Optical hazing in polished porcelain tiles: measuring, thus defining, controlling and defeating a defect

Optical hazing of expensive technical porcelain tiles is principally caused by isolated problems in the industrial polishing process that often occur unexpectedly. As such, the percentage of affected packaged tiles depends on the time for the problem to be recognised and fixed. Quality control is unlikely to detect optical hazing as the viewing conditions are unsuitable. While only a small percentage of defective polished technical porcelain tiles might be packaged, they are unlikely to be detected until after they are installed. It is unrealistic to expect that whole batches of tiles can be laid out and very carefully inspected under appropriate viewing conditions before installation. Optical hazing of cheaper residential porcelain tiles can also be due to its high porosity. Nanocoatings or waxes are typically used to provide stain protection, filling any pores opened by the polishing, as well as any microcracking induced by inappropriate processing conditions1. Where tiles are covered by a waxy coating, any optical hazing is likely to remain undetectable until after the tiles have been installed. Whole batches of polished residential porcelain tiles may be defective. Tile fixers cannot be made responsible for quality control as they will rarely install tiles under circumstances where the optical hazing will be detected. Although some manufacturers use glossmeters as a quality control measure, these instruments are insufficiently precise: too much light is internally refracted within the glass component of the tile surface. The only realistic option is to install appropriate artificial vision equipment at the end of the polishing train in order to detect any problem as soon as it has occurred. The University of Pisa has conducted extensive research into the optical phenomena associated with the polishing and surface modification of stones, using several different analytical techniques. This theoretical understanding has led to various prototype devices, and the granting of patents. A novel scanning reflectometer is now being commercially developed, with one production line model and another for use in the plant or laboratory. Now that we can measure optical hazing, we should be able to reach a consensus as to what is unacceptable, so that we can develop acceptance standards, and adopt quality classifications. Since the reflectometer can be used to control and optimise polishing processes and effectively correlates surface roughness and reflectance, it might also have some potential applications with respect to slip resistance assessments