Cord Testing Using Thermal Shock: Virtue Or Vice?

Cord is defined as a streak of glass whose composition differs from the bulk glass. Due to these compositional differences, certain properties such as thermal expansion, viscosity and density are affected. The most important of these is the coefficient of expansion as it controls the generation of stresses in both the cord streak and the surrounding glass. This article discusses the detection, measurement and testing of cord in glass and the merits or otherwise of using abraded thermal shock tests.