Muffle furnaces are suitable for fine ores which are liable to decrepitate or sinter.
These burners, or modifications of them, have also been applied to muffle furnaces, which are convenient when only a few assays have to be made - the furnace being a mere clay shell and soon brought to a working temperature; but the fuel is too expensive to allow of their being used habitually or on a large scale.
The precipitate is collected in a filter-press, and then roasted in muffle furnaces with nitre, borax and sodium carbonate.
The process is effected either in heaps, stalls, shaft furnaces, reverberatory furnaces or muffle furnaces.
The dry method consists in an oxidizing roasting of the ores, and a subsequent chloridizing roasting with either common salt or Abraumsalz in reverberatory or muffle furnaces.