If they are supported at intervals along a flat side, they are called muffles, and the furnace is known as a Silesian furnace.
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.
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.
As a rule every furnace accommodates ten muffles.
A fresh charge is then put in at once, the muffles being cleared only after three successive distillations.