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.
Refractive Index - The ratio of the velocity of propagation of an electromagnetic wave in a vacuum to its velocity in the glass. This determines how much a light wave is "bent" when entering and leaving the surface of the glass. This property is important in producing certain optical devices or effects, such as lenses. The refractive index differs depending on the specific wavelength of light. Therefore, the refractive index is always specified at stated wavelengths, generally corresponding to characteristic material spectral emission lines, such as the sodium "D" line - the most commonly used wavelength value.
Dispersion - A measure of the separation of light into its component colors, such as a prism dispersing white light into a color band or a rainbow effect.
Abbe Value - The ratio of refractivity to dispersion in an optical medium. Specifically: (nd-1)/(nF-nC), where n is the index of refraction for the spectral lines Fraunhofer d. F and C, respectively.
Transmission - The passage of light through a material; light not reflected back to its source or absorbed is transmitted through the glass. Values are given as a percentage of the original source light energy versus the transmitted light energy. These values vary with wavelength and for each material. For example, soda-lime glass has very low transmission at a wavelength of 265nm, but fused silica transmits well at this wavelength.
Reflectivity - The return of light from a surface with no change in wavelength. This is generally given as a percentage of the source light energy versus the reflected energy and is dependent upon the angel of incidence (the angle between the light source and the glass surface, usually specified from "normal" - a line perpendicular to the glass surface) and wavelength.
Birefringence Constant - Birefringence is the separation of a beam of light into two beams as it passes through a doubly refracting glass. Differing stress layers in a glass will cause birefringence and therefore birefringence may be used to measure the amount of stress in a material. Since the amount of birefringence differs by material, the birefringence constant must be considered when determining stress levels.
Absorption - The amount of light energy converted to heat within a material that is not transmitted nor reflected. The amount of absorption varies by material type and wavelength. Tinted materials will absorb more light than clear materials.
Note on Sodium / Alkali Leaching:
All soda-lime type glasses and some borosilicate glasses contain sodium or alkali metal ions. Prolonged exposure to liquids or vapor, such as water, will cause the sodium / alkali ions to migrate to the surface of the glass. This can cause cloudiness or haze on the surface of the glass. Porous coatings may also incur this phenomenon, causing a disruption of the bond between the coating and the glass surface. In high humidity or critical surface applications, this must be considered when specifying the material. Placing a "barrier" coating, such as silicon dioxide on the glass will limit the amount of reaction.
Acid Resistance (SR) per ISO 8424:1987 - Large quantities of a strong acid (pH 0.3) are applied at 25°C. The time to remove a layer of 0.1 mm thick is measured and classified by SR values.
Alkali Resistance (AR) - The test occurs with large quantities of a warmed (50°C) alkali solution (pH 12). The time required to remove a layer of 0.1 mm is used to designate four alkali resistance classes AR1 to AR 4.
Volume Resistivity - Is the resistance in ohms between opposite faces of a centimeter cube of the glass tested. Its value is stated in ohms-centimeters, and it is tested per ASTM D 257. This is important when glass is used as an electrical insulator.
Dielectric Constant - of a glass is the ratio of the energy stored in a condenser with the glass as the dielectric, compared with the energy stored by the same condenser with air as the dielectric. It is a measure of the ability of a glass to store electrical energy and varies with the frequency of the voltage applied to the condenser. The dielectric strength is measured per ASTM D 150. This is important when the glass is used as a substrate for electrical or electronic devices.
Surface Resistivity - the ratio of the potential gradient parallel to the current along its surface, to the current per unit width of the surface. Surface resistivity is measured according to ASTM D 257. This is usually specified as ohms per square centimeter (O/cm2) or just ohms per square. This method is used to measure the conductivity of coated glasses.
Note on Stress:
Stress is defined as the perpendicular force per unit area applied to an object, in a way that compresses (compressive stress) or stretches (tensile stress) the object. Strength is the ability of glass to withstand these stresses. Non-strengthened glass materials have relatively low tensile strength yet high compressive strength. Therefore, most glass breakage is due to tensile stress failure.
Young's Modulus or Modulus of Elasticity; the amount of stress required to produce a unit change in length (strain). Expressed in pounds per square inch (psi) or MegaPascals (Mpa).
Poisson's Ratio - The ratio of the transverse contraction of a bar of glass to the elongation per unit length. Most glasses have a Poisson's ratio of around 0.22.
Modulus of Rupture - The Modulus of Rupture (MOR)strength of glass is determined by tests conducted in accordance with ASTM C 158. This test measures the bending (flexural) strength of glass. Expressed in psi or Mpa.
Shear Modulus - The amount of force that may be withstood by applying the force such that the glass is sheared or twisted.
Bulk Modulus - The inverse of the compressibility of the material, if the material does not exhibit any time dependent response.
Knoop Hardness - A measure of hardness employing a diamond pyramid indenter. The measure of hardness is known as the Knoop hardness number (KHN).
Density - The mass value per unit of volume, such as grams per cubic centimeter (g/cm3).
Specific Gravity - The ratio of the density of the glass to the density of water, given as a fraction.