In the production of flat glass the molten silica-based mix is cooled slowly under carefully controlled conditions. This annealing procedure removes undesirable stresses from the glass. Cooling occurs in an annealing "lehr"; hence, the glass is termed "annealed" or "ordinary" glass. Annealed glass which has been heated to a temperature near its softening point and forced to cool rapidly under carefully controlled conditions is described as "heat-treated glass." The heat treating process produces highly desirable conditions of induced stress (described below) which result in additional strength, resistance to thermal stress, and impact resistance.

Heat-treated glasses are classified as either fully tempered or heat strengthened. According to Federal Specification DD-G-1403B, fully tempered glass must have a surface compression of 10,000 psi or more or an edge compression of 9,700 psi or more. Heat-strength glass must have a surface compression between 3,500 and 10,000 psi, or an edge compression between 5,500 and 9,700 psi. The fracture characteristics of heat- strengthened glass vary widely from very much like annealed glass near the 3,500 psi level to similar to fully tempered glass at the 10,000 psi level.

HEAT TREATMENT PRINCIPLE

Glass can fracture when its surfaces or edges are placed into tension. Under these conditions inherent surface or edge fissures may propagate into visible cracks.

The basic principle employed in the heat treating process of tempered glass is to create an initial condition of surface and edge compression. This condition is achieved by first heating the glass, then cooling the surfaces rapidly. This leaves the center glass thickness relatively hot compared to the surfaces. As the center thickness then cools, it forces the surfaces and edges into compression. Wind pressure, missile impact, thermal stresses or other applied loads must first overcome this compression before there is any possibility of fracture.

Nanjing SunHouse Glass Co., Ltd. (named Nanjing Shunhao Glass Co., Ltd. in the past), specializes in glass processing for building and home use, since its foundation in 1996. Our main products include: Tempered Glass, Tempered safety Glass, Bullet Proof Glass, Low E Glass, Laminated Glass, Insulated Glass, Fire resistant Glass, Low Emissivity Glass…

Each year glass-topped tables account for 20,000 serious injuries. A report in this month's issue of Pediatric Emergency Care concludes that such tables are an overlooked safety threat. Based on research conducted by Children’s Hospital Boston and written in collaboration with Consumers Union, the study found that common glass used in tables can cause severe injuries to children. Researchers concluded that many of these injuries could have been prevented by the use of tempered glass and the development of safety standards requiring its use.

As we explained earlier on this blog, when tempered glass breaks it shatters into many small fragments. But when common glass breaks, it produces large, jagged pieces. “Huge shards of glass are basically like knives. If they sever an artery, they can cause uncontrolled bleeding, and the injury can be fatal,” says Amir Kimia, MD, of the Division of Emergency Medicine at Children’s Hospital Boston, who led the study

“This is a serious safety hazard with a simple remedy,” says Donald Mays, Senior Director of Product Safety and Technical Policy for Consumers Union.  “The use of tempered glass can significantly reduce the thousands of serious injuries incurred each year from the use of common annealed glass in furniture.”