Q. My clay tiles have small white-centred chips on their surface. What is causing this and will it affect the durability of the tiles?
A. The small pits that are occasionally visible on the surface of clay products are created when pockets of lime immediately below the surface expand, causing the surface above the lime to be pushed up or 'blown'. This expansion takes place as soon as the tiles leave the kiln as the tiles absorb moisture from the atmosphere. Lime occurs naturally in most clays and can usually be neutralised, ie prevented from expanding, by submerging the tiles in water.
Wienerberger makes every effort to prevent lime blows, although occasionally the process can still occur before the tiles have been fully soaked. The expansion action of the lime only occurs immediately after the tiles leave the kiln. The process stops once the tiles have absorbed moisture and can not re-occur. Therefore there is no risk of further 'pitting' to the tile surface after the tiles have been laid on the roof. It is a common misconception that clay products can be attacked by frost action due to irregularities within the surface finish but there is no possibility that the small pits, or 'lime blows', will affect the future durability of the tiles.
All our clay tiles pass the European Standard test for frost resistance, BS EN 539-2: 1998. Under the terms of the European Standard for clay roofing tiles, BS EN 1304: 1998, small pits or chips 7 mm or less in size in the surface of the tiles are not regarded as faults.