Frit
A ceramic glass that has been premixed from raw powdered minerals and then melted, cooled by quenching in water, and ground into a fine powder. Huge quantities and varieties of frits are manufactured for the ceramic industry every year (especially for tile) by dozens of different companies.
Although the fritting process is expensive there are many advantages to using frits in glazes, enamels, etc.
-To render soluble materials insoluble
Often very useful oxides (i.e. boron) are contained in high proportions in raw materials that are either slightly or very soluble. These normally cannot be used in glazes because they have adverse effects on the slurry's fluidity, viscosity, thixotropy, or make it difficult to achieve or maintain the desired specific gravity. In addition soluble compounds are absorbed into porous bodies during glazing and this compromises the body's resistance to bloating and warping and the glaze's homogeneous structure. Fritted mixes containing these materials renders them insoluble and inert. This being said, some frit formulations require crowding the line solubility line, they are thus slightly soluble and over time can precipitate crystals into glaze slurries.
-To improve process safety of toxic metals
Some materials contain undesirable and unsafe compounds. The fritting process drives these off. Many other materials are unsafe in the workplace and fritting decreases their toxicity for ceramic production workers. Lead is a prime example. Lead frits decrease the process toxicity of raw lead compounds. Barium is another example. However the fritting process has no effect on whether or not a fired glaze will leach or not. This is a function of its chemistry, unbalanced and unstable glaze formulas are just as likely with frits as without. The primary safety benefit for frits is thus for workers who use frits in manufacturing.
-To reduce melting temperature and improve melt predictability
Since frits have been premelted to form a glass, remelting them requires less energy and lower temperatures. Frits soften over a range of temperatures (in contrast to crystalline raw materials that melt suddenly) and lend themselves very well to production situations where repeatability and ease-of-use are necessary.
-To avoid volatilization of unstable substances
Most raw ceramic materials contain sulfur or carbon compounds as well as H2O. These vaporize at various temperatures as materials decompose and are driven off as gases during firing. This volatilization activity has a detrimental effect on the glaze surface and matrix. The fritting process drives off these compounds and glazes are thus much more defect free.
-To achieve homogeneity
Other than dissolution and very localized migration, fired raw glaze melts do not mix well to create an evenly dispersed oxide structure. The fritting process employs mechanical mixing to assure a more homogeneous glass that will exhibit the intended properties.
-To achieve oxide blends that are difficult or impossible with raw materials.
Many glaze formulations cannot be achieved with insoluble raw materials (i.e. high borax, high sodium). Frits employ soluble materials to make almost any combination possible.
-Improve the quality of decoration
Over and underglaze colors work better with frits than raw materials because the former are cleaner, less reactive, melt evenly, and have a more closely controlled chemistry. This means colors are brighter by virtue of compatible chemistry, by better glaze clarity. Edges of colors also tend to bleed less and color quality is homogeneous rather than variegated (although variegating materials can be introduced to introduce this quality if desired).
-Frits make it possible to create chemistries that result in phase separations during cooling. This produces special effects, matteness, opacity and specific mechanical properties that the homogenous glass does not have.
-Frits make it possible to create chemistries that can be fired much more quickly than raw glazes because they melt late (allowing body gases of decomposition to pass before the melt is created).
-The particle sizes and surface areas in highly fritted glazes can be more tightly controlled because only one species of particle is present. Particle dynamics are responsible in part or whole for certain glaze properties and effects.
-To produce a material that has a wide softening range (as opposed to a sudden melting temperature)
The Frit market is driven by large customers (especially tile) who need certain formulations and by the prepared glaze industry. Availability of smaller quantities of frits are generally determined by what industry is using. Since the Frit market changes with time, so does the availability of frit types.
Some frit companies, such as Fusion Ceramics, freely supply the chemical analysis of their frits. Others such as Ferro are more guarded and either provide no chemistry or approximate analyses. This practice partially defeats a key purpose of using frits, namely, having control of chemistry. Infact, the lack of chemistry is a key disadvantage of using certain frits. For example, the frit manufacturer might recommend substituting part of one frit for another in a recipe to solve a specific problem (like crazing). The problem with this is that the new frit might have a chemistry that is hostile to the pigments being used, the degree of gloss, the hardness, resistance to devritification, etc. Without the chemistry the new frit can be a bit of a pandora's box. Lack of frit chemistry information works against the general trend of using ceramic calculations to take control of glaze properties. Out Bound Links
In Bound Links
- (Glossary)
Borosilicate
A silicate is an SiO2-centric solid (crystalline o... - (Typecodes)
1: FRT - Frit
- (Project)
Frits
The number of different frits in the world can be ... - (Glossary)
Flux
On the theoretical chemistry level, a flux is an o...
Pictures
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).
Example of how a frit softens over a wide temperature range
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