Stone is a material of variety, nuance and unrivalled durability.
Used appropriately and with regard to its nature it can enhance the
architectural style of a building and last thousands of years. Geological
Consultant, Francis Dimes outlines the geological characteristics
of the material and the principles of its selection for use in conservation
and restoration work.
the most widely used igneous rock in Britain. It was chosen
by Edwin Lutyens for his masterly tour de force of Castle Drogo,
Devon, and was quarried nearby.
considered the aristocrat of building materials, stone is arguably
the traditional building material of Great Britain. Stonehenge, of
a quartzitic sandstone known as sarsen, dates back to about 1800 BC, a lasting testament to its durability. Yet this was a relatively
late arrival compared with the magnificent limestone temples
of Malta which date back to before 4000 BC, or the stone houses
near Jericho, constructed in about 6000 BC.
rock, anywhere, has been used for building if only on a restricted
local scale. A study of the geological map of Great Britain shows
that the country has a great variety of rocks. But not every rock
may be used successfully. Thought must be given to its weathering
properties and to its nature, which may allow the rock to be worked
to a fine surface and detail. It follows that some knowledge of the
local geology will be of help in understanding the relationship of
a building, the country in which it stands and the stone of which
it is built.
stone can be placed geologically (and thus scientifically) into one
of three groups. Only when the nature of the rocks in these groups
is known can the stone be properly used.
rocks are those which cooled and consolidated from a fluid melt (magma)
of rock material. The magma may have cooled quickly to give fine-grained
rocks or slowly, to produce coarse-grained crystalline rocks. The
nature will depend also on their chemical composition.
have been found in Great Britain but only one, granite, has been
used on any scale. South-west England and Scotland are the great granite
producing areas for building.
rocks were originally deposits of sediment (the eroded material from
pre-existing rocks) laid down, mostly, on old sea-floors. When compacted
and cemented, sedimentary rocks result. Their composition depends
on the nature of the original sediment, but only sandstones (composed
of quartz grains) and limestones (chiefly of calcium carbonate) have
been used on any scale. In general terms, limestone predominates in
southern Britain and sandstone in the north, but some limestones such
as Portland stone have been used ubiquitously; and some sandstones,
for example York stone are widely used especially for paving.
rocks result from the modification of pre-existing rocks by heat,
pressure or both. The one which has been most widely used in Britain
is slate. Other indigenous metamorphic rocks have been little used.
Slate is found chiefly in Scotland, the Lake District, North Wales
are constraints inherent in stone which demand that the material is
properly used in accordance with its unique characteristics. Igneous
rocks may contain minerals which on exposure to the atmosphere may
break down with consequent damage to the stone. Rising salts, also,
may cause spalling. Of prime importance with sedimentary rocks is
the placing of the bedding plane so that it is at right angles to
the thrust imposed upon it. Metamorphic rocks may have deleterious
(harmful) minerals present. The greatest restraint in the use of stone
is that of the jointing. All rocks are jointed and the size of a block
that can be wrought from a quarry is controlled by joints.
constraints are minor compared with the benefits: Stone from all these
groups, whether used in classical idiom or in contemporary form, will
have a durability other materials cannot match, provided it is properly
chosen and properly used.
STONE FOR CONSERVATION AND RESTORATION WORK
The first requirement when a stone building is considered for conservation
or restoration is to determine the nature of the stone. Many historic
buildings will have archival records which should be scanned for references
to the source. Where that does not produce any answers a geologist
should be consulted to determine the nature of the stone. The next
stage, the determination of the 'provenance', the source of the stone,
may be much more difficult. Again it is best referred to a geologist
with specialist knowledge of the geology of stone for building and
the provenance is established (which may not always be possible),
a search of the area for still working quarries then follows. Those
quarries in Great Britain known to be producing dimensional stone
are listed in The Natural Stone Directory (see the Recommended Reading section below).
If the stone is still available, can it be obtained within the time-frame
for restoration?; and can it be obtained in suitable sizes? It must
also be recognised that stone from a present day quarry, whilst geologically
the same, may in fact present a slightly different appearance from
stone quarried in the past. Nevertheless, to preserve the integrity
of the building, the same geological stone is always to be preferred.
it is not always possible to find the stone required. In that event
a geologically appropriate stone should be sought, and the re-use
of original or reclaimed stone should be considered. Although many
masons object to 'second-hand' stone, there appears to be no scientific
reasons why the material should not be reused, provided that bedding
and other criteria are observed.
matching of stone from a provenance other than the original is a specialist
task. Again, advice should be sought from geologists with experience
in this field. It may be necessary at this stage, for thin-sections
to be cut for microscopical study, or for X-ray diffraction techniques
to be used. Guidance will be given by the specialist.
Natural Stone Directory, 8th edition, Stone Industries,
Ealing Publications Ltd, Maidenhead, 1991
- John Ashurst and Francis G Dimes, Conservation of Building
and Decorative Stones, Butterworth-Heinemann, London, 1990
- Alec Clifton-Taylor, The Pattern of English Building, 4th edition, Faber
& Faber, London, 1987
- John Penoyre and Jane Penoyre, Houses in the Landscape: A Regional Study of Vernacular Building Styles in England and Wales,
Faber & Faber, London, 1978