Also called strain pattern or anisotropies refers to a specific geometric pattern of iridescence or darkish shadows that may appear under certain lighting conditions, particularly in the presence of polarized light (also called “quench marks”).
The phenomenon are caused by the localized stresses imparted by the rapid air cooling of the heat-treating operation. The quenching process takes place by rapidly cooling the glass with a series of air nozzles arranged in rows. Glass positioned directly in line with a nozzle is induced to develop a slightly greater level of compression than glass positioned between nozzles. These compression differences result in small variations in the density of the processed glass.
Normally, the density variations will have no aesthetic consequences. Occasionally, though, under certain lighting conditions, lines or spots of localized brightness or darkness, known as quench patterns, are observed. Quench patterns are dependent upon the glass thickness, heat treatment equipment and processing conditions, lighting conditions during observation, angle of observation, and other factors. Quench patterns are not considered production or fabrication imperfections according to industry standards.