CLEANING

Terracotta and faience were advertised as sparkling materials which self- cleaned with every shower and which could be “scrubbed like a dinner-plate” back to pristine form. It has therefore been assumed that conservationists should go for as-good-as-new, with this end justifying almost any means. Given that virtually every major terracotta building has been irretrievably changed if not damaged by cleaning, one cannot be certain exactly what the original surface was like. It certainly wasn’t a flat, monochrome. Mired Waterhouse, for one, welcomed the variations in colour of terracotta arising from ‘accidents in the kiln’ and blocks always differ slightly in their hue, as a result of variations in pressing, glazing and firing. Just as with stone, it is natural for blocks on different parts of a facade to take on a slightly different colour with age.

It is best to view all cleaning technologies with caution, until one or more is proven to be effective and essentially non-destructive. Newer systems, such as misleadingly-titled air abrasives, may be more sensitive but can still reap untold damage if used by untrained workers to achieve a squeaky clean image.4 Worst of all, many important buildings have now been cleaned at least three times over the last two decades. Nobodyknows of the accumulated damage being caused to the fireskin or glaze - the protective surfaces crucial to the long term stability of terracotta and faience.

Wet grit blasting was widely discredited following the destruction of the fireskin and much of the detailing on the Royal Albert Hall in 1972. The technique has recently been revived with finer, softer material such as calcium carbonate. The Joss system projects a slurry of marble dust in a swirling action. Crushed nut shells and fruit pips have been tried and good results gained under laboratory and very carefully controlled field experiments, but there is always the risk that the fireskin or glaze will still be opened up and weakened.

Treatment with hydrofluoric acid has also proved destructive, causing etching of the fireskin, opening of pores, and efflorescence by affecting the complex silicates in the body and glazes of terracotta.5 The chemical is now specified in more dilute form and with shorter dwell times, but an operative may still spike up the concentration to achieve a clean facade with least effort.

The use of an alkali cleaner, basically caustic soda, also leads to streaking and

discoloration. As a way of avoiding such damage and the safety hazard of concentrated chemicals, suppliers are now promoting weaker alkali cleaners in the form of pastes which are left to work over a longer period. Neutral organic detergent is a more promising alternative but as yet most formulations have failed to shift heavy accumulations of chemically fixed dirt. Paint strippers and lasers are being used with some success by museum conservators, but there are major problems of control, safety and cost to be overcome before they could be used on complete building facades. For now it is probably best to encourage maintenance cleans with nylon brushes and a neutral detergent rather than deep treatment, accepting that the end result will be brighter rather than pristine. Always check whether cleaning tests have been undertaken and ensure careful monitoring as the main programme progresses.

REPAIRS

The traditional approach to repairing damaged terracotta has been akin to dentistry - consolidate if only minor damage is evident; otherwise cut out and replace. The balance has recently shifted, towards retaining as many blocks as possible for reasons of authenticity, economy and to avoid damaging sound neighbouring pieces. Greater use is being made of resin fillers. An earlier article in Context suggested that rusted steelwork could be stabilised by cathodic protection so reducing the need to hack back and expose columns and beams for treatment.6 However, cracks and crazing often mark underlying pressures caused by rusting or building movement, and the causes of serious deterioration have to be tackled. Highly decorative blocks which must be removed for the treatment of steelwork or the insertion of new non-corroding supports, may be saved by being reset and stitched together with resin. Where the block or slab is shattered, a replacement will be needed. In either case cavities must be filled carefully to prevent the creation of voids which could become reservoirs for water.

Alternative materials to terracotta, such as aluminium, precast concrete and glass-fibre reinforced plastics, are widely used in the United States, even on important landmarks. Such inferior substitutes, which discolour and may decay rapidly over a few years, are rarely accepted for listed buildings in Britain. A public inquiry considered whether GRP could be used to make replacement domes for the Hackney

Empire Music Hall in north London. It was rejected, partly on the grounds that Frank Matcham’s exuberant design was consciously ceramic architecture meant to exploit the decorative potential of terracotta.

The key to conserving rather than tarting-up terracotta lies in maintenance and checking water ingress. Copings and flashings can protect parapets and cuts, and careful attention to roofs, guttering and down pipes can prevent future deterioration. Most terracotta was very tightly jointed with a hard cement mortar. Where open cracks have developed, repointing will keep out moisture, but particular care must be taken in cutting out old mortar in case the edges of the blocks are chipped so creating a wider and more vulnerable joint.

Much research is underway in the field of conserving terracotta, and the new British Standard Code of Practice will become the prime source of advice.7 It will no doubt confirm that caution and the cause of long-term conservation should prevail over desires to create a glistening new- looking frontage, if only because another owner will want to repeat the whole destructive process again within a few years. But will or should the Code propose a moratorium on cleaning to parallelthe halt called by Historic Scotland on the scrubbing of sandstone?

Dr Michael Stratton is Lecturer in Conservation Studies, Institute of Advanced Architectural Studies, University of York. He was forinerly Programme Director of the Ironbridge Institute. He has written books on terracotta, car factory and power station architecture and is currently editing texts on the railway heritage and 20th-century buildings.

STRATTON, MJ: ‘The nature of terracotta and faience: a guideline for conservation’, to be published in ASHURST, N, and MATERO, F:

The History, Technology and Conservation of Architectural Ceramics (Proceedings of a Symposium at HarperAdams College in 1994) (English Heritage, 1996). See also contributions by Nicola Ashurst and John Fidler.

Severn, 1990)

Gollancz (London, 1993)

ASHURST, N: Cleaning Historic Buildings, 2 vols (Donhead, London, 1994)

Moynehan, CR, Allen, G C, Brown,! T, Church, S R, Beavis, J, and Ashurst, J: ‘Surface analysis of terracotta’ Journal of Architectural Conservation, 1, 1995, 56-69

goes underground’, Context 49, 1996, 6-8

1982 (to be revised 1996/7) ‘The cleaning and surface repair of buildings: architectural terracotta and faience’. I am indebted to John Fidler for information concerning cleaning and repair.

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CONTEXT 52