622,530. Glass manufacture. CORNING GLASS WORKS. March 19, 1947, No. 7601. Convention date, March 28, 1946. [Class 56] A substantially pure silica article comprising a skeleton with intercommunicating sub-microscopic pores (formed by melting and shaping glass of a lower silica content and extracting the soluble constituents therefrom by lixiviation, e.g., as in Specification 442,526), is consolidated by heating to a temperature below its melting point while maintaining a partial pressure of oxygen in its pores at substantially zero. Tubes for medium pressure mercury arc lamps made in this way do not evolve gas internally in use, are transparent to u.v. light down to 2,540A (due to reduction of any residual Fe 2 O 3 to FeO), and if a vacuum is used as the means for eliminating the oxygen, are free from any arsenic (used as a fining agent) which might otherwise remain. An electric resistor type vacuum furnace may be used in which the article is heated slowly to 600-900‹C. in a reducing atmosphere, e.g. of 92 per cent N and 8 per cent H, held at about 1,000‹ in vacuo for half an hour, and then heated still in vacuo, to 1,225‹ to complete consolidation to a non-porous state.