How the Aludel Works

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.