GLASS TERMINOLOGY

Glasses differ in such properties as refractive index, lead content, density, scratch resistance and hardness. They also have varying thermal properties such as viscosity and expansion. These terms are explained below:

Refractive index is the relationship between the speed of light in glass and air. It is a measure of the power of a glass to bend light within glass, thereby splitting it into its constituent colors. The higher the refractive index, the more color separation and reflection within the glass.  The lead content of a glass is directly proportional to its refractive index.

Knoop Hardness may be used to judge the scratch resistance and mechanical workability of a glass, in that a glass with a lower Knoop hardness will be easier to grind, but is also more susceptible to scratching.

Transformation Temperature is the temperature range in which glass gradually transforms from a brittle to a plastic state. This information is useful in the determination of the annealing temperature.

Annealing Temperature is the so-called upper annealing point. At this temperature the glass has a viscosity of 10 e13 dPas.s. Below this temperature internal stresses may not be relieved.

Softening Point is the temperature at which the glass begins to deform under its own weight. This is useful to know for such techniques as slumping and fusing.