psssible it can advantageously be rotated in the melt by means of an appropriate mounting by the stirring effect approximately constant exchange conditions are achieved for every point of the substrate this results in uniform diffusion layers and accordingly uniform pressure stress zones it should be noted here that the silver is present in the glass material mainly as metallic silver ag o since silver atoms have a distinctly larger radius than ag ions the observed reduction of ag to ag o leads to an increase of the volume of the glass components and thereby to a correspondingly increased pressure stress in the glass layers with ion exchange if the diffusion profiles of k na and ag in glass substrates treated according to the invention are considered a profile expected per se is found the diffusion of the potassium is not influenced noticeably by the silver diffusion the potassium diffusion profile always reaches about 1 8 to 1 10 of the silver diffusion depth as can further be expected the depth of the diffusion profiles increases with an increasing exchange time and exchange temperature thus a temperature increase by 50 u00b0 k has about the same influence as quadrupling the exchange time namely doubling the diffusion depth accordingly a reduction of the exchange time by about a factor 4 is possible in the.