On the properties of pitch used in working optical glass

Abstract

The paper deals with pitch as a mountant for lenses during the operations of grinding and polishing. It is shown that the viscous nature of pitch entails a number of rather intricate consequences in workshop practice, and that these phenomena require the viscosity to lie between certain well-defined limits. It must be sufficiently high to prevent measurable displacement under the external forces applied in polishing and sufficiently low to relieve rapidly the severer internal strains produced by thermal expansion and contraction. It is also shown that there is considerable difficulty in conserving the necessary properties of pitch even at relatively low temperatures, and means for meeting these difficulties are indicated. The paper is divided into four sections: Section I outlines previous knowledge of the subject; Section II deals with the physical properties of pitch and the alterations produced by low-temperature heating, in so far as these affect its workshop utility. Section III describes a number of workshop phenomena and correlates them with the physical properties described in Section II. In Section IV a brief account is given of some new methods for the mechanical handling of pitch, based on the foregoing studies.