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caused sheets of tile and mortar to
shear away from the thick layer of
render used to even up the face of the
reinforced concrete. As 1960s offices,
universities, and railway stations
become protected through English
Heritage’s post-war listing
programme, so greater attention
needs to be given to ceramics of this
period. Many of the technical
problems associated with the
application of tiles to concrete
structures have been overcome,
partly thanks to analysis by the
Building Research Establishment, and
conservationists need to be taking
more care in trying to retain original
work rather than cutting it out for
poorly matching replacements.
IDENTIFYING THE PROBLEMS
Terracotta was advertised as a water-
proof, frostproof and soot-resisting fac-
ing material. In consequence it is viewed
as being extremely hardy and needing
little maintenance. It has taken several
decades to learn the fundamental lesson
that terracotta, like any other masonry
system, rarely fails due simply to manu-
facturing faults. One has to consider the
way in which it was constructed, the
extent to which it was protected from
moisture, and the likelihood of neglect in
terms of even basic maintenance.
Some terracottas were poorlypressed
in their moulds, resulting in voids and a
tendency to delaminate. Examples dating
to the early and mid Victorian period
are sometimes found to be underfired.
The lack of a properly formed fireskin
can result in rapid deterioration, as in
the case of salmon-coloured ware found
in Mayfair and Kensington.
The glazes on most types of faience
tend to craze over time. This only
becomes serious if the body expands
significantly and the cracks become
large enough for water ingress and
hence a build up of moisture behind
the glaze. Further moisture-induced
problems can arise from the nature of
the mortar joints. Joints were typically
narrow and filled with a hard, dense
cement mortar. Any movement in the
blocks and slabs can lead to cracking in
the mortar. Once water can penetrate
any gaps, it can then freeze and cause
blocks to crack. More catastrophic splits
can occur if the hollow ceramic blocks
were not properly filled with mortar or
breeze - voids will again fill with water
and freeze in winter. Breeze, made
primarily of slag, can itself absorb large
volumes of water and force away the
face of individual pieces. Wherever
moisture passes through a building
facade, there is a danger of salts being
carried and deposited, leading to
discoloration and exfoliation.
Most terracotta ashlar was simply
coursed in with brick to give it a stability
and semi-structural role. But columns,
lintels and large cornices often
incorporated iron cramps. If the mortar
joints fail or water penetrates from
blocked channels behind parapets, then
rust will result in stains, spalling and
then complete failure. The Americans
developed a constructional system
whereby terracotta and faience were
hung off steel and concrete frames
with metal hangars. After several
decades, complete sections of cornice
could fall away from the top of
skyscrapers. Furthermore, as the frame
distorted with age and the hangers
failed, so the non-structural skin could
come under sufficient pressure to cause
long cracks and crumbling.
There is no substitute for careful
inspection. Consultants now use infra-
red and thermographic photography,
magnetometry and scanners. Radio-
graphy can permit an ‘inspection’ of the
internal structure. If small holes can be
made, then borescopes will permit the
inspection of voids and hidden rein-
forcement. Meanwhile, for those lacking
such equipment or large consultancy
budgets, much can be learnt by a careful
visual inspection, ideally from the scaffold
or, as second best, from the ground
using binoculars. Damaged areas can
be recorded on a block-by-block
drawing or large scale photographs, to
highlight patterns of decay and identify
the most probable causes.
Part of the main facade of the Natural
History Museum, London showing evidence
of streaking caused by cleaning with
hydrofluoric acid.
Part of the main facade of the Natural History Museum, London, showing evidence of
streaking caused by cleaning with hydrofluoric acid.
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CONTEXT 52

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