Patent GB-1400823-A (GB)
Title: Process of forming coloured glass bodies
IPC: C03B18/14
Filed: 1972-09-11
Abstract: 1400823 Colouring glass GLAVERBEL 11 Sept 1972 [1 Oct 1971] 42126/72 Heading C1M [Also in Division B5] A process of forming a glass body from a vitrifiable batch and colouring or modifying the colour of such body by causing a substance to diffuse into surface layers of the glass body from a contacting medium is characterized in that during or after forming of the glass body a reducing agent is introduced into the surface layers of the glass body and the body is then contacted with a medium comprising a mixture of (a) at least one salt providing reducible metal ions capable of being reduced by said reducing agent and (b) a diluting agent constituted by one or more salts of another metal or metals under temperature conditions, typically 400-600 C., such that such reducible metal ions diffuse into said body and at least part of such ions becomes reduced by said reducing agent. The glass body may be formed from a soda-lime glass. Suitable reducible metal ions which colour the glass by the diffusion type process are of Ag, Cu, or Au. The said reducible metal ions preferably derive from a metal salt selected from nitrates, chlorides and sulphates. The diluting agent, e.g. KNO 3 , may provide metal ions, preferably alkali metal ions, which diffuse into said body in exchange for smaller ions, thus causing compressive surface stresses to be set up in the glass body. The second stage treatment medium may contain zinc salts which serve to promote the diffusion of the the reducible metal ions. By introducing the reducing agent into the body of glass during or after the forming process the agent becomes concentrated in surface layers of the glass where colouration is to be effected. The reducing ions may be introduced into the glass by causing such reducing ions to diffuse into the surface of the body from a contacting medium. The reducing agent can be introduced into the glass in the course of the drawing process. However it is preferred that the reducing agent diffuses into said body from a quantity of material of higher specific gravity on which said body is formed or treated. For example during the production of flat glass, molten glass is supplied into a tank containing molten metal, e.g. molten tin so that the molten glass forms a float layer ("float glass") on the metal and reducible ions (e.g. Sn<SP>++</SP>) diffuse into the surface layers of the glass in contact with the metal. Other materials may be used for floating glass, e.g. molten lead. An electric potential difference between the metal and the glass may be used to control the ion exchanger. The contacting with the medium containing the reducible metal ions may be carried out in the float-tank in which the float glass is formed or this second stage may be performed after the glass has left the float tank. The introduction of reducing agent into already formed flat glass may take place in the same way as into float glass. Other reducible metal ions which may be introduced into the surface comprise ions of the elements Pb, Cu, Sb, Bi, S, Ce, Fe, Se, V, Cr, Mn, Mo, W and As, and in this respect the glass may be contacted with alloys such as Ce-Sn, Cu-Sb, Mn-Sn or Pb-W. Preferably the concentration of reducing agent in at least one surface portion of the glass body is at least 1% by weight. The treatment medium, apart from the diluting agent, consists wholly or predominantly of a silver compound(s) in a concentration of less than 10% by weight. The use of silver imparts a yellow colouration and the so-coloured glass may serve as an actoric light screen. A further preferred medium consists (apart from said diluting agent) wholly or predominantly of a copper compound(s) in a concentration not more than 50% by weight of said medium. Preferably the treatment medium is a salt medium in molten state, the medium being agitated if necessary which may be by 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. In one Example (10) float glass already having a bronze colouration due to presence of Fe and Se and obtained using a float tank containing molten tin is immersed in a bath containing silver ions, thereby intensifying the colour to a deep brown. The composition of specific glasses treated in accordance with the present invention are as follows in per cent by weight:- (a) 71% SiO 2 , 1% Al 2 O 3 , 14% Na 2 O, 9% CaO, 4% MgO and minor amounts of Fe 2 O 3 , SO 3 , K 2 O; (b) 80% SiO 2 , 2% Al 2 O 3 , 3À5% Na 2 O, 13% B 2 O 3 , 1% K 2 O and a minor amount of Fe 2 O 3 ; and (c) 72À5% SiO 2 , 1À5% Al 2 O 3 , 14% Na 2 O, 7À5% CaO, 4% MgO and minor amounts of K 2 O, Fe 2 O 3  and SO 3 .