If they are supported at intervals along a flat side, they are called muffles, and the furnace is known as a Silesian furnace.
In the Silesian process the distillation is conducted in specially constructed muffles of a prismatic shape arched above, which are arranged in two parallel rows within a low-vaulted furnace, similar to the pots in a glass furnace.
A fresh charge is then put in at once, the muffles being cleared only after three successive distillations.
The chief improvements in the plant of these processes are concerned with the manufacture of the retorts or muffles, and especially with the introduction of gas-firing.
This requires more time and fuel than the work in " open " furnaces, but in the muffles the gaseous hydrochloric acid is separated from the fire-gases, just like that evolved in the pot, and can therefore be condensed into strong hydrochloric acid, like the pot-acid.
It is not easy to keep the muffles permanently tight, and as soon as any leakages occur, either hydrochloric acid must escape into the fire-flue, or some fire-gases must enter into the muffle.
It is now usually performed by a series of muffles, superposed one over another, so that the whole forms a kind of shelfburner, with internally heated shelves (the" Rhenania "furnace).