The precipitate is collected in a filter-press, and then roasted in muffle furnaces with nitre, borax and sodium carbonate.
High, and heated either in a muffle or in a crucible furnace at a gradually increasing heat for forty or fifty minutes.
Types of muffle furnaces are figured in the article Annealing, Hardening And Tempering.
"Ultramarine rich in silica" is generally obtained by heating a mixture of pure clay, very fine white sand, sulphur and charcoal in a muffle-furnace.
The door of the muffle is then opened and the current of air which is drawn over the scorifier rapidly oxidizes the lead, while the melted litharge gradually closes over the metal.
Metallurgical furnaces of the first class are termed crucible, muffle or retort furnaces, and of the second shaft and reverberatory furnaces.
The conversion of the sulphide into oxide is adopted where the Douglas-Hunt process is employed, or where hydrochloric or sulphuric acids are cheap. The calcination is effected in reverberatory furnaces, or in muffle furnaces, if the sulphur is to be recovered.
In the .scorification method the ore is mixed in the scorifier (a shallow dish of burned clay) with from ten to twenty times its weight of granulated metallic lead (test lead) and a little borax glass, and heated in a muffle, the front of which is at first closed.
It is, therefore, decidedly preferable to employ " muffle-furnaces " in which the heating is performed from without, the fire-gases passing first over the arch and then under the bottom of the muffle.
The scorifier is taken from the muffle in a pair of tongs and the contents poured into a mould, the lead forming a button in the bottom while the slag floats on top. When cold, the contents of the mould are taken out and the lead button hammered into the form of a cube, the slag, which is glassy and brittle, separating readily from the metal, which is then ready for cupellation.
In consequence the fire-gases, when arriving there by the chimney shaft (a), have already a good upward draught, and when circulatung round the muffle are at a lower pressure than the gases within the muffle, so that in case of any cracks being formed, no hydrochloric acid escapes into the fire-flues, but vice versa.
(a) Heating chamber fixed and forming part of furnace= muffle furnaces.
These are known as muffle or chamber furnaces; and by supposing the crucibles or retorts to represent similar chambers of only temporary duration, the ordinary pot melting air furnaces, and those for the reduction of zinc ores or the manufacture of coal gas, may be included in the same category.