Technical Specifications

Physical properties of terracotta and faience, tested in accordance with the British Standards (BS) and the American Society for Testing and Materials (ASTM), By Ceram Research, British Ceramic Research Ltd.

Compressive Strength
BS3921, ASTM C67, 70-90 Mpa, 10283psi

Thermal Measurement
BS3921, ASTM C67, 3.5 X 10-6 in 20-100 degrees centigrade range. Thermal Conductivity 1.3 W/Mk

Thermal Shock Resistance
ASTM C484 No Damage

Absorption
BS3921, ASTM C67, 5 Hour Boil 12%, 24 Hour Soak 5.8%

Frost Resistance
BS3921 (“F” category) ASTM C67 No Damage

Acid Resistance
Resistance to chemicals found in normal service conditions. Susceptible to damage in  contact with Hydrofluoric Acid

Craze Resistance
BS6431, ASTM C424

Surface Hardness
Min 6 on Mohs Scale

Tolerance
BS3921, BS5385, ASTM C67 Generally +/- 2% on Linear Dimensions

Fire Resistance
Incombustible

Bulk Density
Generally in the range of….

Durability
Vitrified ceramic materials are unaffected by ageing, weathering, ultraviolet radiation or atmospheric pollution.
For some restoration projects or specific design requirements, it may be more important to achieve a good aesthetic match than to achieve specific physical properties and tolerances. Each project should be individually considered.

Specifications for the Manufacture and use of Architectural Terracotta & Faience

1. General
Architectural Terracotta and Faience can either be 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 or cast to an even wall thickness. Strength to the hollow void so formed is provided by adding strengthening webs of equal thickness. Once the clay is stiffened and the mould has been stripped away the unit is allowed to cure for approximately 24 hours before the surfaces to be exposed on the building are smoothed using hand tools.

2. Definitions
Architectural 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, eg. Red, buff, tawny, grey, etc.

Architectural Faience prepared clay with an applied coating of glaze or engobe which when fired provides a coloured surface finish.

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.

Glaze 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.

Engobe an opaque, coloured clay slip coat applied to the surface of a block or slab.

Block a load bearing unit, either Terracotta or Faience, formed by building clay walls approximately 35m (1 ¼ “) thick. Hollow cells are sometimes formed in the back of the block by the addition of strengthening webs which generally require filling with a weak concrete mix.

Slab a non load bearing unit, either Terracotta or Faience, in the range of 25 to 50mm thick used to clad a structural wall. If mechanical restraint fixings are required to provide stability a minimum 40mm thickness is recommended. Typically units are up to 600 x 300 m on face and are supplied cut after firing to length and height as required.

Kiln firing 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 degrees centigrade are achieved to make the units hard and durable. With Architectural Faience it is sometimes necessary, although rarely, to twice fire the units  as some glazes mature at lower temperatures than the body.

3. Manufacturing Tolerances
Overall Size generally +/- 2 % in linear dimensions
Twist – 5mm per 300mm measured across the diagonal

Out of Squareness – 5mm per 300mm length

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 dependant upon 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 pinholes, 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.

Note: it is sometimes more important to achieve a good aesthetic match, for example restoration work, than achieve specific physical properties and tolerances. 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. Please advise us of your requirements in order to 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, ie. Colour and glaze finish.

4. Packing and Delivery
Domestic – units are packed with polystyrene on non-returnable timber pallets and shrink wrapped. Delivery will be made by flat bed lorry, size to suit load and site conditions. Carriage, packing and handling is charged extra unless otherwise stated.
Export – units are packed in plastic or timber crates with polystyrene. Shipping or air freight can be arranged as required which will charged as an extra unless otherwise advised. Insurance’s, taxes and local duties can be allowed for to suit customer requirements.

5. Site Work
Inspection – Units are to be checked against delivery notes supplied with driver and any discrepancies reported. If pallets are to be set aside unopened for later use a suitable storage area is to be provided to prevent any 3rd party damage. Upon un-wrapping any units thought not acceptable are to be advised to us in writing. Upon investigation, units will be replaced free of charge but without cost for consequential damage provided that the blocks have not been filled with concrete or installed on the building.

Filling – The hollow cellular voids of the blocks are generally expected to be filled with concrete. Unless the concrete fill is expected to add structural strength it is not our recommendation that a weak mix be used. A proprietary ‘Light – Ag’ concrete is satisfactory under normal conditions. Where an improvement in structural strength has been identified compliance with the specification of the structural engineer is advised. It is not our recommendation that blocks are used as permanent shuttering for cast in situ structural concrete elements.

Installation – units are to be wetted prior to installation allowing 24 hours for the fill to cure. Units should be gauged horizontally and vertically to ensure joints can accommodate manufacturing tolerances. Where necessary units are to be cut on site to ensure a satisfactory fit and this is part of the installation contractor’s responsibility.

Check-outs  - are sections of block which are required to maintain the shape during firing but which have to be removed by the fixing contractor prior to installation.

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 terracotta due to corrosion.

Cleaning Down – Units to be washed down during installation. Chemical cleaning compounds are to be avoided unless professional advice on their use has been sought. Compounds containing hydrofluoric acid are known to damage terracotta and must not be used.

6. Maintenance
Visual Inspection – Should be carried out on a regular basis and any failure to 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.

Cleaning – Specialist advise should always be sought before undertaking any cleaning work. The avoidance of damage to the terracotta should be the primary consideration. If possible compounds containing Hydrofluoric acids should be avoided.

7. Summary
The selection of clay and glazes is often dictated by the choice of colour and finish required particularly with restoration work. It is therefore not always possible to commit ourselves to achieving specific physical properties and tolerances.
We are able to submit test results under the criteria defined above through our Technical Services Department. 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.