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Glasses containing lead oxide as an ingredient are subject to reduction when exposed to flames of a carbonaceous nature. The carbon partially reduces the lead oxide to its metallic state, forming a black deposit. On this account, lead glasses cannot be used in blow-pipe working with the ease with which soda-lime glasses can be worked, without reduction taking place. English crystal glass, which contains a high percentage of lead, is usually melted in hooded or covered pots to prevent the carbonaceous flames of the furnace reducing the lead and otherwise destroying the clearness of the glassware. Soda-lime glass and others without the presence of lead can be melted in open pots without any fear of reduction. Modern gas-fired recuperative furnaces, in which more complete combustion of the carbon takes place, can now be used for melting lead glasses in open pots, thus presenting a great saving in the fuel required to melt and produce such glass, besides permitting the use of a cheaper form of pot. This cannot be done with the ordinary English coal-fired furnaces.