How the Philosophical Egg Works

A philosophical egg starts as an ordinary piece of blown glass, rounded and egg-shaped, with a long neck left open at one end for filling. Sealing it is the entire point of the design: once the material to be worked was placed inside, the glassblower heated the neck until it softened and then pinched and fused it completely shut, leaving no opening, no stopper, and no seam through which anything could enter or escape. That total seal was what let a substance be carried through the whole sequence of the work, whatever stages a given operator's practice called for, without the risk that opening and reclosing the vessel at each stage would let something vital in the process leak out or let outside air contaminate it.

The sealed egg sat inside an athanor or another steady, low-heat furnace, since the whole point of the vessel was to be left alone for an extended stretch, sometimes described as running for the length of a full alchemical cycle, while the substance inside underwent its transformations unobserved and untouched. Because the vessel could never be opened to check on its progress, alchemists relied entirely on the color and appearance of the material visible through the glass to judge how the work inside was proceeding.