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Frit

Frit master

Alternate Names: Ceramic Frits

Frits 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. The frit topic in the glossary section itemizes the many reasons why frits are so useful. Many things that are impossible with raw materials can be done with frits (the converse is also true), demonstrating that we must consider more than just chemistry when evaluating why glazes fire the way they do.

The Frit market is driven by large customers (especially tile) who use recipes given them by the prepared glaze industry, the engineers at these companies work at the recipe level and often do not even know all the details of the chemistry of the frits they use. Availability of smaller quantities of frits are generally determined by what industry is using. The frit market changes with time. Frit companies make many more products than what their literature or websites display, these are legacy formulations or custom mixes.

Some frit companies provide the chemistry of their products, others did in the past but do not do so now. Some provide approximate analyses. In the eyes of someone interested in the chemistry of the glazes they make, this practice or non-disclosure 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 brand-names. 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. Another factor is the general ignorance of how to use ceramic chemistry software to manipulate recipes to target or maintain a specific chemistry. But this is changing and we are sure that pressure will come to bear on manufacturers as expertise improves.

Admittedly, each manufacturer makes specialized frits (i.e. strontium, lithium compounds) that they invest heavily in R&D to develop. Keeping the makeup of these secret protects 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 competely evident. Thus the actual production of a duplicate can be a more elusive goal than it at first seems.

One other factor to consider is the old principle: 'You get what you pay for'. Two frits may appear equal, being pure white powders with a given chemistry. Like any other industry, some companies, especially in developing countries, take shortcuts that affect their adherence to the stated chemistry, the homogeneity of the mix, solubility, particle size or presence of impurities and unmelted particles.

BaO, SrO, Li2O, ZnO special purpose frits are commonly used in industry (in middle and low temperature glazes) however potters focus on glazes that employ only the typical KNaO, CaO and MgO. This is because they find the raw materials that source the former are either toxic, troublesome to use or cause firing faults. While they are more expensive than typical boron frits, using them you can produce glazes with better fit, melting, surface, brightness, color, clarity and with fewer firing faults.


Frits work much better in glaze chemistry

Frits work much better in glaze chemistry

The same glaze with MgO sourced from a frit (left) and from talc (right). The glaze is 1215U. Notice how much more the fritted one melts, even though they have the same chemistry. Frits are predictable when using glaze chemistry, it is more absolute and less relative. Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide in a glaze a different system with its own relative chemistry is entered. But when changing form one frit to another to supply an oxide or set of oxides, the melting properties stay within the same system and are predictable.

LOI is not important? Think again!

LOI is not important? Think again!

This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling). Notice that talc is not finished until after 1650F (many glazes have already begin melting by then).

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By Tony Hansen

XML for Import into INSIGHT

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