A seventeenth-century goldsmith sat at his bench in Prague, engraving a brass plate with a design copied from a manuscript he could not read. The lines he cut would seal a vessel, he was told, against the intrusion of air and the escape of spirit. He did not know he was making one of the earliest documented hermetic seals. The distinction between hermetic and non-hermetic seals is not a matter of aesthetics. It is a functional divide rooted in alchemical practice, one that separates containers meant to hold volatile substances during transformation from those meant simply to enclose.
Hermetic sealing creates an airtight barrier, preventing any exchange between the contents of a vessel and the outside atmosphere. Non-hermetic sealing allows varying degrees of airflow, pressure release, or contamination depending on the intended outcome of the operation.
Where the Term Comes From
The word “hermetic” traces directly to Hermes Trismegistus, the legendary figure who fused the Greek god Hermes with the Egyptian god Thoth. By the early medieval period, Hermes Trismegistus was credited as the founder of alchemy and the author of the Corpus Hermeticum, a collection of philosophical and magical texts composed between the first and third centuries CE in Alexandria.
The connection to sealing came later. In alchemical texts of the Renaissance, particularly the works of Paracelsus (1493–1541) and Basil Valentine (pseudonym, ca. 1500), practitioners described the need to close vessels so tightly that no vapor could escape. The seal itself became “hermetic” because it invoked the name of Hermes, the master of boundaries, messengers, and the hidden arts. A vessel sealed in his name was closed to the world. Nothing got in. Nothing got out.
By the time Sir Thomas Browne used the phrase “hermetically sealed” in his 1646 work Pseudodoxia Epidemica, the term had entered English as a technical description. Browne wrote of the method used “to seal up glasses hermetically,” referring to the practice of melting the neck of a glass vessel shut with a flame — a technique alchemists had refined over centuries.
The non-hermetic seal has no such mythic parentage. It is simply a closure that does not aim for absolute isolation: a lid, a cork, a wax seal that cracks, a stopper that breathes.
What the Seal Actually Does
In alchemical operations, the purpose of a hermetic seal is containment with transformation. The alchemist places a substance inside a glass vessel — often the phial or egg-shaped flask called the ovum philosophicum or philosophical egg — then seals it completely. The vessel is heated for days or weeks. The sealed environment forces the contents to cycle through stages of decomposition and recomposition without losing the volatile spirits that would escape into open air.
Non-hermetic seals serve a different logic. When an alchemist or herbalist wants a substance to release its essence gradually, or when pressure must be vented, the seal is deliberately loose. In distillation, for example, the receiving flask must be non-hermetic at one point: vapors condense and need somewhere to go. In fermentation, a sealed vessel that cannot vent will explode. The distinction is not better or worse. It is a choice governed by the material’s behavior under heat and time.
In practical terms, a hermetic seal in the laboratory required:
- A glass vessel with a narrow neck
- Heat applied to the neck until the glass softened
- Twisting or pinching the softened glass closed
- Slow cooling to prevent cracking
A non-hermetic seal required only whatever kept the contents from spilling while allowing air to pass: a cloth tied over the mouth of a jar, a wooden stopper left slightly ajar, a lid with a hole drilled through it.
The Symbolic Weight of the Seal
The hermetic seal carries meaning beyond its function. In alchemical symbolism, the sealed vessel represents the womb of the prima materia, the raw, chaotic substance that must be contained and cooked until it transforms into the philosopher’s stone. The vessel is the cosmos in miniature. The seal is the boundary that holds chaos inside so order can emerge.
Non-hermetic containers symbolize the opposite: openness, exchange, influence from outside. In magical practice, an unsealed container invites spirits or energies to enter. A sealed one either traps what is inside or keeps external forces at bay. The distinction maps onto the broader hermetic principle of “as above, so below”, the microcosm of the vessel mirrors the macrocosm of the universe, and its seal determines whether the two communicate or remain isolated.
This is why medieval grimoires insist on sealing certain materials. A sealed flask of alchemical water kept its “virtue”, its spiritual potency, intact. A bottle left unsealed would dissipate that virtue into the air, rendering the preparation inert.
How the Practice Entered Science
The hermetic seal moved from alchemy to early chemistry through the work of Robert Boyle (1627–1691), who used sealed vessels to study air pressure and the behavior of gases. Boyle’s air pump experiments relied on creating vacuums inside glass containers sealed with a mixture of beeswax and turpentine. He called the process “hermetically closing” the vessel, borrowing the alchemical term directly.
By the 18th century, the hermetic seal had become standard in pharmaceutical and chemical laboratories. The apothecary’s phial, a small glass bottle with a ground-glass stopper, was the non-hermetic counterpart: the stopper fit tightly enough to prevent spillage but allowed a slow exchange of air, sufficient for storing dried herbs but not volatile tinctures.
Today, the terms survive in industry. Hermetic compressors, hermetic connectors, and hermetic switches describe components sealed against moisture and gas. Non-hermetic packaging, by contrast, relies on preservatives or refrigeration to compensate for the air that inevitably enters.
Common Misconceptions
“Hermetic means magical.” It does not. The term describes a physical property, airtightness, that happens to have originated in alchemical practice. A mason jar with a rubber gasket is hermetic. A Tupperware lid creates a non-hermetic seal. The distinction is material, not mystical.
“All alchemical seals were hermetic.” False. Alchemists sealed some vessels to trap spirits and left others open to allow evaporation or fermentation. The balneum Mariae (water bath) used a non-hermetic lid so steam could escape. The athanor furnace had a chimney to vent smoke. The hermetic seal was one tool among many, not a universal rule.
“Wax seals on letters are hermetic.” Only if the wax forms an unbroken barrier covering the entire opening. A traditional sealing wax stamp pressed onto paper creates a decorative non-hermetic closure. Air can still pass around the edges. True hermetic sealing of a letter required melting wax over the entire edge of the folded paper, then pressing it flat, a technique the Vatican diplomatic corps used for sensitive correspondence until the 20th century.
FAQ
Is a cork stopper hermetic or non-hermetic?
Non-hermetic. Cork is porous and allows slow gas exchange, which is why wine bottles are stored on their side to keep the cork moist and the seal tight, but it still breathes.
Does a hermetic seal require melting glass?
Not always. Modern hermetic seals use compression gaskets, welded metal, or vacuum-tight adhesives. But the original alchemical method did require melting the glass neck closed.
Can a non-hermetic seal be used in alchemy today?
Yes, for operations that require evaporation, oxidation, or pressure release. The choice depends on the material and the goal of the work.
The hermetic seal is not a decoration and never was. It is a solution to a problem that alchemists encountered every time they heated a volatile substance: how to hold the spirit inside while the body transforms. Non-hermetic seals provide another kind of utility, letting air and pressure move where they must. The distinction between them is practical, historical, and occasionally cosmological. To understand it is to see that not every container is built for the same work, and that the seal we choose determines what happens inside.
Explore how sealed vessels function in the broader framework of alchemical practice, or examine the symbolic role of containers in Western esotericism. For a closer look at how specific tools shape ritual work, see our guide to the philosophical egg and the sealed flask.