Resources

Fully tempered glass is created in a process that is similar to heat-strengthened glass. Cut-to-size, annealed, float glass is heat-treated and air-cooled, creating an edge compression greater than 9700 psi (67 MPa) and a surface compression greater than 10 000 psi (69 MPa). Fully tempered glass may show more visual distortion of effected images than heat-strengthened glass. Its key performance characteristics are increased strength and the ability to meet the requirements of safety glazing standards (i.e., CPSC 16 CFR 1201 or ANSI Z97).

Fully tempered glass when fractured tends to break into small irregular shaped fragments that meet the criteria of the aforementioned safety glazing standards. Under uniform static loads, fully tempered glass is about four times stronger than annealed glass of the same thickness, and twice as strong as heat strengthened glass of the same thickness. It also has significant resistance to breakage by blunt projectiles. The increased strength of fully tempered glass (due to its compression stresses) makes it an option for almost any exposure.

The increase in compression stresses and equilibrium center tension stress in fully tempered glass also contribute to infrequent occurrences of spontaneous breakage (see related article April 1998 USGlass magazine page 66). All heat treated glass will break when the compression layer is penetrated. Surface or edge damage, which does not completely penetrate the compression layer, may be propagated by thermal or wind loads, building creep and static fatigue, resulting in spontaneous breakage. This breakage may occur days or even months after the initial damage, therefore the cause is not readily apparent.

Spontaneous breakage may be the result of one or more of the following: surface or edge damage to the glass; deep scratches or gouges in the glass surface; severe weld splatter on the glass surface; glass to metal contact; thermal loading; and nickel sulfide inclusions. Nickel sulfide inclusions refer to the existence of certain types of rare and very small, undissolved nickel sulfide stones that are extremely difficult to detect. Glass manufacturers take extraordinary steps to minimize the potential for nickel sulfide inclusions. Considering that a large furnace may produce up to 600 tons of glass per day, total elimination of con-taminants is impossible.

More in Types Of Flat Glass

POG / Resources / Types Of Flat Glass / Fully Tempered Glass