Strength of glass can be ascertained by different methods, such as: Modulus of Elasticity, Modulus of Rupture, Shear Modulus, Bulk Modulus, Hardness, Density, and Gravity. This article will focus mainly on the Modulus of Rupture – the structural strength of glass.
Glass, as we know, is a very strong material. The established mean Modulus of Rupture for tempered glass is of 166 MPA, whereas for annealed glass it is only 41 MPA, 4 times weaker than tempered glass! Yet, glass is extremely fragile in the presence of a weak point. The strength of the glass is highly dependent on the integrity of its surface. Theoretically, the strength of the glass can be determined by the atomic bonds. If it weren’t for the micro flaws and imperfections, “perfect” glass could be up to 500 times stronger than what it is in reality.
A common misconception is that acid-etched glass is weaker than float glass. However numerous studies prove just the opposite. Acid-etched glass is in fact just as strong or stronger. This is due to the fact that there are microscopic defects or flaws on the float glass surface. By adding stress on the glass, the micro flaws grow and end up creating a critical flaw that will break the glass. Therefore, by removing a surface layer of the glass, it also removes some of those flaws, in essence making the glass stronger. Even though the acid-etched glass may be thinner, it is stronger, because of its healing effect on the glass surface.
A study done by Corning Incorporated states that, “Frequently it becomes desirable, if not essential, to remove a surface layer on a glass article. One reason may be to provide a smooth surface. Another reason is to remove surface flaws, and thereby strengthen or fortify the article.”1
And according to the University of Illinois, “The fracture strength of the glass was greatly improved by the chemical etching process.” “From this observation it must be concluded that the etching treatment greatly reduced the number and size of surface flaws present.”2
Furthermore, as the hydrofluoric acid removes a layer of glass during the acid etching process it also has the effect of dulling the sharp edges of the flaws. This process removes the severe 90 degree angle of penetration of the flaws in the glass surface and merges the multiple flaws in an evenly spread series of bumps and valleys. By creating smaller “waves”, this fundamentally heals the surface of the glass, making the glass stronger than what it was originally.