CERAMICS

Cerramics are materials often used in construction, made from a mixture of minerals, typically silica sand, with a clay binder and some impurities, and up to 30% water. They are fired at a higher temperature than bricks, so that the silica re-crystallises to form a glassy material that has greater density, strength, hardness, resistance to chemicals and frost and a greater dimensional stability.

During firing, the water is driven off, though this may be reduced from 30% to 2-5% by drying before firing. At this reduced water content products are moulded as powder before being fired at 1,800-2,000 degrees for days or weeks at a time, depending on the ceramic and process details. Ceramics may have an as-fired appearance or be glazed (a glass-like coating).

These materials are environmentally stable - they will not oxidise further in the atmosphere, therefore, they are economical in terms of maintenance costs. Problems are likely to occur when they are combined with other materials, typically fixings which are highly stressed and subject to corrosion. If fixings fail, the result can be dramatic. Unlike metals, ceramics are not capable of ductile behaviour. They fail in a brittle manner, directly after their elastic limit.

Types of ceramics

Fire clays and shales

These products include ordinary bricks, clay roof tiles, flooring quarries and pavers.

Terracotta

This is literally ‘burnt earth’. It is made from yellow to brownish-red clays with a uniformity and fineness between brick and vitrified wall tiles. Terracotta is often used for unglazed chimney pots, air bricks, copings and planters.

For more information, see Terracotta.

Faience

This is a glazed form of terra-cotta or stoneware. The base material may be fired to the ‘biscuit’ stage before glazing and re-firing, or a ‘once-fired’ process may be used. The latter improves resistance of the glaze to crazing (the spread of lines or cracks on the glazed surface), but reduces the range of colours available.

For more information see: Faience.

Fireclay

This contains a high proportion of clay resistant to high temperatures (kaolin). It is used for chimney flue linings and firebacks.

Stoneware

This is similar in composition to fireclay, but is fired at a higher temperature than fireclay and contains a higher proportion of glass. As a result it is harder and less absorbent. Modern manufacturing processes mean that stoneware no longer has to be glazed for use in drainage pipes.

Earthenware

The raw materials are blended and may contain a considerable proportion of limestone. It is a finer product than stoneware and is used as the body for glazed wall tiles and table ‘china’. Water absorption may be up to 15%, however, making it less suitable for sanitaryware than vitreous china.

Vitreous china

This has a higher glass content than earthenware, and its water absorption is only about 0.5%, which makes it suitable for sanitary fittings. It is stronger than earthenware.

Porcelain

Porcelain is very similar to vitreous china, but is often made from purer materials under more strictly controlled conditions. It is used for special uses, such as electrical insulators.

New ceramics

These are also called ‘technical’ or ‘engineering’ ceramics. Their purity is far higher than traditional ceramics, not using raw clay mined directly from the ground. Powders are formed which are then cast, pressed, extruded or moulded into shape. The powders may be set in organic binders. The combination of pure materials and exacting production techniques ensures the very high strength of these materials.