Hermes’ seal the hermetic seal which closes the alchemical vessel and keeps it airtight by either fusion or welding. The sealing not only keeps the mixture in the glass vessel secure from the intrusion of outside influences, but also makes sure the mercurial contents do not escape.
Abraham defines Hermes’ seal as the operative alchemical closure that simultaneously guards the Work from external contamination and prevents the volatile spirit from dissipating — a double function of containment and protection central to the opus.
, A Dictionary of Alchemical Imagery, 1998thesis