Experimental Studies on ‘Stains’ Formed on Polished Surfaces of Optical Glasses

III. Geometry of Stained Area

Abstract

Surface topography of YAKE, or stained areas, artificially produced on polished surfaces of optical glasses by dipping the glass samples into alkaline, acid or neutral solutions, is studied with the aid of a phase-contrast microscope and by interferometric techniques. One half of the surface under investigation is protected by covering with a layer of asphalt, and the other half exposed to the etchant.
In the case of alkaline corrosion, the glass is uniformly dissolved into the solution, and the height of the exposed surface is diminished at a constant time rate. In F 2, KF 2, KF 6 and SF 3, a peculiar attack by the etchant is found in a zone under the asphalt layer just next to the boundary line between the protected and unprotected areas. A kind of groove due to the. intense corrosion is formed in this boundary zone in the case of F 2, KF 2 or KF 6.
AOYAKE layers exhibiting interference colours are formed by acid corrosion in all kinds of glasses examined but LaK 13, which is dissolved into acid solutions. Just after the corrosion, the height of the AOYAKE surface is the same as that of the untreated (protected) surface, but the former is diminished by desiccation in vacuo or by heat treatment. The amount of this diminution in height of the corroded surface is not proportional to the duration of corrosion. The AOYAKE layers of SF 3 and SK 5 are cracked when the corrosion treatment is prolonged, and small fragments come off when dried. If the samples of F 2, KF 2, KF 6 and SF 3 attacked by acid are kept in a humid atmosphere, many microcrystals are seen to appear on the AOYAKE surface.
A very soft AOYAKE layer is formed on the surface of LaK 13 by pure water corrosion. Again in this case, a peculiar attack is observed in a zone along the boundary line.