Glass used in architecture today commonly includes clear and tinted glass substrates, low-emissivity and solar-control coatings, decorative ceramic-frit patterns and safety glazing considerations that require glass to be heat-treated. Heat-strengthened and fully tempered glass is designed to meet increased thermal and mechanical stresses, or other specified physical criteria. Tempered glass is also used to meet safety glazing code requirements. As a result of the heat-treating process, iridescence, or what is often referred to as a quench pattern/mark, strain pattern or anisotropy, may be visible in heat-strengthened and fully tempered glass under certain polarized lighting conditions.

The Technical Services Division of the National Glass Association (NGA) has produced this Glass Technical Paper solely for informational purposes. This Paper was developed by dedicated member volunteers and subject matter experts. The original version of this document was approved and published in 2008. This version of the document was updated and published in April 2022. All purchases will be electronically accessed or delivered.