Dmg tables Tempered glass is about four times stronger than annealed glass. However, tempered glass gets its strength from stresses in the glass, which also gives it the property of breaking up into many small cubes when it does break. Except for outdoor patio furniture which usually uses thinner glass, most glass tables are made with annealed glass. If a steel hammer is used to hit the edge of 3/4″ thick annealed glass, the edge will probably chip or even crack. If the hammer were used to hit the edge of 3/4″ tempered glass, the glass would either hold together with no visible damage, or instantly shatter into hundreds of little cubes. There will be no glass left to support anything that was on the table.
When glass is heated to about 1150В°F it becomes soft, and if cooled quickly the outer surfaces of the glass obtain a surface compression of about 10,000 psi. Because annealed glass is cooled slowly it doesn’t have the stresses in the glass that tempered glass has. Tempered glass gets its strength from this compression, and will break at about 24,000 psi whereas annealed glass breaks at about 6,000 psi. This is the comparison people most often use when speaking about tempered glass being four times stronger than annealed glass. However, the compressed surface layer is only about 20% of the thickness of the glass, and the inner 60% of the glass is under tension. All that has to be done to break tempered glass is to penetrate the compressed surface layer, which releases the tension within the glass by thoroughly shattering it.