The retorts of the prisoners were notable.
This is mixed with small coal, and when redistilled gives an enriched dust, and by repeating the process and distilling from cast iron retorts the metal is obtained.
The retorts are charged with molten sulphur from an upper reservoir, which is kept at the requisite temperature by means of the lost heat of the retort fires.
The pyrites is subjected to dry distillation from out of iron or fire-clay tubular retorts at a bright red heat.
These distilling vessels are called retorts if they are supported only at the ends, and the furnace using them is termed a Belgian furnace.
