Series editors' preface - Wood Tools

324 Conservation of Furniture
Sudden changes in temperature, especially
sudden rapid cooling, may cause glass to
crack. It can be resistant to many chemicals
but water is an enemy of most common glass
and vitreous glazes. Glass, which is a network
of negatively charged polysilicate ions containing
positively charged Al +++ , Ca ++ , Mg ++ , Na +
and K + ions, is ‘corroded’ by the penetration
of water which replaces oxygen in the silicondioxide
matrix with hydroxyl (OH) groups
thereby hydrating the glass. This hydration
layer is always present on glass causing a
range of effects depending on the stability of
the glass composition. The more noticeable
manifestations of this layer on less stable types
of glass range from surface opalescence
through crazing to deep pitting and opacity. If
the proportion of Ca ++ , Mg ++ and Al +++ ions is
low, a potential exists for the migration of Na +
and K + ions. Na + and K + ions are replaced by
H + ions from water leading to the formation
of highly alkaline, hygroscopic and damaging
sodium and potassium hydroxides on the
surface which then react with carbon dioxide
in the air to form sodium and potassium
carbonates. The resulting film absorbs
moisture from the air producing droplets of
moisture in a process called weeping or sweating.
Calcium migrates to the surface forming
opaque calcium sulphate (gypsum) crusts. If
left untreated, this can continue to the point
where the glass becomes opaque and badly
crizzled, with tiny scales of glass flaking off
when the glass is touched. To avoid the risk
of this type of glass disease a constant RH of
about 40% is recommended, but care should
be exercised since it is possible that a very
low humidity may also be damaging to certain
types of glass. The more severe forms of glass
disease are found on vessel glass and are
unlikely to be encountered in furniture. The
softness of some glass and glazes makes them
surprisingly liable to scratching and wear.
For further information on the deterioration
of ceramics and glass see Buys and Oakley
(1994) and Newton and Davison (1989).
8.6 Stone and related materials
Any kind of stone which contains calcium
carbonate, for example marble and limestone,
will react with sulphuric acid formed from
gaseous sulphur pollution to produce calcium
sulphate, carbon dioxide and water. The
calcium sulphate is left behind in a loose and
finely divided state where it is easily eroded
away, exposing a fresh surface to attack. This
process also tends to create a finely pitted
surface, thus not only vastly increasing the
surface area for fresh attack but also providing
pockets for the collection of acid which then
begins to tunnel into the stone. Carbon dioxide
(CO 2) which is present in the air in concentrations
of around 600 000 μg/m 3 (much higher
than SO 2) is also capable of attacking calcium
carbonate in all its forms. Calcium carbonate is
almost insoluble in water (solubility 0.018 g/l).
Carbon dioxide dissolves in water approximately
1:1 by volume at 15 °C (cf 40 parts of
SO 2 will dissolve in 1 part of water) to form
carbonic acid (H 2CO 3) which ionizes to a small
extent and is a weak acid. This can react with
calcium carbonate to form the bicarbonate
which is soluble in water and in this way as
much as 2.29 g of calcium carbonate can be
dissolved in a litre of water. Although sulphuric
acid is a more strongly ionized and much more
powerful acid than carbonic acid, there is a
correspondingly larger amount of CO 2 in the
air. However, CO 2 mainly presents a problem
in outside environments. Calcareous stone is
also attacked by nitric acid. These types of
reactions present serious problems for stone in
the open but normally proceed only very
slowly inside buildings.
Besides pollution, a major risk to stone (and
to those handling it) comes from inappropriate
handling. Natural planes of weakness and
fault lines in stone can easily be stressed
beyond breaking point if large heavy but
relatively thin flat slabs are picked up by their
ends in a horizontal position. Table tops
should always be supported by carefully
easing them off on to a wooden support that
should extend over the full area of the stone
slab. They should then be carried on edge,
vertical rather than flat. For further information
see Amoroso and Fassina (1983).
8.7 Colorants – pigments, dyes and
stains
Many pigments are extremely stable and
remain unchanged for centuries. However, the