Experimental results to prove the existence of a thin surface layer in glass fibers of alumina silicate group are presented and the reasons for the increased strength of the surface layer as compared with the strength of the core of glass fibers and massive glasses are discussed. Five different levels of strength of glass, connected with different surface flaws, are discussed. The chemical composition of the glass fiber has a different level of strength, and this is very marked in more perfect materials. © 1969.