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 waterproof, frostproof and soot-resisting facing 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 manufacturing 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 infrared and thermographic photography, magnetometry and scanners. Radiography can permit an ‘inspection’ of the internal structure. If small holes can be made, then borescopes will permit the inspection of voids and hidden reinforcement. 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