GB626008A - Improvements in and relating to enamelling - Google Patents

Abstract

626,008. Enamelling. ELMES, A. W., GLASS, H. M., and IMPERIAL CHEMICAL INDUSTRIES, Ltd. May 7, 1947, No. 12346. [Class 56] In enamelling metal articles, heat and reduced pressure are simultaneously applied to (1) the metal base prior to coating, (2) the ground coat, and (3) the cover coat or coats; the invention is limited to (1), (1) + (2), (1) + (2) + (3) and (2) + (3), the number of pinholes found by high voltage spark test in these cases being 12 per cent or less of the number present in articles enamelled in the normal way. Reference is made to a pressure of 4mm. Hg. absolute for a period of 3-4 mins. (other times are also mentioned); the heating of (1) is 50‹C. above the normal enamelling temperature (800 to 950‹ ) used for (2) and (3), and the vacuum is maintained during cooling to ensure that the metal remains "degassed"; in the case of (2) and (3), the presence of oxygen, necessary for adherence, is ensured by applying the vacuum only after the enamel has glossed. The process may be carried out in a fused silica closed end tube within a tubular wirewound electric furnace; the open end, protruding from the furnace, is closed by a bung and two side arms provide connections to a vacuum pump and to a valve to atmosphere. Ground and cover coat enamel slip compositions are given respectively as having the following analysis (per cent by wt.): SiO 2 52.6, 46.5, CaO 4.3, 2.9, Co 3 O 4 1.4, -, Al 2 O 3 9.0, 3.0, TiO 2 0.26, 7.0, K 2 O 2.6, 0.5, F 1.1, 2.5, Fe 2 O 3 0.43, - MgO 0.72, 0.3, BaO 2.3, -, Mn.,0, 0.64, -, Na 2 O 13.4, 16.8, B 2 O 3 11.9, 6.2, SnO 2 , -, 5.6, Sb 2 O 5 , -, 8.0.