CONTEXTi The Association of Conservation Officers No 52 December 1996 SPECIAL FEATURE: Terracotta and tile PPG15 and recording Redundant historic chapels in~ed
ijtubor ~oof ijtHe @o. 1fiimiteb Manufacturersof HANDMADECLAYROOFTILES Strengthand Frosttestedto BS402 0~ ~ AC=IJI OFCIRTIF~ATION IOMES Certificate No. FM 11616 BS EN ISO 9002:1994 Registered Company Suppliedwithfullrangeof fittings,Ornamentals,Oastand KentPegs,Flat and CrossCamberedAvailablein fivecolours,SussexRed,SussexBrown, Red,MediumAntiqueand DarkAntique. NOWAVAILABLE TudoruniqueINVISIBLEUNDERTILEVENTINGSYSTEM. Theessential complemento a HandMadeclaytileroof. St Maryin CastroChurch,DouerCastle,refurbishedin the DarkAntiquecolourof TUDORTraditional HANDMADECLAYROOFTILES Acceptedin sensitiveareasand regardedbythe mostdiscerningspecifiersas the finest handmadetileon the market.Tudorgivethe highestgradeof serviceto our customerswho can takepleasurein theirinvestmentknowingthat theyare addingto the qualityof the environmentand our heritage. TUDORROOFTILECO. LIMITED DengeMarshRoad,Lydd,KentTN29 9JH 1991 Telephone01797 320202 Historic Houses Association Award Winner Fax01797 320700
CONTEXT Editor: Bob Kindred (Ipswich Borough Council, Tel: 01473 262934, Fax: 01473 262974) 4 Henley Road, Ipswich IPl 3FS, Tel: 01473 259441 Deputy Editor: Liz Marten (North Herts District Council, Tel: 01462474411) 10 Kristiansand Way, Letchworth, Herts SG6 1UE Production Editor: Jonathan David, Tel: 01582 763415, Fax: 01582 467480 Context is published quarterly and is distributed to all members of the Association of Conservation Officers. © Association of Conservation Officers 1996 ISSN: 0958-2746 Published for the Association of Conservation Officers by Hall-McCartney Ltd, Heritage House, PO Box 21, BALDOCK, Herts SG7 5SH. Tel: 01462 896688 Fax: 01462 896677 Nonmember subscriptions to Context Context is available to nonmembers of ACO at an annual subscription rate, including postage, of: United Kingdom Elsewhere £36.00 US$93.00 Subscription orders, together with remittances payable to HallMcCartney Ltd, should be sent to the address shown above. OFFICERS CONTENTS No 52 News December 1996 4 Terracotta & tile Understanding and conserving terracotta. Dr Michael Stratton Manufacturing faience and terracotta. A view from Ibstock Hathernware Specification for the manufacture and use of architectural terracotta and faience Victorian mosaics: conservation and repair. Leslie Durbin Conserving ceramic tiles. Tony Herbert PPG15 and recording: the Oxford experience. Part 1. Carol Rosier Traditional building materials: Scottish slate. Robin Kent There is more to lime mortar than ... Michael Farey Redundant historic chapels in Wales. Roger Wools Saving old churches: another way. Anthony Barnes Sir Titus: conservation, yesterday, today and tomorrow. David Morton Mandatory double glazing. An apology Gazebo: conservation and the role of the County Garden Trusts.Judith Roberts The work of the 20th Century Society. Blain Harwood M'Learned Friend. Charles Mynors ACO Business News Letters Book reviews 6 10 13 14 17 20 23 26 28 31 33 34 35 36 37 39 40 41 THE ASSOCIATION OF CONSERVATION OFFICERS PHOTOGRAPHS IN THIS ISSUE Front cover: making a model for a terracotta moulding. Photo: Ibstock Hathernware pp 6-9: using terracotta. Photos: Michael Stratton p I I: restored terracotta in Leeds. Photo: Ibstock Hathernware p 12: preparing terracotta. Photo: Ibstock Hathernware pp I 5 and I 6: restoring mosaics. Photos: Leslie Durbin pp 17-18: conserving ceramic tiles. Photos: Dudley MBC; Tony and Kathy Herbert pp 20 and 2 I: Cornbury stables. Photo: Oxford Architectural Unit, drawing Jo Cormier p 22: building in Abingdon. Drawing: Oxford Architectural Unit pp 23 and 25: slate in Scotland. Photos: Robin Kent p 24: map by Dr Michael Burgoyne p 29: chapel, Carmarthen. Photo: Roger Wools The views expressed in Context are not necessarily those held by the ACO or the publishers. Neither the publishers nor the ACO shall be under any liability whatsoever in respect of contributed articles. The products and services advertised in this publication are not necessarily endorsed by the Association. Chairman: Mary King (Liverpool City Council, Tel: 0151-225 5678), 24 Beech Road, Holt Green, Ormskirk L39 6SJ Consultations Co-ordinator: Graham Arnold (Sefton Borough Council, Tel: 0151-934 3574), 13 Sefton Drive, Liverpool LS 3SD Vice-Chairman: Alan Taylor (Staffordshire County Council, Tel: 01785 277282), 15 Village Gardens, Walton on the Hill, Stafford STl 7 OLL Secretary: Dr Richard Morrice (English Heritage, Tel: 0171-973 3132), PO Box 301, Brighton, E Sussex BN2 lBQ Treasurer: Robert Parkinson (West Oxfordshire District Council, Tel: 01993 770247), The Old Priory, Priory Lane, Bicester, Oxon OX6 7BG Membership Secretary: Gus Astley (Bath City Council, Fax: 01225 477674), Manor House, Wells Road, Hallatrow, Bristol BS18 5EJ, Tel/ Answerphone: 01761 453047 BRANCH OFFICERS North: Ian Ayris, Tel: 0191-232 8520 ext 6151 North West: Graham Arnold, Tel: 0151-934 3574, Fax: 0151-934 3587 Yorkshire: Bob Scriven, Tel: 01775 761161 ext 4275 CONTEXT 52 West Midlands: Colin Richards, Tel: 01584 874941 ext 2321 or David Baxter, Tel: 01432 36474 East Midlands: Graham Beaumont, Tel: 0115-982 3823 South West: Paul Dadson, Tel: 01823 255425 Education Co-ordinator: John Selby (South Cambridgeshire District Council, Tel: 01223 443000), 77 Deacons Lane, Ely, Cambs CB7 4PS Technical Panel Co-ordinator: Marion Brinton (Basingstoke & Deane Borough Council, Tel: 01256 844844), Development Department, Civic Offices, London Road, Basingstoke, Hants RG21 4AH. Publicity Officer: Kathy Baird (North Shropshire DC, Tel: Business: 01939 232771; Other times Answerphone/Fax: 01691 656516), Edinburgh House, New Street, Wern, Salop SY4 4DB South: Julia Smith, Tel: 01296 382823 East Anglia: John Preston, Tel: 01223 463341 ext 2630 South East: Rob Fraser, Tel: 01273 712619 London: Rosemarie MacQueen, Tel: 0171-798 2455 Scotland: Ronnie Robertson, Tel: 0131-5570019 Wales: Neil Sumner, Tel: 01222 820028; Claire Deacon, Tel: 01437764591ext5195 Northern Ireland: Laurence Manogue, Tel: 01232 23500 ext 279 3
4 NEWS COMMENT CONSULTATIONS: WHO WANTS TO LISTEN? One aspect of the growing maturity of the Association has been its status as an official consultee on proposed Government legislation, policy directives and regulations. It is hoped that this will carry further weight when the Association becomes the Institute for Historic Building Conservation in 1997. Despite the Government seeming not to have any imminent legislative programme for better regulation of the heritage (beyond the Lottery), the last nine months has seen a flood of directly and indirectly relevant initiatives -some more welcome than others. On the plus side, 'Protecting our Heritage' took a generally positive and sensible approach to listing procedures but an absence ofDoE input on planning related aspects and no acknowledgement of the resource implications were two regrettable omissions. On the minus side, the deregulatory study commissioned by the DoE into the 'Reconciliation of Legislation on Historic Buildings' (on which many Conservation Officers provided an early input) appeared not only to be seriously flawed but its preliminary conclusions - on which the ACO had only the briefest of opportunities to comment - could not be supported. The Government steering group responsible for it also appeared not to contain a single representative with direct local government experience. One characteristic which was common to all the consultations (except 'Protecting our Heritage') was the disgracefully brief time given to respond. Pressure to answer without time for proper internal consultation or mature reflection was a matter of great concern not only to ACO but also to other bodies including the national amenity societies and the local government associations. At times there was a suspicion that consultation exercises were merely cosmetic and responses might carry little weight. Some important and fundamental consultations were muddled, poorly drafted and smacked of unduly hasty preparation - particularly the draft Directions set to replace those still in force under Circular 8/87. Poor drafting leads to poor implementation, but will the Government carry the can or will the local authorities who have to use them get the blame? Despite many unreasonable deadlines, your Consultations Sub-Committee has performed heroically to represent the views of ACO members. Thanks are due to them for the quiet efficiency, considerable time and voluntary effort this has taken since the Spring. FORTHCOMING ISSUES:DEADLINES COVER STORY great importance to painting stucco with lime-wash because of the way it mellows with age and frequently sent builders his recipe made up of 100 parts slaked lime, 1 part copperas (or pyrites), 2 parts salt and 2 ANDTHEMES parts Russian tallow, with powdered colour as required - yellow ochre, red oxide, Venetian red and burnt umber." If any readers have similar interesting or favourite recipes or have views on this matter generally, they may care to write to the Editor. CONTEXT BACK ISSUES AVAILABLE The Editor finds that space in his 'office' is now at a premium and his better half is dropping hints that it is time for a rationalisation. Although early issues, Nos 1-11, are no longer available, there are reasonable quantities from No 17 (December 1987) onward apart from Nos 21 and 28, both of which are also out of print. Back copies are available on a first-come-first-served basis at a cost of £4.00 each (inc p+p) while stocks last. If you are missing back issues, now is your only chance! AREYOUOUT THERE? There have been complaints, (although not to the Editor!) that there is no regular Letters page in Context. How odd that no one has written to me about it! Well we do publish letters! Usually we print all the relevant letters that we receive. Perhaps, if we had actually received some letters with a view to express, we might consider a recurrent Letters page! Copy deadline for the Spring 1997 issue will be 15 January. Please adhere to this deadline. Issue 53 will look at the problems and opportunities of enabling development. Other contributions, experiences, technical notes and case studies are welcome (space permitting) but articles under 2,000 words are much preferred. We continue our recent practice of devoting our winter issue to a particular building material and devote this issue to terracotta and faience. Considerable damage has been done to the material by inappropriate and inexperienced cleaning, a topic dealt with by Dr Michael Stratton. Equally, there are many cases where replacements can be the manufactured but this requires an understanding of the manufacturing constraints. These are explained by lbstock Hathernware Ltd. One example of the company' expertise in this field is shown in our cover photo. THE CONSERVATION PORTFOLIO In the December 1996 edition of the Portfolio are: Copy in word-processed form on 3·5" floppy disk would be appreciated where possible. Please be sure to indicate what software it is written in on the label. Don't just give the title of the file! Do not sent faxed copy only - it cannot be scanned directly into Context or the Newsletter as it will be too blurred at the Editor's end. Any faxed copy not immediately followed up with a clear hard copy in the post will be rejected for publication. RECIPE COLUMN Lucy Archer's 1985 biography about her father, Raymond Erith, the highly accomplished post-war classicist architect, made passing reference (page 94) to a recipe for limewashing of stucco which readers might find of interest. "He attached Eurocom Enterprise Ltd Uginox Terned Stainless Steel for Roofing, Flashings and Gutters Britannia Architectural Metalwork & Restoration Standard and Special Castings in Iron and Aluminium Cliveden Conservation Workshop Advisers to the National Trust. Specialist Contractors in Stone and Plaster Conservation Conservation Specialists Ltd Conservators of Historic Buildings and Artefacts CONTEXT 52
NEWS ACO CANTERBURY SCHOOL: 9-13APRIL 1997 RECONSTRUCTION AND REGENERATION:THE PHILOSOPHY OF CONSERVATION Every working day, every member of this Association makes decisions against a philosophical background which has developed over many years and which is still developing. Often we take those decisions without thinking about that background and it is intended that the 1997 Annual School, based in Canterbury, should stimulate discussion of the philosophies and principles which underpin our work in this year when we move from Association to Institute. We all recognise the immense debt which historic building conservation in Britain owes to John Ruskin and William Morris, and their debt will be underlined. There have, however, always been variations in the tradition and their contribution needs highlighting. Can one talk of conservation before the 19th century? If speculative reconstrnction should be avoided, is post-disaster reconstruction also taboo? Are Uppark, Warsaw Old Town and Ypres (the latter which it is hoped we will visit) authentic? Is an architecture which is traditional in detail but contemporary in scale and servicing an adequate alternative? Can we learn from related philosophies, such as those used in conservation archaeology, garden history and sustainable development? What happens to our philosophies when the scheme goes on site? Are owners allowed philosophies? Canterbury is a very suitable site for these explorations. Heavily bombed during the last war, it was partially rebuilt with buildings which are now coming to the end of their useful lives. How should such a thriving tourist centre redevelop? Thanet (the towns ofRamsgate, Margate and Broadstairs) is, however, following a different route, regeneration being harnessed symbiotically to conservation. Sandwich, a small town apparently with the highest density CONTEXT 52 of listed buildings in England, is different again. Need it develop further at all? These issues of regeneration will be accompanied by visits to the three centres to assess the philosophical points raised. It will be followed by the Annual School, taking the form of lectures, and Saturday will be devoted to a visit to Ypres, the south Belgian historic town reconstructed after being so badly shattered by the various battles of the Flanders campaign. On Sunday morning the development of historic sites will be continue to be assessed. Our thanks must go to Canterbury City Council, especially Mansell Jagger, John Chater and David Kincaid, who have at very short notice, assisted by Ray Harrison of Thanet DC, Clive Alexander of Dover DC and Richard Morrice of English Heritage, organised a stimulating and unusually cerebral programme. Accommodation will be on one site, just outside the city walls in part of Christ Church College. The School will be over three full days, rather than the two of recent practice, so as to include the visit to Ypres. As usual early booking is essential. David Kincaid or Joy Burton at Canterbury CC can give further details (see the advert for the address). UPDATE CONSERVATION CHARTERS Further to the article in Context 51 (p20), which detailed the many international conservation charters and standards relating to historic buildings and sites, the author is prepared to supply copies of the Charters referred to. Anyone interested in obtaining copies of the charters can contact Jack Gillon at: 12 Kirkhill Road, Edinburgh EH16 5DD. The 1997 ACO Annual School Canterbury Reconstruction and Regeneration The Philosophy of Conservation 9th - 13th April 1997 Christchurch College, Canterbury The Day School Friday 11th April 1997 TheACO is becomingan institute. The Schoolwill examine the principlesthat underpin our work. The Day Schoolwill explorethe philosophyof conservation,preservationand reconstruction Information and booking forms from: David Kincaid or Joy Burton (01227) 763 763 Fax: (01227) 763 727 CANTERBURY O t::J CITY COUNCIL~ The Association of Conservation Officers 5
6 Terracotta tile UNDERSTANDINGAND CONSERVINGTERRACOTTA CERAMICSTANDARDS Conservation Officers in most towns and cities will be confronted with buildings faced in terracotta. Some will be listed but most depend on being in conservation areas for any protection. Terracotta -comprising large blocks and slabs of moulded clayware -was used most widely on commercial and public architecture dating to the latel 9th and early 20th centuries. Such buildings have to earn their keep. Owners will expect any refurbishment to create a bright and commercially more attractive appearance. Costs will be vetted carefully against increased value or rentals. Only in exceptional cases can grant-aid be used to encourage the highest standards in any conservation project. In this situation where conservationists have to work hard to convince owners, architects and builders of the need for sensitivity, the challenges presented by terracotta are compounded by the fact that there is still widespread ignorance about the nature of different types of architectural ceramic, and the merits and dangers of different approaches to cleaning. This depressing scenario can be Dr Michael Stratton discusses the essential features of the material taken further. Most of the key examples of terracotta architecture in Britain have been irreparably damaged not by pollution or decay but by human intervention in the name of conservation. The fireskin and much of the decorative detailing on the Royal Albert Hall was blasted away by grit cleaning in 1972, and the surface of the nearby Natural History Museum was scored and bleached by hydrofluoric acid a few years later. This trail of abrasive destruction continues, with some commercial buildings now being cleaned every 3-5 years, as the rate of soiling increases due to loss of the smooth protective fireskin. A new British Standard for the conservation of terracotta is awaited. Until it is ready, only broad principles can be put forward, along with a series of cautions about the damage which can inadvertently result from decisions over cleaning and partial replacement. This article urges a more conservative approach; offers some guidelines as to how Conservation Officers can try to understand ceramic architecture and the nature of terracotta; to then guide owners and architects through the maze of different technologies and options. UNDERSTANDING TERRACOTTA As a prelude to any recommendations or decisions, one needs to understand the nature of the material and the context in which it was used. 1 Unglazed terracotta, typically with a natural colour of buff, grey or red, was made in the Tudor period but then, far more widely, from the late 18th century. It can be readily distinguished from glazed faience, where the body is covered by one or more glazes. Confusion only arises with some of the vitreous finishes dating to the turn of the century, where a clear, matt glaze was applied to the fired body. Faience became widely used from the 1890s. The most usual colour for faience was a white or cream, bright polychromy inspired by Art Nouveau and Art Deco being reserved for more exotic projects. The unglazed surface of terracotta may be more vulnerable to soiling and is hence often the subject of more vicious cleaning - but the protective fireskin, achieved by smoothing over a block once it has been extracted from the mould and by giving it a full, high-temperature firing, is the key protective shield for the piece, as well as giving it its distinctive colour and texture. To understand the nature of the surface, be it fireskin or glaze, it is necessary to appreciate the nature of the body, which, in turn, will depend on the clays used by the manufacturer. In most simplistic terms, firms either worked a single clay or created a mixture which (once mixed with water) would have a smooth texture and plasticity. Terracotta clays had to carry fine detail and then be fired to a high temperature without distortion or excessive shrinkage. During the Tudor period, young surface clays from the south east of EngBuilding by numbers: hollow blocks of terracotta being set onto brickwork for the Bournemouth Pavilion, c1930. CONTEXT 52
A section of the cornice of the Royal Albert Hall, London, showing evidence of moss growth in open joints, and disco/oration of blocks following grit blasting. Badly split cornice blocks have been cut away prior to replacement. land were used to create the buff or pale red blocks as found on Sutton Place, Surrey, Hampton Court or the church monuments in East Anglia. The revival of terracotta in the late 18th century was dominated by Coade stone made in Lambeth, London, from 1769. China clay was used to create a highly durable stoneware, called artificial stone and intended to be mistaken for high quality limestone. Coade stone can be identified by its maker's stamp, its smooth texture and through checking the gazetteer, covering the whole of Britain and the colonies, researched and published by Alison Kelly.2 There is far greater potential for confusion with architectural dressings and garden ornaments dating to the early 19th century. Some ofCoade's competitors produced artificial stones which were moulded in clay while others were based on cement, mixing ground limestone and casting it to produce vases, statues and rockeries.Just to add to the complexity, a couple of firms, for example James Pulham of Broxbourne, Hertfordshire, produced both ceramic and cement based artificial stones. True clay-based terracottas dominated from the mid-19th century, partially due to outbursts against 'dishonest' sham materials from the Ecclesiologists and, later, from members of the Arts and Crafts movement. Buff terracottas were widely used in the cultural centre of South Kensington during the 1860s, for the Victoria and Albert Museum and the Royal Albert Hall, and on the Natural History Museum in the following decade. Widespread appreciation of these buildings resulted in terracotta being adopted for many provincial art schools. Close study of the mouldings and sculptural decoration shows that textures, colours and modelling vary widely, influenced as much by the manufacturer, whether Blanchard of Bishops Waltham, Blashfield of Stamford, or Daulton of Lambeth, as the designers involved. Each firm used different clays, different types of kiln and highly individualist modellers. By the 1880s the bulk of British terracotta was being made on the coalfields of the Midlands and the North, from clays brought out of mines and fired with the coal with which they were geologically associated. Curiously, the CONTEXT 52 rapid growth of the industry and the use of railways for delivery did not result in any nationwide standardisation, but more of a vernacular relationship between manufacturer and local architect. It is always worth being aware of such regional ties, partly so that one can also appreciate the significance of the prestigious exceptions where material was transported by rail from many miles away. The red terracotta used for libraries and schools in Birmingham came from Ruabon in north Wales. The same firms also supplied many contracts in Cheshire, though John Douglas tended to build up his designs in smaller solid blocks. On the other side of the Pennines, Leeds architects were more likely to collaborate with the major manufacturer in Leeds: Wilcocks (later titled Burmantofts), which specialised in producing an orangey buff material. There are other more localised and hence highly distinctive outbursts of terracotta on other parts of England: red Tudor style window mouldings and chimney stacks in Norfolk made by Gun ton of Costessey, salmon-coloured ware made by Doulton for apartments in London, and grey dressings for villas and terraces around Bournemouth made by Jennings of Poole. Given that the terracotta on a particular building may have been discoloured and abraded by soiling and cleaning it is always worthlookingoutforcomparablework and checking surviving trade catalogues before agreeing to a particular colour, texture and type of modelling for any replacement work. This regional pattern was shaken up by the introduction of faience for external facades and the emergence of new producers in the last years of the 19th century. Frost-proof glazes could be applied most readily to buffburning glazes. Doulton, Burmantofts and Carter of Poole prospered while the Ruabon firms, such asJ C Edwards, drifted into decline. One might think that glazes made for greater uniformity, and hence make for easier decisions concerning cleaning and re-manufacture. In practice, firms offered a multitude of finishes, such as semi-clear vitreous glazes, matt glazes, combed surfaces, and mottled granite effects. It can be extremely difficult to identify whether blocks were initially glazed, or finished with clay slips, once they have been cleaned several times and even given a shiny anti-graffiti coating. 3 During the 1930s, faience was used in bright jade green and orange colours for cinemas and chain-stores. Historians have tended to see the Odeon cinema as the swansong of British terracotta, but there was a further revival in the 1950s. Oversized tiles were given abstract low relief patterns in pale blue and lemon colours. In the following decade architects followed the example ofLe Corbusier in applying plain white tiles to structural concrete. Shaws of Darwen dominated this market with its Twintiles. The company supplied several prestigious complexes designed by Yorke, Rosenberg and Mardall, most notably St Thomas' Hospital, London, and most notoriously, given the area of tiling that fell away after little more than a year, the new campus for the University of Warwick. Ingress of moisture and differential shrinkage 7
8 Terracotta tile 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 isviewed as being extremely hardy and needing little maintenance. It has taken several decades to leam 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 constmcted, the extent to which it was protected from moisture, and the likelihood of neglect in terms of even basic maintenance. Some terracottas were poorly pressed 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. Part of tbe main facade of tbe Natural History Museum, London, sbowing evidence of streaking caused by cleaning witb bydrofluoric acid. 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-stmctural 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 mst 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 stmcture. 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 visualinspection, ideallyfrom 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 tbe main facade of tbe Natural History Museum, London sbowing evidence of streaking caused by cleaning witb bydrofluoric acid. CONTEXT 52
~ SHAWS ~A~~~ V Terracotta & Faience Celebrating 100 years of continuous manufacture, combining latest production technology with the skills of our dedicated craftsmen. For advice, surveys or quotations, please contact: Jon Wilson SHAWS of DARWEN Waterside, Darwen, Lancashire BB3 3NX Tel: 01254 775111 Fax: 01254 873462 Decorative CAST IRON Call the experts We have designed and made ornamental cast iron since the nineteenth century. Our expertise has proved invaluable for countless restoration projects. F.J.Thornton & Co Ltd, Jenner Street, Wolverhampton, WV2 2AE. Tel: 01902 352390 CONTEXT 52 STONEHEALTH LIMITED 73 London Road, Marlborough, Wiltshire. SN8 2AN Telephone & Fax: 01672 511515 The most gentle way to clean limestone, sandstone, terracotta, brick, faience etc. Chemical free environmentally friendly swirling vortex No other system can match the over-all performance of JOS JOS is gentle and cleans without damage - Retain the patina and mellow character of an old building without making it look new Paint removal problems'? Use Stonehealth's new steam system I>OF'F will remove oil based, them10plastic and masonry paints, bitumastic coatings, heavy deposits of oil and grease etc........ even chewing gum without damage to the substrate A Safer to use, yet effective, range of products without using chlorine or hazardous solvents - biodegradable - water soluble MICONE Cleaners, degreasers, biocides, paint and graffiti removers Soot removal, lirnescale, nicotine and other pollutants 11 HIKST Consen·atio11 ~~ ART CONSERVATORSAND HISTORICBUILDING CONSULTANTS Conservation of painted and applied decoration on plaster, stone, canvas, wood and metal substrates. Also specialist building works including joinery, sculpture, marble, stonework, stucco, pargetting, wall and floor plaster. Restoration and recreation of historic decorative schemes. Surveys, specifications and analysis services available Laughton, Sleaford, Lincolnshire NG34 OHE Telephone: (01529) 497449. Fax: (01529) 497518 9
10 CLEANING Terracotta and faience were advertised as sparkling materials which selfcleaned 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. Alfred 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. Nobody knows 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 ofthefireskin 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 cills, and careful attention to roofs, guttering and clown 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 newlooking 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 parallel the 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 formerly 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. This approach is developed more fully in 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 Harper Adams College in 1994) (English Heritage, 1996). See also contributions by Nicola Ashurst and John Fidler. 2 KELLY,A:MrsCoade'sStone(SPA, Upton-onSevern, 1990) 3 STRATTON, M J: The Terracotta Revival, Gollancz (London, 1993) 4 For an overview on cleaning technologies see ASHURST, N: Cleaning Historic Buildings, 2 vols (Donhead, London, 1994) 5 For a scientific study of these issues see Moynehan, CR, Allen, G C, Brown, IT, Church, SR, Beavis, J, and Ashurst, J: 'Surface analysis of terracotta' ,Journal of Arcbitectural Conservation, 1, 1995, 56-69 6 Duffy, A, and Wrightson, D: 'Conservation goes underground', Context 49, 1996, 6-8 7 British Standard 6270 Part 1 Amendment G, 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. CONTEXT 52
Terracotta tile MANUFACTURINGFAIENCE AND TERRAC·OTTA A view from Ibstock Hathernware, the terracotta and faience company within the Ibstock Group The two top storeys of this 1930s terracotta building in Leeds were completely destroyed by fire which also damaged the rest of the building. Jbstock Hathernware supplied replacement terracotta for its successfu,restoration. CONTEXT SY Before embarking on the various stages involved in the manufacture of terracotta and faience, it is worth first taking the following considerations into account. The manufacture of these two building materials is more akin to art than engineering. As every piece is hand made, no two pieces, even from the same mould, will be identical and this has to be borne in mind during design. Blocks will vary slightly from the design size due to the contraction factor during manufacture. There is normally an 8% shrinkage which occurs during drying and firing, and regular joints are needed to accommodate these slight discrepancies. Very large blocks should be avoided wherever possible and, at the very least, a proportional balance between length, breadth and height should be aimed for. Variation in colour shade with both terracotta and faience can be expected from block to block, which greatly adds to its aesthetic appeal. Both materials are extremely strong in compression but weak in tension. Every effort must therefore be exercised to take advantage of their load-bearing capabilities and avoid tensile stress. Similarly, it can be expected that glazes will craze at some time during their life. This is purely cosmetic and does not detract in anyway whatever from the durability of the material. Finally, it has to be remembered that after manufacture terracotta and faience expand slightly whereas concrete shrinks. When they are used in conjunction therefore, allowance for movement joints should be made. DESIGN The first stage in the manufacturing process is to prepare elevational drawings at I :20 scale or larger, based on the architect's design requirements. This would, of course, necessitate a site survey in the case of a restoration 11
12 I Terracotta tile project. These drawings would then be used as the basis of a 'jointing' scheme, breaking the elevation down into individual blocks. The aim of this exercise is to create the highest repeat factor possible with blocks of a size and shape which will not cause difficulties in manufacture. Constructional design is also considered at this stage and large scale details are prepared showing how the terracotta works in conjunction with the other elements of construction. This design package is then submitted, via the architect, to all other parties in the design team, main contractor, fixing contractor, structural engineer etc. for their approval. _ MODEL MAKING Working from the clay drawings which provide an exact profile to make the model, the craftsman produces a zinc template by cutting, dot by dot, punches straight through the drawing, the reverse of which is mounted on a timber frame called a 'horse'. This is designed so that it can be pushed along the edge of a bench. Underneath this profile, liquid plaster is added by degrees with the 'horse' being moved back and forth to transfer the profile shape to the drying plaster. The template is mounted on a centre point for curved blocks or fixed to a lathe for circular items. This is not a mechanical process but rather requires skilled artists to complete any ornate additions to the blocks. MOULD MAKING Again using plaster, a mould is now made around the model, the latter being coated with shellac and soft soap to prevent sticking. Sides are then built up by hand one at a time until all faces other than the back are formed. When dry the mould can be stripped from the model, which is set aside. Once reassembled without the model, the mould is dried and can then be filled with clay. CLAY PREPARATION There are many different types of clay, some more suited than others for the different type products man has devised - china for pottery and earthenware for pipes. No one clay falls under the category of 'terracotta'; rather the physical properties of a particular clay make it suitable or not. It is in the processing that the important characteristics such as colour, strength and firipg qualities are developed. First the clay must be allowed to stand outside for a period of time to allow weathering. It is then processed. One method is the 'press' technique -simply reducing the clay to a water mix slurry, which is sieved to remove unwanted particles and then pressed, being forced through nylon cloths for approximately five hours at a pressure of 90 lb/in 2 . Alternatively, the 'pan mill' method involves a mixing machine where the clay is ground through a sieve by large heavy metal wheels, with iron particles being extracted by passage under an electromagnet. CLAYWORKING There are three methods of working the clay, of which hand pressing is the most common. Clay is smeared by hand into the plaster mound, ensuring that no pockets of air are formed, to an even thickness of approximately 30 mm. Webs are formed in the hollow void to give support and maintain shape during drying and firing. The hygroscopic nature of the plaster is such that water is literally sucked from the clay, starting the process of drying and causing contraction to begin. After a few hours, the clay is stiff enough to be turned out of the mould, which is then dried for later re-use. After a day or two, the surface of the block is worked on. Using knives, leather, rubber and wooden tools the surface is smoothed off, pushing the coarser particles of clay down and bringing the finer up to the surface. This not only improves the appearance of the block but creates a 'skin' which imparts much of the wearing characteristics of the finished piece. Slip casting involves the pouring of liquid clay into the mould, again with the hygroscopic effect drawing the water out. After a predetermined time, the remaining liquid can be poured off leaving a desired thickness of set clay in the mould. Extrusion, as the name implies, is suitable for the production of many, straight shaped blocks. When such are needed, there is a considerable cost advantage in this latter method. DRYING Half of the shrinkage occurs in the drying process and so extreme care must be taken in ensuring a slow and even curing. Warm currents ofair, from underfloor heating, are allowed to circulate around and through the blocks. Sometimes a bed of rollers is set under the block to allow this often considerable amount of movement to be accommodated. GLAZING It is at this stage that the application of a glaze, when required, is carried out. Early methods included brushing on or even dipping in a trough full of liquid glaze. Today, however, compressed air is used to spray the glaze onto the block. There might be several layers of glaze applied and various techniques used to give a particular effect, such as speckling and mottling. FIRING AND INSPECTION The process of firing causes the finer, more fusible, particles of the clay in a body to turn to liquid which, when cool, then bonds the coarser particles together. This vitrification essentially ensures that the blocks will be hard wearing and durable. There are basically two types of kiln. One is a continuous type, often a tunnel kiln, which works on a set cycle of reaching full heat and then cooling, and is suitable for items of low volume such as wall tiles. Terracotta, however, with the large masses often involved, is usually fired using an intermittent kiln where the product is fired in two separate stages, the burners firing in two separate durations to reach predetermined temperatures - often as high as 1200°C. To finish there is the final inspection where attention is given not only to accuracy of size and colour but the detection of any occurrence of copper or iron contamination. Spares, at the rate of one in twenty, are usually allowed during manufacture to ensure that replacements are in hand should blocks be damaged in transit or on site. lbs tock Hathernware Ltd, Normanton on Soar, Loughborough LE12 5EW CONTEXT 52
SPECIFICATION FOR THE MANUFACTURE AND USE OF ARCHITECTURAL TERRACOTTA & FAIENCE 1 GENERAL 1.1 Architectural terracotta and faience can be either load bearing or cladding building materials made from refined selected clays which when fired attain hardness and durability. They are made by shaping clay, whilst wet and pliable, drying, applying a glaze in the case of faience and firing in kilns. Traditional methods of manufacture involve making a plaster 'model' around which is formed a plaster 'mould'. With the mould assembled without the model, a negative of the required block is formed into which clay is pressed by hand to an even wall thickness. Strength to the hollow void so formed is provided by adding strengthening webs of equal thickness. Once the clay his stiffened and the mould has been stripped away, the unit is allowed to cure for about 24 hours before the surfaces to be exposed on the building are smoothed using hand tools. Alternatively, clay can be produced in a liquid consistency and poured into the moulds. During the curing period the clay in contact with the moulds gradually thickens. Surplus liquid is then poured away. Finishing as hand pressing is similarly required. Slip casting is suitable for limited sizes and shapes and most useful for objects where the hand can notreach, e.g. balusters, tight necked urns etc. A more modern approach is the use of mechanical extrusion. A column of clay is forced under pressure through a metal die which imparts shape. Cutting beds subdivide the column into units of required linear length. This process has the advantage of not requiring the preparation of models and moulds but does impose limitations on the size and complexity of block. 2 DEFINITIONS 2. lArchitectural terracotta: prepared clay, or blend of clay, which with natural pigments or by the addition of stains and oxides, gives a through-body colour when fired; Colours are typically earthy, e.g. red, buff, tawny, grey etc. 2.2 Architectural faience: prepared clay with an applied coating of glaze or engobe which when fired provides a coloured surface finish. 2.3 Body: clay, or blend of clays, prepared to the required consistence from which certain elements and impurities have been removed and to which selected constituents have been added. 2. 4Glaze: a coloured coating, either opaque or translucent, applied to clay blocks or slabs which when fired become an integral surface finish. Glazes can be plain, mottled or textured and can range from high gloss to egg:shell finish. CONTEXT 52 2.5 Engobe: an opaque, coloured clay slip coat applied to the surface of a block or slab. 2.6 Block: a load-bearing unit, either terracotta or faience, formed by building clay walls approx. 35 mm (1 1 / 4 in) thick. Hollow cells are formed in the back of the block by the addition of strengthening webs which generally require filling with a weak concrete mix. 2. 7 Slab: A non-load-bearing unit, either terracotta or faience, in the range of 25 to 50 mm thick, used to clad a structural wall. If mechanical restraint fixings are required to provide stability, a minimum 40 mm thickness is recommended. Typically units are up to 600 x 300 mm on face and are supplied cut after firing to length and height as required. 2.8Firing: Blocks or slabs are subjected to high temperature in a controlled environment kiln using gas as the fuel source. Optimum firing temperatures depend on the clay and glazes used. Typically temperatures around 1200°c are achieved to make the units hard and durable. With Architectural faience it is sometimes necessary to fire the units twice as some glazes mature at lower temperatures than the body. 3. PHYSICAL PROPERTIES 3 .1 Clay body Compressive strength: >25 N/mm2 when tested in accordance with BS 3921: 1985 Frost resistanc: Classified in the 'F' category of BS 3921: 1985 Thermal conductivity: 0·5-0·9 Wm/K dry (k value) 1-1·5 Wm/K@ 5% moisture Fire resistance: Incombustible Acid resistance: Not adversely affected by atmospheric pollutants. Susceptible to damage in contact with hydrofluoric acid. Bulk density: Generally in the range of 1900-2750 kg/m3 Water absorption: Generally in the range of 3-15% by volume when tested in accordance with BS 3921:1985 3.2 Glazes Craze resistance: To comply with the requirements of BS 6431: Part 17 Surface hardness: Between 4 and 7 on Moh's scale of hardness 3.3 Engobes Craze resistance: To comply with the requirements of BS 6431: Part 17 Note: The ability of a glaze or engobe to resist crazing is very much dependent on the constituents in its make up. Where a specific colour is required, e.g. restoration work or meeting design requirements, it is not always possible to meet the requirements BS 6431: Part 17. 4.0 MANUFACTURING TOLERANCES Overall size: ±2% in linear dimensions Twist: 5 mm per 300 mm measured across the diagonal Out of squareness: 5 mm per 300 mm length Colour: to be in keeping with the body or glaze sample submitted. As with all claybased products, shade variations are to be expected, the range of which is dependent on the materials used and firing temperatures required. Surface quality: units are to be free of imperfections which are known to have an adverse effect on the durability of the finished product. Visual imperfections, such as pin holes, crawls, minor copper spots etc. are permissible provided that they do not detract significantly from the overall appearance of the work when inspected at normal viewing distances. Note: It is sometimes more important to achieve a good aesthetic match,Jor example in restoration work, than to achieve specific physical properties and tolerances. Where specific requirements are stated, the manufacturer should undertake to carry out tests to confirm compliance either at its own NAMAS certified testing facility or by other independent means. Please advise the company of your requirements in order that any cost implications can be determined. In the event that adjustment to the body or glaze is required to achieve stated properties or tolerances, this may require some compromise in other regards, i.e. colour and glaze finish. 5. PACKING AND DELIVERY 5.1 Domestic: Units are to be packed with either polystyrene or woodwool on nonreturnable timber pallets and shrink wrapped. Delivery will be made by flat-bed lorry, size to suit load and site conditions, with crane off-loading if requested. Carriage, packing and handling is charged extra unless otherwise stated. 5.2 Export: Units are packed in timber crates with polystyrene orwoodwool packing. Any restriction on use of packing materials imposed will be taken account of. Shipping or air freight can be arranged as required which will be charged as an extra unless otherwise advised. Insurance, taxes and local duties can be allowed for.to suit customer requirements. 6. SITE WORK 6.1 Inspection: Units are to be checked against delivery notes supplied with driver and any discrepancies reported. If pallets 13
ihbc.org.ukRkJQdWJsaXNoZXIy MjgyMjA=