There are two types of heat-treated glass—heat-strengthened (Kind HS) and fully tempered (Kind FT)—as defined in the American Society for Testing and Materials (ASTM) C 1048-Standard Specification for Heat-Treated Flat Glass–Kind HS, Kind FT Coated and Uncoated Glass. 

A majority of the heat-treated glass produced over the last 30 years has been fabricated in horizontal roller hearth furnaces. The preparation stage for the heat-treatment process requires annealed float glass to be cut to the required final size, the edges to be treated according to the specified finish (commonly seamed or polished) and the glass to be washed. The process then requires the glass to be transported on horizontal rollers through an oven and heated to approximately 1,150 degrees Fahrenheit (621 degrees Celsius). Upon exiting the furnace, the glass is rapidly cooled (quenched) by blowing air uniformly onto both surfaces simultaneously. The cooling process leaves the surfaces of the glass in a state of compression and the central core in compensating tension.

The color, clarity, chemical composition and light transmission characteristics of glass remain essentially unchanged after heat-treating. Likewise, hardness, specific gravity, expansion coefficient, softening point, thermal conductivity, solar optical properties and stiffness remain unchanged by the heat-treating process. The only physical properties that change are improved flexural and tensile strength, and improved resistance to thermal stresses and thermal shock. 

Under uniform loading, heat-treated glass is stronger than annealed glass of the same size and thickness. The heat-treating process does change the break pattern of the glass, i.e., fully tempered glass disintegrates into relatively small pieces meeting the safety glazing requirements.

As mentioned, the heat-treating process typically involves the transport of very hot glass on rollers.

As a result of this soft glass-to-roller contact, some glass surface changes will occur. Minute glass particles (fines) from the glass cutting and edging process, typical manufacturing plant air-borne debris or dust, refractory particles from the tempering oven roof, as well as external air-borne dirt and grit carried into the plant by the large volumes of quench air used in the process, may adhere to one or both glass surfaces. Also, the physical contact of the soft glass surface with the rollers may result in a marking or dimpling of the glass surface. 

Current glass quality specifications contained in ASTM C 1036-Standard Specification for Flat Glass establish the size and number of glass imperfections allowed based on specific visual inspection criteria. The glass surface conditions listed previously are not usually visible to the eye under normal visual circumstances. These surface conditions do not threaten the visual nor structural integrity of the product and are not reason for rejection of glass under the ASTM consensus standards. 

This material is excerpted from the Glass Association of North America (GANA) Informational Bulletin Heat-Treated Glass Surfaces Are Different, developed by the GANA Tempering Division Construction Subcommittee with permission of GANA. Copyright, GANA. All rights reserved. For more information, visit www.glasswebsite.com.

Architects' Guide to Glass & Metal
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