Thermal Properties

Coefficient of Thermal Expansion (CTE) - The property of a material to expand in size as the temperature is raised. Most glasses have a relatively linear expansion rate between 0 and 300°C. This is an important factor to consider when placing the glass in a frame, since glass expands much less than most metals and plastics and may cause breakage upon cooling. Glass such as Borofloat® is designed with CTE to match silicon and when bonded together will expand and contract with minimal stress to the device.

Thermal Conductivity - The ability to conduct heat through the glass or away from a heat / light source. This is important when considering glass as a view port for high temperature or high infrared applications.

Maximum Operating Temperature - The maximum temperature recommended for use in an application. This may be a long or short-term rating. Note: For strengthened glass, never exceed a temperature of 100°C below the Strain Point for short-term or 200°C below the Strain Point long-term in the application.

Specific Heat - A property that indicates the amount of energy glass stores for each degree increase in temperature, on a per unit mass basis. Its units are J/kg-K.

Surface Emissivity - The relative emissive power of a body compared to that of an ideal blackbody. In other words, the fraction of thermal radiation emitted compared to the amount emitted if the body were a blackbody. By definition, a blackbody has a surface emissivity of 1. The emissivity is also equal to the absorption coefficient, or the fraction of any thermal energy incident on a body that is absorbed.

Thermal Shock - The amount of temperature differential that can be withstood by glass when subjected to a rapid change. Heating the glass to a specific temperature and then exposing to a cooler temperature, usually ice water, verifies the specific rating. Strengthened glass, whether heat-treated or chemically strengthened, significantly improves the rating over annealed glass.

Thermal Gradient - Defined as the maximum temperature differential between the front surface and the back surface that will cause a 1000 PSI tensile stress on the cooler surface.

Working Temperature - This is the temperature at which the glass may be fully re-shaped. The glass may be formed into any shape and will sag completely under its own weight.

Softening Point - The temperature at which unsupported glass will begin to sag. If the temperature is reduced slightly, the glass will remain in the sagged shape. For soda-lime glass, this point is 726°C (1340°F).

When heat treating glass, the softening point should not be exceeded during heating to prevent roller wave or other shape distortion. Generally, heat treat processing equipment targets approximately 1180-1200°F maximum glass temperature during the glass heating phase.

Annealing Point - This is the temperature at which glass will relieve stresses (either compressive or tensile) in a matter of minutes. For soda-lime glass, the annealing point is 546°C (1015°F).

When heat treating glass, all portions of the glass must exceed this temperature in order to prevent "cold-cracking" during quench.

When fabricating glass and to prevent warping or bowing due to unequal front and back surface stresses, the glass must be properly annealed. To anneal glass, whether during the manufacturing of the glass or after processing of the glass, all portions of the glass must be cooled uniformly form over the annealing point to under the stain point. (See Strain Point below)

Strain Point - This is the temperature at which glass will relieve stresses over a period of hours. For soda-lime glass this temperature is 514°C (957°F). Since this is a point of reference only, stresses will continue to relieve over extended periods of time below this temperature. The stress relief is cumulative, so this is a gradual degradation of stress if glass remains in the area of the strain point for extended periods.

Therefore, heat treated glass should not be used at a temperature exceeding 250°C (482°F) for short periods of time of 230°C (446°F) for extended periods of time (See Maximum Operating Temperature).

Glass is chemically strengthened at temperatures slightly below generally 25°C (77°F) below the stain point or else the compressive stresses introduced into the glass by ion-exchange will be relieved as they are produced, resulting in no significant temperature at which ion-exchange occurs by more than the 25°C.