Vitrification is the transformation of a substance into a glass, that is to say, a non-crystalline amorphous solid. In the production of ceramics, vitrification is responsible for its impermeability to water. Vitrification is usually achieved by heating materials until they liquidize, then cooling the liquid, often rapidly, so that it passes through the glass transition to form a glassy solid. Certain chemical reactions also result in glasses. In terms of chemistry, vitrification is characteristic for amorphous materials or disordered systems and occurs when bonding between elementary particles becomes higher than a certain threshold value. Thermal fluctuations break the bonds; therefore, the lower the temperature, the higher the degree of connectivity. Because of that, amorphous materials have a characteristic threshold temperature termed glass transition temperature : below Tg amorphous materials are glassy whereas above Tg they are molten. The most common applications are in the making of pottery, glass, and some types of food, but there are many others, such as the vitrification of an antifreeze-like liquid in cryopreservation. In a different sense of the word, the embedding of material inside a glassy matrix is also called vitrification. An important application is the vitrification of radioactive waste to obtain a substance that is hopefully safer and more stable for disposal. During the eruption of Mount Vesuvius in 79 C.E. a victim's brain was vitrified by the extreme heat of the volcanic ash. However, this claim is not supported by rigorous chemical analysis and explanation in the cited or original reference sources, and should be viewed with caution. The conditions described are not typically associated with the formation of a glass, but the Vesuvius eruption produced vast quantities of silicious volcanic glass.
Ceramics
Vitrification is the progressive partial fusion of a clay, or of a body, as a result of a firing process. As vitrification proceeds, the proportion of glassy bond increases and the apparent porosity of the fired product becomes progressively lower. Vitreous bodies have open porosity, and may be either opaque or translucent. In this context, "zero porosity" may be defined as less than 1% water absorption. However, various standard procedures define the conditions of water absorption. An example is by ASTM, who state "The term vitreous generally signifies less than 0.5% absorption, except for floor and wall tile and low-voltage electrical insulators, which are considered vitreous up to 3% water absorption." Pottery can be made impermeable to water by glazing or by vitrification. Porcelain, bone china and sanitaryware are examples of vitrified pottery, and are impermeable even without glaze. Stoneware may be vitrified or semi-vitrified; the latter type would not be impermeable without glaze.
Applications
When sucrose is cooled slowly it results in crystal sugar, but when cooled rapidly it can form syrupy cotton candy. Vitrification can also occur in a liquid such as water, usually through very rapid cooling or the introduction of agents that suppress the formation of ice crystals. This is in contrast to ordinary freezing which results in icecrystal formation. Vitrification is used in cryo-electron microscopy to cool samples so quickly that they can be imaged with an electron microscope without damage. In 2017, the Nobel prize for chemistry was awarded for the development of this technology, which can be used to image objects such as proteins or virus particles. Ordinary soda-lime glass, used in windows and drinking containers, is created by the addition of sodium carbonate and lime to silicon dioxide. Without these additives, silicon dioxide will require very high temperature to obtain a melt, and subsequently a glass. Vitrification is used in disposal and long-term storage of nuclear waste or other hazardous wastes in a method called geomelting. Waste is mixed with glass-forming chemicals in a furnace to form molten glass that then solidifies in canisters, thereby immobilizing the waste. The final waste form resembles obsidian and is a non-leaching, durable material that effectively traps the waste inside. It is widely assumed that such waste can be stored for relatively long periods in this form without concern for air or groundwatercontamination. Bulk vitrification uses electrodes to melt soil and wastes where they lie buried. The hardened waste may then be disinterred with less danger of widespread contamination. According to the Pacific Northwest National Labs, "Vitrification locks dangerous materials into a stable glass form that will last for thousands of years."
Vitrification in cryopreservation
is used to preserve, for example, human egg cells and embryos. Currently, vitrification techniques have only been applied to brains by Alcor and to the upper body by the Cryonics Institute, but research is in progress by both organizations to apply vitrification to the whole body. Many woody plants living in polar regions naturally vitrify their cells to survive the cold. Some can survive immersion in liquid nitrogen and liquid helium. Additives used in cryobiology or produced naturally by organisms living in polar regions are called cryoprotectants.
