Heat Strengthened and
Fully Tempered Glass

What is the difference between heat-strengthened (HS) and fully tempered (FT) glass? How do I determine which is needed for my project?

Heat-strengthened (HS) glass has been subjected to a specifically controlled heating and cooling cycle, and is generally twice as strong as annealed glass of the same thickness and configuration. HS glass must achieve a residual surface compressive (RSC) level between 3,500 and 7,500 PSI for thickness up to 6mm, according to ASTM C 1048. It has a greater resistance to thermal loads than annealed glass and when broken, the fragments are typically larger than those of fully tempered glass and initially may remain in the glazing opening. Heat-strengthened glass is not a safety glass product as defined by building codes. It is intended for general glazing, and is usually the choice for commercial applications where additional strength is desired to withstand wind load and thermal stress, but the strength of tempered glass is not necessary or required by building safety codes. HS glass can’t be cut or drilled after heat-strengthening and any alterations, such as edge grinding, sand blasting or acid etching, can cause premature failure.

Tempered glass is approximately four times stronger than regular annealed glass of the same thickness and configuration. Its residual surface compressive (RSC) level must exceed 10,000 PSI for thicknesses up to 6 mm per ASTM C 1048. When broken, it usually breaks into many relatively small fragments, which are less likely to cause serious injury in most applications. Tempered glass is often referred to as “safety glass” because it meets the requirements of various code organizations for safety glazing. This type of glass is usually intended for sliding doors, storm doors, building entrances, bath and shower enclosures, interior partitions and other uses requiring superior strength and safety properties. Tempered glass cannot be cut or drilled after tempering, and any alterations, such as edge grinding, sand blasting or acid etching, can cause premature failure.

It is important to note that the heat-treating process creates optical distortion that can be reduced but not eliminated. Guardian recommends that the glass manufacturer or fabricator be consulted and mock-ups be viewed and approved whenever possible.