing additional strain and damage to the
remaining upholstery and frame. Further information
on this topic is provided by French
(1990), Gill and Eastop (2001) and Williams
(1990).
Hardware
The main chemical factors that effect the
metals fasteners are environmental. High levels
of relative humidity, the acidity of the
surrounding wood and the sulphur compounds
in rubber, leather, wool and certain
textile finishes will cause some metals, particularly
iron and copper alloys (brass and
bronze), to corrode. Tack and nail heads and
staple bars become weakened and eventually
the corrosion products (copper carbonates
from copper and its alloys, rust from iron)
bond with the surrounding upholstery making
removal difficult. Similarly the shanks become
locked into the wood. In this condition it is
quite difficult to remove the tack or staple
without the head or bar snapping off. Fibres
become acidic as they degrade. Metal ions
catalyse the degradation of cellulose in the
presence of oxygen and moisture. Carbohydrates
present in cellulose base polymers
may be a contributing factor in corrosion
reactions with copper ions. Sulphuric acid
produced by degradation processes and the
burning of fossil fuels corrodes metals which
in turn act as catalysts for further degradation.
Wood produces organic acids which attack
metals and textile fibres. Metal support systems
(compression and tension springs) are vulnerable
to corrosion attack and metal fatigue