Aludel

An aludel is unusual among the vessels in this category because it has no bottom at all: it is essentially an open tube, wider at the base and narrowing toward the top, designed to be set over another vessel rather than to hold anything on its own. The lowest aludel in a stack sits directly over a heated pot containing the solid material to be purified, and as that material is heated it does not melt into a liquid but sublimes, passing directly from solid to vapor.

The rising vapor travels up into the aludel's open interior, where the cooler air inside causes it to condense, or resublime, directly back into a solid again, this time as fine, purified crystals clinging to the inner walls rather than mixed with whatever impurities remained in the original material below. Multiple aludels could be stacked one on top of another, each diminishing slightly in size toward the top, giving the rising vapor a longer path and more cool surface to condense against, with a closed head at the very top to keep the finest vapor from escaping entirely. Because nothing ever passes through a liquid phase, sublimation in an aludel produces an unusually clean separation of a volatile solid from whatever nonvolatile material it started mixed with.

The aludel's name traces to Arabic al-uthal, itself derived from an earlier Greek word for "smoky" or "sooty," a fitting origin for a vessel built to handle vapor. It is documented in detail in the Arabic alchemical corpus attributed to Jabir ibn Hayyan, and a description and illustration closely matching later medieval depictions survives in an Arabic manuscript titled 'Ainu-s-San'ah, written in Baghdad in the year 1034, giving the aludel one of the more securely dated early descriptions in this entire category.

The design passed from Arabic into Latin alchemical practice, where it remained the standard sublimation apparatus for purifying substances such as cinnabar and mercury throughout the medieval and early modern periods, playing a significant role in the development of early experimental chemistry. The aludel is widely regarded by historians of chemistry as a direct forerunner of later distillation apparatus, since it works on the same underlying principle of driving off a vapor and recondensing it elsewhere in a purer form. It did not survive into the modern laboratory under its own name or shape, but sublimation itself remains a standard purification technique in chemistry today, carried out in purpose-built sublimation apparatus that performs, with more precise temperature control, exactly the physical process the stacked aludel was built to do.