Series editors' preface - Wood Tools

Early in the fourteenth century small mirrors
were produced in Nuremberg by introducing
tin, bismuth and a resin into hot globes of glass.
This method was superseded around 1500 by
a cold process employing liquid mercury to
affix a sheet of tin leaf to the polished back of
a flat glass sheet. This tin/mercury amalgam
method continued to be used for common
broad-glass mirrors as well as the luxurious
plate glass expanses of the aristocracy until a
cold chemical method involving the in-situ
reduction of a silver containing solution to form
a thin metallic silver layer was developed in the
mid-nineteenth century.
At the beginning of the twentieth century the
Lubbers process had made it possible to draw
a cylinder up to 40 feet long from a molten
reservoir of glass. The advantages of drawing
out a flat sheet of glass rather than producing
a cylinder which was then flattened were appreciated
during the nineteenth century but introduced
with only limited success by Fourcault in
France and Colburn in the United States. By
1913 the Fourcault process had been perfected
in France and in the US the Libby-Owens
Company, working from Colburn’s patents, was
producing flat drawn-glass. Innovations in the
casting of plate glass were made in England and
the US but it wasn't until the development of
the float glass method by Pilkington Bros in
England in 1952 that truly flat glass was
produced without grinding and polishing. In this
process a drawn sheet is floated on a molten
tin bath in an inert atmosphere and kept soft
until both surfaces are planar and defect free.
Virtually all clear window glass is made by this
process today. For further information on glass,
see Newman (1977).
Identification of glass and ceramics
For a description of the characteristics of
different ceramic materials see Buys and
Oakley (1994). Frank (1982) provides information
about the analysis of archaeological
glass. Advanced analytical methods such as
XRF can be applied to the analysis of materials
used in reverse glass paintings and mirror
foilings (Ryser, 1991; Schott, 1988).
5.7 Stone and related materials
The all-encompassing term stone includes
rocks and minerals which are the natural
Other materials and structures 217
products of geological processes. Minerals are
named elements and compounds of specific
composition and crystalline type, and rocks
are mixtures of minerals. Common usage of
the term stone may also include organic
materials, such as amber and jet, altered by
geological conditions. Stone is commonly used
in furniture as polished slabs forming surfaces
such as table tops and occasionally as precious
and semi-precious stones used in decoration.
Rocks and minerals are also the origin of
materials such as bole, clay and pigments that
are components of materials such as gesso,
paint and fillers found on objects. Minerals
have also been used as abrasives.
Rocks can be classified into three general
types: (i) igneous, solidified from the molten
state; (ii) sedimentary, formed by the erosion
of existing rocks followed by redeposition;
and (iii) metamorphic, formed by the action of
heat and pressure on existing rocks (Cox,
1972). The composition, structure, formation
and properties of the rocks that make up these
classes and their important degradation
products, formed by mechanical and chemical
degradation of the parent rock, are described
by Dietrich (1989), Press and Siever (1986) and
Webster (1987). This short section can only
attempt to review some of the stony materials
encountered in furniture conservation.
Marble
Stone used in slabs to form surfaces such as
table tops is often described as marble. This
term has acquired a loose trade definition
meaning any decorative facing stone cut into
slabs to cover furniture or architectural surfaces.
Marble also has a more specific meaning,
which excludes much of this material. True
marble is completely metamorphosed limestone
in which all of the original structures
such as fossils have been changed into
crystalline calcite. Marble may be characterized
by its colour, by its texture (e.g. serpentine),
or by its origin (e.g. breccia di tivoli – rock
from Tivoli). Marble may be pure white, as is
much of the material from Italy, Greece,
Vermont and Georgia (USA) or a variety of
colours. Colours, imparted by impurities in the
form of metal salts, include green ‘Connemara’
from Ireland, black (‘noir Belge’) from Belgium
and red (antico rosso). Marble is relatively soft
and easy to work but is hard enough to