A matrass is built simply: a rounded or slightly oval body, wide enough to hold a useful volume of liquid, drawn out at the top into a neck narrow enough to be stoppered, sealed, or fitted with a still head as the operation required. Used as a digesting vessel, the matrass was sealed at the neck and left standing, undisturbed, over gentle heat for an extended stretch, sometimes weeks, allowing whatever slow chemical change was underway inside to proceed without evaporation loss or outside contamination, the narrow neck minimizing the surface area exposed to air even before it was sealed.
Used instead as a distilling vessel, the same matrass took the place of a cucurbit: it was set over heat with its neck left open and a separate head fitted onto that neck, so vapor rising from the liquid inside condensed in the head exactly as it would in any alembic assembly. That dual use, digestion when sealed, distillation when capped, made the matrass one of the most generally useful vessels an alchemist owned, and its long neck, whether left open, stoppered, or extended into the particularly long form called a bolthead, gave the operator room to grip, seal, or fit apparatus onto the vessel without disturbing the body holding the liquid.