“Gutwirthova Marie, Private, Prague. A Method of Producing a Powder Mixture Releasing Heat and consisting of iron filings and ammonium chloride, especially for heaters using paper containers; iron fillings are mixed with ammonium chloride and acidified water in the ratio of ca. 20: 1: 1 and left to react; this mixture is subsequently mixed with washing soda and another dose of water in the same ratio of ca. 20: 1: 1 and left to react while stirring. Application October 15 1934 ” Patent Bulletin 1936, p. 492