Vitrification (from Latin vitreum, „glass“ via French vitrifier) 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 vitrified solid. Certain chemical reactions also result in glasses. In a wider sense, the embedding of material in a glassy matrix is also called vitrification. An important application is the vitrification of radioactive waste to obtain a stable compound that is suitable for ultimate disposal.
In terms of chemistry, vitrification is characteristic for amorphous materials or disordered systems and occurs when bonding between elementary particles (atoms, molecules, forming blocks) 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 (Tg): below Tgamorphous materials are glassy whereas above Tg they are molten.
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.“
Glazing alone does not make pottery impermeable to water. 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.