Heat-Treated Glass

processed glass

What is Heat-Treated Glass?

Types of Heat Treated Glass.

Heat-treated glass is a term used to describe glass that has been processed through a tempering furnace to alter its strength characteristics, provide greater resistance to thermal and mechanical stresses and achieve specific break patterns for safety glazing applications as compared to annealed glass.

The process of heat-treating glass is taking annealed glass, cutting it to its desired size, transferring the glass to a furnace and heating it to approximately 1,150°F. Once at this temperature, the glass exits the furnace and is then rapidly cooled or quenched. Air is blown on to the glass surface on both sides simultaneously. This cooling process creates a state of high compression at the glass surfaces while the central core of the glass is in a compensating tension.

The only physical characteristics of the glass that change are the improved strength and resistance to thermal stress and shock. There are two kinds of heat-treated glass, heat-strengthened(HS) and fully tempered(FT).

Fabrication requirements, tolerances, and testing procedures for heat treated glass are defined in ASTM C1048 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass.

Annealed (AN)

Raw glass that has not been heat treated is annealed glass. In a specification, the designation for annealed glass is AN. Due to the fragile nature of annealed glass, the break pattern becomes large pieces and shards of glass. Please see the image indicated.
Double strength

Heat-Strengthened (HS)

Heat-strengthened glass is twice as strong as annealed glass of the same thickness, size and type. If broken, heat-strengthened glass will break into large shards similar to annealed glass. The surface compression of heat-strengthened glass with thicknesses of 1/4″ (6mm) and less is 4,000-7,000 psi. Surface compression for 5/16″ (8mm) and 3/8″ (10mm) heat-strengthened glass is 5,000-8,000 psi. While improving the strength and resistance to thermal shock and stress, heat-strengthened glass does not meet safety glazing requirements as outlined by the American National Standards Institute (ANSI) Z97.1 or the federal safety standard Consumer Products Safety Commission (CPSC)16CFR1201, and therefore should not be used in these situations. In a specification, the designation for heat strengthened glass is HS.
Four time Stronger

Fully Tempered (FT)

Glass with fully tempered surfaces is typically four times stronger than annealed glass and two times as strong as heat-strengthened glass of the same thickness, size and type. In the event that fully tempered glass is broken, it will break into fairly small pieces, reducing the chance for injury. In doing so, the small glass shards make it more likely that the glass will become separated from the opening.

The minimum surface compression for fully tempered glass is 10,000psi. In addition, it complies with the safety glazing requirements as outlined by the American National Standards Institute (ANSI)Z97.1 and the federal safety standard Consumer Products Safety Commission (CPSC)16CFR1201. In a specification, the designation for fully tempered glass is commonly abbreviated as FT. Please see index for full copy of ASTM C-1048, the specification that covers heat-treated glass.

Avoid spontaneous breakage

Heat Soaking

Fully tempered glass may break without warning due to the expansion of nickel sulfide inclusions (NiS) present within float glass. To avoid the risk of spontaneous breakage, the heat soak oven will cause pieces to with NiS present to break within the eight hour heating process. Glassfab can perform a heat soak test to provide the added assurance that significant spontaneous breakage will not occur. Glassfab Tempering Services adheres to BSEN14179-1:2016, A European Standard that specifies the heat soak process system together with tolerances, flatness, edgework, fragmentation and physical and mechanical characteristics of monolithic flat heat soaked thermally toughened soda lime silicate safety glass for use in buildings. Please see index for full specification
meausring optical distortion

Lite Sentry

LiteSentry’s revolutionary Osprey® inspection systems include the only high preforming systems that meet and exceed growing requirements of leading architects and glass suppliers, including mandated distortion and anisotropy measurements. The Osprey® is a sophisticated, on-line visual inspection system that measures optical distortion of all types, including roll wave, edge kink, picture framing, corner lift, belly banding, vertical kink, bi-stability, saddle bow, bird’s eye, pocket, hammer and any other distortion in heat-treated and laminated glass, solar PV and sheet plastic. The Osprey® 10 Distortion by LiteSentry™ exceeds all industry requirements, providing optimal process control.


GTS Inc. provides fabricated glass that, at minimum, will meet ASTM C1048 Standard Specification for heat treated flat glass. GTS Inc. warrants that its tempered glass will meet the safety criteria of CPSC 16 CFR #1201 categories 1 & 2 and ANSI Z97.1 1984, as in effect from time to time. GTS Inc. fabricates and heat soaks to BS EN 14179-1 and internal standards, and that should set the criteria for measurable acceptance. GTS Inc. can fabricate to internal Enhanced Quality Control (EQC) for Bow and Warp; Rollerwave Distortion standards if required. Any GTS Inc. heat treated/tempered glass may not be modified, drilled, sandblasted, ground, polished, or fabricated. Such modification can seriously weaken the glass, impair its safety characteristics, and shall void any warranty by GTS Inc. All GTS Inc. heat treated glass will have a logo per code. Purchaser must specify and assume responsibility to pass building compliance inspection for NO LOGO orders. All heat-treated and tempered products are subject to minimum seamed edge, unless otherwise specified by the purchaser for ground or polished edge finish. Anisotropy is inherent in all heat-treated flat glass and is not a reason for rejection.
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