STANDARD SPECIFICATION OF THE MANUFACTURE & USE OF HATHERN GLAZED OR UNGLAZED ARCHITECTURAL TERRA COTTA

Transcription

1 STANDARD SPECIFICATION OF THE MANUFACTURE & USE OF HATHERN GLAZED OR UNGLAZED ARCHITECTURAL TERRA COTTA 1.0 DEFINITIONS 1.1 Architectural Terracotta (alts:terra-cotta, Terra cotta)- a body of selected and prepared clays made plastic by the addition of water, hand pressed into moulds, dried, finished and fired in a kiln to make hard and durable. Blocks are generally of a size and shape which exceeds that of brick, e.g. 450mm long x 215 x 300 high. used in load-bearing masonry construction. Blocks can be either ashlar, moulded, enriched or ornate. Different colours are typically earthy, e.g. Red, buff, tawny, grey, etc. 1.2 Architectural Faience - all as terra-cotta but with the addition of a surface coating of either a glaze or engobe, typically a different colour and texture to the underbody. 1.3 Body - Clay, or blend of clays, prepared to the required consistency from which certain elements and impurities have been removed and to which selected constituents may have been added, i.e. mineral stains and metallic oxides. 1.4 Glaze - A coloured coating, either opaque or translucent, applied to clay blocks or slabs which when fired become a hard and durable finish. Glazes can be plain, mottled or textured and can range from high gloss to egg-shell finish. 1.5 Engobe - an opaque, coloured clay slip coat applied to the surface of a block or slab and fired to a hard and durable finish 1.6 Hand pressed block - a load bearing unit, either Terra-cotta or faience, formed by building clay walls approx. 35mm (11/4") thick. Hollow cells are formed in the back of the block by the addition of strengthening webs. They can be straight, curved or circular Blocks fall into the following categories. Ashlar - a block of rectangular shape with one or more finished faces. I.e. ashlar quoin; two faces, ashlar double quoin; three faces. Moulded - a block with a uniform linear profile along one or more faces. E.g. string, cill, cornice, etc. Enriched - an otherwise ashlar or moulded block with the addition of a simple sculpted decoration pattern. E.g. egg & dart, acanthus leaf, etc. Ornate - a block bearing a complex piece of free hand sculpture e.g. coats of arms, statuary, urns and vases, etc. Charnwood, Old Station Close, Shepshed, Nr Loughborough, Leicestershire LE12 9NJ T: +44 (0) F: +44 (0) sales@mbhplc.co.uk MICHELMERSH Brick UK Ltd, Freshfield Lane, Danehill, Haywards Heath, Sussex RH17 7HH Registered No (England & Wales)

2 1.7 Extruded block - linear units of either ashlar or moulded description machine formed by forcing clay around a shaped die. Continuous voids may also be formed in the process in the direction of extrusion. 1.8 Slab - A non load bearing unit, either terra-cotta or faience, in the range of 40 to 75mm thick made by either pressing, extrusion or casting, used as a cladding to a structural wall. Mechanical, non ferrous, fixings are required to provide support and restraint. Typically units are up to 750 long (depending upon thickness) x 300mm wide and are cut to length and height after firing. 1.9 Firing - Clay, in the blocks or slabs, are subjected to high temperature in a controlled environment kiln using gas or other fuels. Optimum firing temperatures depend on the clay and glazes used. Typically temperatures around c are achieved to make the units hard and durable. With Architectural Faience it is sometimes necessary to twice fire the units as some glazes mature at lower temperatures than the body Efflorescence - is the gradual migration of soluble salts, which are sometimes present in clay bodies, mortars and fill, being drawn to the surface of the block by the climatic action of wetting and drying then crystallizing and forming white deposits. 2.0 PHYSICAL PROPERTIES OF CLAY BODIES 2.1 Compressive Strength - Of the material is generally greater than 40 N/mm 2 when tested in accordance with BS 3921:1985. Typically a strength in the range of 50-70N/mm 2 can be anticipated. Note: the ultimate failure strength of a block should not be confused with the compressive strength of the body from which it has been made. For example an ashlar block made from a fired clay body of 75N/mm2, 450mm long x 100mm deep x 300mm high, with walls and webs of approx.30mm thick and unfilled hollow voids when tested to destruction failed at 25N/mm2 approx. We disregard the effect of any concrete filling in the voids as, due to shrinkage, it can not be guaranteed to perform structurally. 2.0 Frost resistance - Classified in the 'F' category of BS 3921: Liability to efflorescence - Due to the high temperatures and long firing cycles generally employed in the production of Architectural terra-cotta and Faience their liability to efflorescence can generally be assumed to be low to nil when tested in accordance with BS 3921:1985. However certain clays together with low firing temperatures necessary to achieve required colours may cause the units to exhibit a tendency towards efflorescence. It should also be borne in mind that the presence of soluble salts in the cements, aggregates and sands used in the installation can contaminate Terracotta and Faience causing efflorescence.

3 2.2 Thermal Conductivity Wm/ 0 c dry (k value) Wm/ 0 5% moisture 2.3 Fire resistance - Incombustible 2.4 Acid resistance - Not adversely affected by atmospheric pollutants. Susceptible to damage in contact with Hydrofluoric acid. 2.5 Bulk Density - Generally in the range of kg/m Water absorption - Generally in the range of 3 to 15% by volume when tested in accordance with B.S. 3921: PHYSICAL PROPERTIES OF GLAZES AND ENGOBES 3.1 Craze resistance - To comply with the requirements of B.S. 6431: Part Surface hardness... Between 4-7 on Moh's scale of hardness Note: The ability of a glaze or engobe to resist crazing is very much dependent upon 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 B.S : Part 17. Even if a glaze is deemed to be craze resistant 4.0 MANUFACTURING TOLERANCES 4.1 Overall size - + / - 2% in linear dimensions 4.2 Twist - 5mm per 300mm measured across the diagonal 4.3 Out of Square - 5mm per 300mm length 4.4 Colour... to be in keeping with the body or glaze sample submitted. As with all clay based products shade variations are to be expected. The range of which is dependent upon the materials used and firing temperatures required. 4.5 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 they do not detract significantly from the overall appearance of the work when inspected at normal viewing distances. 5.0 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.

4 5.2 Carriage, packing and handling is charged extra unless otherwise stated. 5.3 Export - units are packed in timber crates with polystyrene or woodwool packing. Any import 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's, taxes and local duties can be allowed for to suit customer requirements. 6.0 SITE WORK 6.1 Acceptance on site - Units are to be checked upon delivery comparing the reference numbers imprinted in a bed face against the delivery notes supplied and any discrepancies reported. If pallets are to be set aside, unopened, for later use a suitable and secure storage area is to be provided to prevent any 3rd party damage. Upon unwrapping pallets any units thought not to be in compliance with manufacturing tolerances are to be advised to us in writing and set aside for collection or inspection. Units damaged in transit will be replaced free of charge, in accordance with our conditions of sale, provided that the blocks have not been filled with concrete or installed on the building. 6.2 Check outs - used to maintain the shape of the unit during firing are to be carefully removed by the fixing contractor prior to filling and installation. 6.3 Void filling (non structural) - The hollow cellular voids of the blocks are traditionally filled with concrete. Unless the concrete fill is expected to provide structural strength it is our recommendation that the following specification be used. 8:1 sand/cement with clean broken brick aggregate trowel filled into voids and not otherwise compacted. A light weight fill based on the use of 'Light-Ag' is satisfactory under normal conditions provided that the possibility of being the potential cause of efflorescence has been fully discounted. 6.4 Void filling (structural) - Where a need for the concrete fill to provide structural strength has been identified by the structural engineer caution during filling should be exercised. Any potential problems due to stresses caused by thermal expansion of the fill during setting should be fully accommodated 6.5 Permanent Shuttering - It is not our recommendation that blocks are used as permanent shuttering in cast in situ structural concrete elements. 6.6 Setting out - due to the potential of inherent size and variation of units it is important to establish the best setting out of features before commencing installation. Joint sizes should be expected to vary to some extent to assist in

5 6.7 accommodating manufacturing tolerances. Where necessary units can be trimmed on site by the fixing contractor to achieve a satisfactory fit. 6.8 LAYING - traditionally terracotta blocks were set with a mortar joint nominally 4mm wide, this being subject to +/- variations due building and manufacturing tolerances. 6.9 Mortars - the selection of the type of mortar to be used is dependent on various factors. For example:- matching existing mortars (Restoration projects) colour of mortar required (To match existing or design requirements) construction (e.g. cladding or load bearing) structural considerations, solid masonry, steel frame construction, etc. Whilst traditionally a strong mortar would have been specified 3:1 sand/cement current practices favour a weaker mix more capable of accommodating building movement. The specification of the type of mortar to be used is therefore a matter for the architect and structural engineer to decide Preparation - Units should be checked for fit and any adjustments, i.e. cutting, made. Units are to be filled 24 hours prior to installation allowing for the fill to cure. Units should be wetted, not soaked, at the time of laying Fixings - Generally in stainless steel provide bearing and restraint. Any structural design requirements are the responsibility of the supplier or the structural engineer. Any existing ferrous metals are to be treated to prevent possible damage to the terra-cotta due to corrosion Cleaning down Units are to be washed down during installation. Any mortars stains, etc. are to be washed off as work progresses. Chemical cleaning compounds are to be avoided unless professional advice on their use has been sought. Compounds containing hydrofluoric acid are known to damage terra-cotta and must not be used. 7.0 MAINTENANCE 7.1 Visual inspection - Should be carried out on a regular basis and any failure of joints rectified. Associated works such as flashings, lead dressing, asphalt roofing should be checked for signs of deterioration. Any weep holes incorporated in the units should be rodded clean to prevent water retention in the fabric of the building.

6 7.2 Cleaning - Specialist advice should always be sought before undertaking any cleaning work. The avoidance of damage to the terra-cotta should be the primary consideration. If possible compounds containing Hydrofluoric Acids should be avoided. 8.0 SUMMARY 8.1 Compliance with relevant British Standards, specified tolerances, etc. - It is sometimes more important to achieve a good aesthetic match, for example restoration work, than specific physical properties and tolerances. 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. 8.2 Testing - Where specific requirements are stated we would undertake to carry out tests to confirm compliance either at our own testing facility or by other independent means. Any requirement for testing should be advised to us prior to submitting our quotation so that costs can be included and our manufacturing programme formulated accordingly.