GB995212A - Improvements in or relating to moulded devitrified glass elements - Google Patents

Abstract

<PICT:0995212/C1/1> A moulded glass element is made by mixing a powdered aluminous silicate glass component with a powdered magnesium oxide component, shaping, heating and sintering the mixture, thereby causing the components to react together and to devitrify. The element contains predominantly one or more of cordierite (2MgO, 2Al2O3, 5SiO2), spinel (MgO, Al2O3), or forsterite (2MgO, SiO2), with or without anorthite (CaO, Al2O3, SiO2). The aluminous silicate component preferably contains at least 10% by weight of Al2O3, and may consist, in per cent by weight, of SiO2, 54-68; Al2O3 17-27; MgO, 4-12; CaO, 0-10; BaO, 0-1; B2O3, 0-3; TiO2, 0-3; F, 0-0.5; alkali metal oxide, e.g. Na2O + K2O, up to 0.4, and the grain size may be less than 60 m . The MgO component may contain: MgO, 87.14; SiO2, 3.08; CaO, 1.93, with heating loss of 7.00%, and the grain size may be less than 49 m . The mixture may contain up to 45% MgO. Examples of element specimens A1 to A10 are given whose compositions are indicated in the Figure; the ranges of components are as follows: SiO2, 29.8-52.1; MgO, 47.6-8.5; Al,O3, 13.7-25.2; B2O3, 1.5-2.8; CaO, 6.2-9.7; BaO, 0.5-0.9; Na2O, 0.1-0.3; K2O, 0.1; F, 0.1-0.3. The aluminous silicate/magnesium oxide mixture is formed by grinding the two powders with an anhydrous medium, e.g. acetone, amyl alcohol or methylated spirit, to less than 5 m , filtered, dried, and forced through a 60 m mesh screen. 4-8% plasticizing agent, e.g. a cellulose derivative or wax emulsion, is then added. The mixture is then shaped or moulded by means of vacuum continuous extrusion presses or vacuum piston presses. The binder is then burnt out and the elements sintered while horizontal or suspended in furnaces electrically heated in stages up to 1400 DEG C. The lowest sintering temperature is 30-50 DEG C. above the glass softening point. During the sintering step, devitrification occurs.