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Frit

Frit master

Huge quantities and varieties of frits are manufactured for the ceramic industry every year by dozens of different companies. They are made by melting mixes of raw materials in special kilns, then pouring the mix into water and finally grinding it into a fine powder.

Frit suppliers refer to the use of their frits in 'partially fritted' and 'all-fritted' glazes. The latter generally refers to glazes with 90% or more frit, the former to 90%.

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.

-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 normal 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. Barium and strontium frits are good examples, barium and strontium carbonate produce a lot of glaze defects because of the gases of decomposition they produce.

-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 completely homogenous 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. One interesting group is the 'specific oxide' borosilicates, they contain borosilicate and one other oxide (i.e. calcium, barium, sodium, strontium, lithium). Frits GF-125, 129, 143, 154, 156 are examples.

-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).

-Fast fire technology
Industry now measures firing time in minutes instead of hours. Frits can be formulated to melt quickly and completely after body gases have been expelled, thus greatly reducing glaze imperfections.

-Matte glazes
High zinc, barium, calcium, strontium, and alumina frits can be used and blended to create quality matte glazes that are very difficult to make with raw materials that do not melt enough and produce gases of decomposition.

-Opaque glazes
When zircon is added to a frit during the smelting process it is a more effective opacifier. Clear and opaque frits can be blended to give excellent control over opacity.

In recent years frits are even being used at high temperatures. For example defect free high strontium, barium and even calcium glazes can be made more easily with added frit, especially in fast fire operations.

The Frit market is driven by large customers 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 some frit types.

In recent years some frit companies, such as Fusion Ceramics, freely supplied the chemical analysis of their frits. Others, such as Ferro who were forthcoming with data in the past, have been more guarded and either provide no chemistry or approximate analyses. There is now a situation of 'legal chill' in the industry because of law suits against the frit manufacturers for deviance of their products from the published formulas. Now most companies are hesitant to supply formula data. Since most standard frits have been made for many years users are doing 'intelligence work' to get the formulas by finding them in older magazines and books and by deducing them from charts of equivalent frits from different manufacturers.

However each manufacturer makes specialized frits (i.e. strontium, lithium compounds) that they invest heavily in R&D to develop. The makeup of these are usually kept secret to protect against the formulations be copied by other manufacturers. Even though powdered samples of these frits could be analysed by competitors to deduce their approximate makeup, the tightly controlled chemistry required to achieve the intended effect may not be evident. Thus the actual production of a duplicate can be a more elusive goal than it at first seems.

These factors are of great interest to people using ceramic calculations. The secrecy makes little sense since it partially defeats the whole purpose of using frits, namely, having control. It also works against the general trend toward using the oxide viewpoint to take greater control of glaze properties.

Richard Willis

Natural or composed materials of very high silica content melted to a glass, shattered by rapid cooling, and (usually) milled to a fine powder prior to use in clay fabrics or glazes.
The usual procedure is to fill a crucible with dry-blended ingredients as either a finely powdered or coarsely granulated matter, heat the crucible until the matter melts, pour it (or let it drip from a hole in the bottom of the crucible) into cold water while molten, whereby it will explode as shattered glass, collect the “shattered glass” granules to a mill, and grind to desired fineness.
Fritting materials prior to their use in recipes affords several benefits, the major ones being: the mix will be more homogeneous; the fritted form of a material always fuses at lower temperatures than its unfritted form; and many undesirables (corrupting and/or toxic and/or gaseous agents) will be made inert, neutralized, or burned away. As well, factors such as thermal dilations will be greatly improved. For these reasons, among others, volcanic equivalents to metamorphic materials are often preferred whe
re man-made frits are not so available or practical to obtain or make. Nevertheless, natural frits, such as “volcanic ash” (i.e., “pumicite”), often contain trapped gases in need of calcination prior to use.

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Pictures
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).


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