Page 95 - The Building Conservation Directory 2013

t w e n t i e t h a n n i v e r s a r y e d i t i o n t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 9 5
3.2
Structure & Fabric :
Masonry
fits the ‘petrological fingerprint’ of the
original stone. The risk is that an unsuitable
replacement may well weather in a different
manner, and in the worst case it may actually
be physically incompatible, leading to the
deterioration of either the new or the original
stone. It is inevitable that sourcing sandstones
for sympathetic use in new build projects and
for future building conservation will become
increasingly difficult should sandstone
supplies continue to diminish and become
much less diverse.
How, therefore, from a geological point
of view, do we persuade the planners and
quarry-owners to improve the situation?
We clearly need to make sure that mineral
planners, producers and users all understand
why this diversity is so important. Sandstones
do have generic similarities but most show
marked differences, from simple colour
variations to contrasting mineralogies. We
need to explain exactly what a sandstone is,
how differences between sandstones arise and
the consequences if mismatched sandstones
are used in a building. Such differences can
subsequently have a significant impact on the
rate of decay and long-term durability of a
sandstone in the building fabric.
What is a sandstone?
The term sandstone is used to describe a hard
sedimentary rock formed by the weathering,
erosion and accumulation of ‘sand-sized’
fragments from any pre-existing rock-type –
igneous, metamorphic or sedimentary. Sand
accumulations principally form as wind-
blown desert dunes, coastal beaches or the
beds of rivers and streams. Geologically, sand
includes all particles between 00.063 microns
and 2mm in size. Most sand deposits include
a range of grain sizes within these limits
(deposits which contain a wide range of grain
sizes are described as ‘poorly sorted’). Over
time, such sand deposits become gradually
buried, compacted and cemented to form the
sandstone beds quarried today. Geologists
refer to this transformation from sand to
sandstone as diagenesis, a process which takes
place over millions of years.
Understanding the mineral composition
of building sandstones is not just of academic
interest and should be of value to both the
producer and user of the stone. The term
sandstone provides no indication of the
mineralogical composition of the rock. Most
building sandstones comprise a mineralogical
mix of grains including quartz, feldspar, mica
and diverse rock fragments. They might also
include other components in their framework
such as fossil fragments, pebbles or clay layers.
The presence of particular minerals in a
sandstone can have important implications
when considering its end-use as a building
stone. Concentrated layers of micaceous clay
minerals, for example, give natural fissility
to the sandstone, allowing it to be split
into flagstones or stone roofing slates. The
sand grains are held together by a range of
natural mineral cements that may include
silica, calcium and magnesian carbonates
or iron compounds. The type of cement
present can markedly affect not only its visual
appearance but its durability and workability.
When describing sandstones it is usual to
first determine their dominant grain size
(coarse-grained, fine-grained, etc) and then
to identify any other distinctive features
present. These may include colour variations
(red, grey, green, purple sandstones, etc), grain
composition (feldspathic, pebbly, fossiliferous,
etc) or cement composition (calcareous or
ferruginous for example).
Sandstones dominated by quartz
framework grains and cemented by silica,
although likely to be hard and very weather
resistant, may also prove to be difficult to cut
or shape without suitable equipment, which
can significantly increase production costs.
By contrast, more porous sandstones with a
variable mix of grains, together with a less
pervasive cement (probably the most typical
British building sandstone characteristics),
can be worked and dressed by hand and still
provide a durable stone cost-effectively.
Sandstone beds may be thick and
massive (freestones), show inclined layers or
cross-bedding, parallel laminations or may
include pebble layers, fossils and mineral
concentrations. Such features give many
building sandstones a distinctive visual
character which in historic structures often
helps in the determination of their original
quarry source.
Porosity and permeability
Sandstones commonly show considerable
variability in their natural porosity and
permeability, thus either restricting or
encouraging fluid flow through the rock.
Many porous British sandstones are
important natural water aquifers, while
others form reservoirs holding large reserves
of oil and gas. Natural fluid trapped in
pores within a sandstone generally reaches
a chemical equilibrium with its framework
minerals. However, the fluid may also absorb
airborne pollutants such as sulphur dioxide.
The negative impact of the presence of the
airborne pollutants produced by industrial
processes during the 19th and 20th centuries
in encouraging sandstone decay has been
extensively documented. Such acidic or
alkaline pore waters can enlarge, damage or
block pore systems and cause the disfiguring
precipitation of gypsum and other mineral
cements on stone surfaces. Mismatched
sandstones used as part of a conservation
repair can inhibit fluid flow, further enhancing
stone decay.
England
England’s sandstones have been exploited for
building stone for hundreds of years and much
of its surviving stone heritage is constructed
of sandstone. In London, which doggedly
Averham Church, Nottinghamshire: The fearsome
profile of this massive gargoyle is enhanced by its
‘mask of pollution’
St Gregory the Great Church, Dawlish: Permian
red sandstone from the local Dawlish Sandstone
Formation with customised cross-bedding
Coleorton Church, Leicestershire: Cavernous decay
is a common feature of the weakly cemented, fine-
grained, white Permo-Triassic sandstones of the
English Midlands
House, Niton, Isle of Wight: Decoratively coursed and
galleted, green-grey, glauconitic sandstone from the
local Upper Greensand Formation (Cretaceous)