Frit is a generic term for a material that is generated by blending minerals, oxides, and other inorganic compounds, heating to at least melting temperature, cooling, and pulverizing [HAY90]. An example frit production process is given as follows: Melting of the ingredients above 1150 °C, holding at the temperature for 4 h, cooling down in water, crushing, and pulverizing of the material. A glass is an amorphous substance without significant crystal formation [HOLL95]. In thermodynamics, glass is defined as frozen, undercooled fluid. All substances that were molten and cooled down appropriately follow this definition. The quick cooling results in the formation of crystal seeds during solidification, but the time is too short for the crystallization process.
Frits for the manufacturing of grinding wheels consist commonly of boron silicates or magnesium glass [BOTS05]. In addition, frits also contain TiO-2, fieldspar, borax, quartz, soda ash, red lead, zinc oxide, whiting, antimony trioxide, sodium silicofluoride, flint, cryolit, and boric acid [HAY90]. In producing grinding tools, frits act as flux agents and change the properties of the vitrified bond, for example by lowering the sintering temperature [JACK11, p. 92].
Frit manufacturers characterize their products by melting behavior (temperatures for beginning of fusion, melting, and sintering) and thermal expansion. A hot stage microscope analyzes the melting behavior.
3.2.2.4 Flux Agents
Siliceous clay with low melting point is considered as ‘flux agent’ and minimizes the surface tension at the interface between bond and abrasive grit [JACK07]. Lithium for example is one of the most expensive flux agents, but can decrease softening and melting point, viscosity, as well as heat expansion coefficient [BOTS05].