Heat strengthened glass is similar to tempered glass except that the cooling is done at a much slower pace. Annealed glass is heated to approximately 650-700 º C, but the cooling process is slower than that for tempered glass. Heat strengthened glass is about twice as strong as annealed glass of the same size and thickness. Heat strengthened glass is a semi tempered glass which retains the normal properties of ordinary float glass. Heat Strengthened glass has been strengthened thermally by inducing a surface compression of 6000 to 9000 psi as compared to a range of 11000 to 20000 psi in case of fully tempered glass. Heat strengthening adds strength to the glass while limiting the change in its breakage characteristics. The cooling process places the surfaces of the glass in a state of high compression and the central core in a state of compensating tension.
HS glass - in which heats the glass to a uniform temperature of approximately 650º C to 700 º C in an electrically heated horizontal furnace. Ceramic rolls convey the glass through these furnaces at speeds regulated to ensure temperature uniformity and minimal optical distortions. The heat strengthened process parallels the traditional process of glass tempering, except that the cooling cycle is less rapid. However, the heating temperature remains the same for both processes. The residual stress on the edge and the glass surface differ in both cases with tempered glass having a higher level of stress. Due to the relatively lower rate of cooling during the strengthening process, heat strengthened glass develops less stress as compared to fully tempered glass. The process increases the mechanical and thermal strength of heat strengthened glass, making it twice as tough as annealed glass.