METHOD FOR EXCHANGING ION IN GLASS

PURPOSE:To improve and stabilize the optical performance of an optical transmission element by bringing a glass into contact with a specified molten salt of a nitrate and nitrite to exchange the ion in the molten salt for the ion in the glass. CONSTITUTION:A glass is brought into contact with a molten salt, the ion in the molten salt is exchanged for the ion in the glass, and the following molten salt is used. Namely, a mixture of a nitrate and nitrite in which the nitrate content is controlled to 0.05-2.0wt.% is used for the molten salt. Since a nitrate contg. a small amt. of the nitrite is used for the molten salt, the condition at the interface between the glass and molten salt can be appropriately controlled in conformity to the exchange of glass ion. Consequently, the glass surface is microscopically eroded, the network constituting the glass skeleton is cut, the diffuse ion is easily migrated, and the ion exchange proceeds smoothly. Consequently, an appropriate refractive index distribution is obtained, and the optical performance of optical transmission parts formed by ion exchange is improved and stabilized.