An alembic is built in two parts that are set one on top of the other rather than blown as a single piece. The lower vessel, the cucurbit, is a rounded pot that holds the liquid to be distilled and sits directly over a heat source, traditionally a bed of coals or a small furnace. As the liquid warms, the more volatile fraction turns to vapor first and rises into the domed head fitted over the cucurbit's mouth. The inside of the head is cooler than the rising vapor, so the vapor condenses on its curved wall the moment it touches it, the same way steam beads on a cold window. Gravity pulls the condensed liquid down the inside of the dome to a channel that narrows into a spout, and the spout carries it out of the apparatus and down into a waiting receiver, drop by drop, without ever letting the collected liquid touch the hot vessel again.
Because the vapor that rises first is enriched in whatever component boils at a lower temperature, a single pass already partly separates a mixture, and alchemists and early chemists ran the same liquid through an alembic more than once, a practice called rectification, to concentrate it further. The head could be swapped for other cap designs, the tapered rosenhut for essential oils or the wide moor's head for bulk distillations, while the cucurbit and the basic principle stayed the same. Nothing about the process requires anything the eighth century did not have: a heat source, two shaped vessels, and an understanding that a cool surface will condense a hot vapor.