As was described inthe preceding paper (this Journal, Vol. 30, No. 2, p. 86 (1961)), the scratch made by a steel point on a glass surface is composed of two regions: the “fractured” and the “abnormal region.” The latter is not different from the original surface when observed by the naked eye, but its physical as well as chemical properties have been modified rather drastically through severe stress during scratch formation.
The “abnormal region” is different in refractive index and is more easily attacked by diluted hydrofluoric acid as compared with the original glass. These “abnormalities”, however, completely disappear when annealed.
When a plane surface of a glass sheet is scratched it becomes convex owing to the expansion caused by scratch formation. If the o;iginal scratches are invisible, i-e., if they are composed of the “abnormal region” exclusively, the sheet recovers from deformation on annealing. The deformation due to “secondary cracks” in the scratch is not influenced by annealing.
When a glass surface is coated by glue, and the glue film is desiccated by heating, thin flakes of glass are stripped accompanying the crack formation in the glue film. This “glue coating method” can be conveniently applied to the study of nature of the scratch. Stripping can start from visible “secondary cracks” as well as from the “abnormal regions” of the scratch. When annealed, however, the latter can no longer be the cause of stripping.
It is believed that the so-called “latent scratches” are nothing but the “abnormal regions”.
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