GB1400824A - Process of colouring glass bodies - Google Patents

Abstract

1400824 Colouring glass GLAVERBEL 11 Sept 1972 [1 Oct 1971] 42127/72 Heading C1M [Also in Division B5] A process of colouring or modifying the colouration of a glass body containing reducing ions by causing a substance to diffuse into surface layers of the glass body from a contacting medium is characterized in that the said medium comprises a mixture of metal salts containing not more than 50% by weight of a salt providing reducible metal ions of a type capable of being reduced by said reducing ions, and in that after diffusion of reducible metal ions into said body from said medium, the final colouration is achieved by heating the body to a temperature above the strain point of the glass. The glass body may be formed from a soda-lime glass. The reduction is not relied upon for achieving the final colouration and in some cases there is no significant colouration of the glass body at the end of the first treatment stage. The process finds particular application in colouring bodies of flat soda-lime glass which may be produced by the so-called "float" process in which the reducing ions (Sn<SP>++</SP>) diffuse into the glass from the molten material the glass floats on. Alternatively the glass contains the reducing ions (Fe<SP>++</SP>) prior to being drawn into a glass body using a conventional drawing machine. The treatment medium may contain less than 10% by weight of salt(s) providing the reducible metal ions and such ions are wholly or predominantly silver ions. Such ions produce a yellow colouration. Other reducible metal ions used are of copper and gold. Preferably the treatment medium is a salt medium in molten state, the medium being agitated if necessary as by propellers or bubbling gas therethrough. However the treatment medium may comprise the mixture of metal salts in vapourifed condition. The molten medium may be sprayed on to the body, but preferably the body is immersed in a quantity of the molten medium. Only part of the surface of the body (flat or curved sheet) may be coloured or modified in colour and in such cases those parts of the surface not to be so modified are shielded from the treatment medium by a temporary coating. In further alternatives the medium may be kept in contact with the surface portion to be treated by retaining such medium within an endless retaining wall contacting the body along the boundary of the surface area to be treated or the medium may be caused to stream along the surface area to be coloured. The treatment medium advantageously contains one or more zinc salts which promote the diffusion of the reducible metal ions into the surface of the glass body. The said reducible metal ions preferably derive from a metal salt selected from nitrates, chlorides and sulphates. The balance of the treatment medium comprises alkali metal salts, e.g. NaNO 3 , KNO 3 , Na 2 SO 4 , K 2 SO 4 . When silver salts are used in the treatment medium the first treatment step is preferably carried out while the body is at a temperature less than 350‹ C.; in the second step the body is heated to a temperature of at least 500‹ C. after the diffusion of reducible metal ions thereon. The colouration development in the second stage may be promoted by bringing the treated surface of the body into contact with a reducing gas, e.g. H 2 . The salt with which the salt providing the said reducible metal ions is mixed may comprise a metal salt providing metal ions which diffuse into the glass body in exchange for other ions. After heating the glass body the body may be rapidly cooled to produce compressive surface stresses therein. In one embodiment during the heating of the body while at a temperature above the strain point of the glass ions, e.g. Li<SP>+</SP>, which confer a lower coefficient of thermal expansion on surface layers of said body are caused to diffuse into such surface layers in exchange for other ions. The body may be tempered either during or subsequent to the colouring treatment. During the final heat treatment the body may be bent.