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
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