GB1241254A - Improvements in and relating to the formation of glass-ceramic articles - Google Patents

Abstract

1,241,254. Glass-ceramic articles. A. T. RANSON (Owens-Illinois Inc.). 24 April, 1969, No. 20886/69. Heading C1M. [Also in Division G2] A shaped glass-ceramic article (17) having at least one cavity (19) therein communicating with the surface via an opening (18) whose crosssectional area is less than that of the cavity, is formed by solidifying a thermally-crystallizable glass 21 (Fig. 1, which shows a single cavity) around a cavity-forming part 22 in a mould 20, the cavity-forming part 22 being formed of a refractory material which will yield to the stresses set up during the glass crystallization step, and crystallizing the glass to form a glassceramic, the part 22 being subsequently removed. During the crystallization the moulded glass 21 increases in density and therefore shrinks, and consequently the cavity shrinks. Thus the glass-ceramic articles may crack unless the material 22 will yield under stress. A suitable part 22 is made by mixing a composition comprising 10-55 wt. per cent calcium sulphate and 35-90 wt. per cent silica with water (e.g. 15-50 parts by weight water to 100 parts composition), casting, and firing to 1200-2500‹ F. The silica is preferably 200- 400 U.S. mesh. Minor amounts of B 2 O 3 , Al 2 O 3 , K 2 SO 4 and NaCl may be included. Calcium sulphate hemihydrate is preferably used. The mixture may be formed around a rod 23, e.g. of metal, ceramic or graphite, which may be threaded, to aid removal of the part 22 from the formed glass-ceramic article. Preferably the part 22 is washed with water, and thus disintegrates and is removed from the cavity formed. The mould 20 may be of metal or graphite with a coating of fibrous silica-alumina material. The glass-ceramic may be as disclosed in Specifications 1,176,522, 1,124,001 or 1,124,002.