The hidden perils of glazing building ceramics

»1 Clay roofing tile with discolouration from the 1990s

»2 Current damage (right) of the discoloured glaze compared with the non-afflicted glaze (left)

»3 SEM image showing the clouded glaze/cavities

»4 Reproduction of the damage pattern with a leaching test

Anyone having anything to do with glazing building ceramics, especially clay roofing tiles, on a daily basis is no doubt well aware of the perils and intricacies that must be taken into consideration. These are down to the many factors that influence the ultimate quality of the glaze. Sources of defects can be the glaze itself (in the form of the glaze powder), the glaze slip, the application process as well as the drying and firing technology used. In the “best” case the production errors show up immediately after firing. However, cases occur in which the defects or changes only become noticeable during use of the product, i.e. during its storage or after it is installed on a roof.

1 Problem description

In the late 1990s, an incident was reported in the roofing tile industry in which a large number of around one- to two-year-old glazed roofing tiles had become discoloured, as shown in »1. The peculiar thing about the damage was that not all the roofing tiles on a roof were affected, and of the damaged roofing tiles themselves only those areas exposed to direct sunshine had become discoloured.

In many tests, the cause for the discolouration was established as clouding of the glaze caused by anhydrite leaching (calcium sulphate). The resulting cavities formed a sponge...