A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method

Test method

A test method is a definitive procedure that produces a test result.A test can be considered as technical operation that consists of determination of one or more characteristics of a given product, process or service according to a specified procedure. Often a test is part of an experiment.The test...

 and conditions of usage.

Etymology

The word hermetic comes from the name of the Greek

Greek mythology

Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

 god

God

God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

 Hermes

Hermes

Hermes is the great messenger of the gods in Greek mythology and a guide to the Underworld. Hermes was born on Mount Kyllini in Arcadia. An Olympian god, he is also the patron of boundaries and of the travelers who cross them, of shepherds and cowherds, of the cunning of thieves, of orators and...

, via the vocabulary of alchemy

Alchemy

Alchemy is an influential philosophical tradition whose early practitioners’ claims to profound powers were known from antiquity. The defining objectives of alchemy are varied; these include the creation of the fabled philosopher's stone possessing powers including the capability of turning base...

. The alchemists invented a process for making a glass tube airtight, which was used in distillation

Distillation

Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

. The process used a secret seal whose invention was attributed to the legendary patron of alchemy, Hermes Trismegistos.

Uses

Several types of packaging need to maintain a seal against flow of gasses: foods, pharmaceuticals, several chemicals and some consumer goods. The term can be used to describe the Food preservation

Food preservation

Food preservation is the process of treating and handling food to stop or slow down spoilage and thus allow for longer storage....

 practices such as Vacuum packing

Vacuum packing

Vacuum packing or vacuum packaging is a method of packaging that removes air from the package prior to sealing. It can involve both rigid and flexible types of packaging...

 and Canning

Canning

Canning is a method of preserving food in which the food contents are processed and sealed in an airtight container. Canning provides a typical shelf life ranging from one to five years, although under specific circumstances a freeze-dried canned product, such as canned, dried lentils, can last as...

. Barrier packaging includes containers such as glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, aluminum can

Aluminum can

An aluminum can, or can, is a container for packaging made primarily of aluminum .They are commonly used for foods and beverages but also for products such as oil, chemicals, and other liquids. The common 12-ounce size can weighs 15 grams when empty.-Usage:Use of aluminum in cans began in 1957...

s, metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 foils, and high barrier plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

s.

Building envelopes designed with sustainable architecture

Sustainable architecture

Sustainable architecture is a general term that describes environmentally conscious design techniques in the field of architecture. Sustainable architecture is framed by the larger discussion of sustainability and the pressing economic and political issues of our world...

 use airtight technologies for energy conservation

Energy conservation

Energy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...

 projects. Under the low energy building, passive house

Passive house

The term passive house refers to the rigorous, voluntary, Passivhaus standard for energy efficiency in a building, reducing its ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling. A similar standard, MINERGIE-P, is used in...

, low-energy house

Low-energy house

A low-energy house is any type of house that from design, technologies and building products uses less energy, from any source, than a traditional or average contemporary house...

, self-sufficient homes, zero energy building

Zero energy building

A zero-energy building, also known as a zero net energy building, Net-Zero Energy Building , or Net Zero Building, is a popular term to describe a building with zero net energy consumption and zero carbon emissions annually. Zero energy buildings can be independent from the energy grid supply...

, and superinsulation

Superinsulation

Superinsulation is an approach to building design, construction, and retrofitting that dramatically reduces heat loss by using much higher levels of insulation and airtightness than normal...

 standards, structures are required to be extremely air-tight compared to conventional construction. Air barriers, careful sealing of every construction joint in the building envelope, and sealing of all service penetrations through it are all used to achieve this. Airtightness minimizes the amount of warm (or cool) air that can pass through the structure, enabling the mechanical ventilation system to recover the heat before discharging the air externally. In green building

Green building

Green building refers to a structure and using process that is environmentally responsible and resource-efficient throughout a building's life-cycle: from siting to design, construction, operation, maintenance, renovation, and demolition...

, windows combine triple-pane insulated glazing with the airtight void between panes filled with argon

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 or krypton

Krypton

Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

 gas to reduce thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 and increase R-value (insulation)

R-value (insulation)

The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it or R = \Delta T/\dot Q_A.The R-value being discussed is the unit thermal resistance...

 efficiency. In landscape

Landscape

Landscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...

 and exterior construction projects airtight seals are used for general service and landscape lighting electrical connections and splices; and in other specific applications, needing to meet "airtight" standards to be "waterproof" and/or "vapor-proof" for human safety and proper functioning.

Applications for hermetic sealing include semiconductor electronics, thermostat

Thermostat

A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...

s, optical devices, MEMS, and switch

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

es. It is used for electrical or electronic parts that are designed and intended to secure against the entry of water vapor

Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 and foreign bodies in order to maintain the proper functioning and reliability of their contents.

Hermetic sealing for airtight conditions is used in archiving significant historical items. In 1951, The U.S. Constitution, U.S. Declaration of Independence and U.S. Bill of Rights were hermetically sealed with helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 gas in glass cases housed in the U.S. National Archives in Washington, DC. In 2003, they were moved to new glass cases hermetically sealed with argon

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

.

Testing

Standard test methods are available for measuring the moisture vapor transmission rate

Moisture Vapor Transmission Rate

Moisture vapor transmission rate , also water vapor transmission rate , is a measure of the passage of water vapor through a substance....

, oxygen transmission rate

Oxygen Transmission Rate

Oxygen transmission rate is the measurement of the amount of oxygen gas that passes through a substance over a given period. It is mostly carried out on non-porous materials, where the mode of transport is diffusion, but there are a growing number of applications where the transmission rate also...

, etc. of packaging materials. Completed packages, however, involve heat seals, joints, and closures which often reduce the effective barrier of the package. For example, the glass of a glass bottle

Glass bottle

A glass bottle is a bottle created from glass. Glass bottles can vary in size considerably, but are most commonly found in sizes ranging between about 10ml and 5 liters....

 may have an effective total barrier but the screw cap closure and the closure liner might not.