Notes
Frits are made by melting mixes of raw materials in special kilns, then pouring the molten 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 can also be 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 to 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. The availability of smaller quantities of frits is 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. In fact, 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. The 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 a secret protects against the formulations being copied by other manufacturers. Even though powdered samples of these frits could be analyzed 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 having 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 the 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.
Related Information
Frits melt so much more evenly and trouble free
These two specimens are the same terra cotta clay fired at the same temperature (cone 03) in the same kiln. The chemistry of the glazes is similar but the materials that supply that chemistry are different. The one on the left mixes 30% frit with five other materials, the one on the right mixes 90%+ frit with one other material. Ulexite is the main source of boron (the melter) in #1, it decomposes during firing expelling 30% of its weight as gases (mostly CO2). These create the bubbles. Each of its six materials has its own melting characteristics. While they interact during melting they do not mix to create a homogeneous glass, it contains phases (discontinuities) that mar the fired surface. In the fritted glaze all the particles soften and melt in unison and produce no gas. Notice that it has also interacted with the body, fluxing and darkening it and forming a better interface. And it has passed (and healed) most of the bubbles from the body.
Frits melt so much better than raw materials
Feldspar and talc are both flux sources (glaze melters), they are common in all types of stoneware glazes. But their fluxing oxides, Na2O and MgO, are locked in crystal structures that neither melt early or supply other oxides with which they like to interact. The pure feldspar is only beginning to soften at cone 6. Yet the soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high-MgO frit is melting beautifully at cone 06! The frits progressively soften, starting from low temperatures, both because they have been premelted and have significant boron content. In both, the Na2O and MgO are free to impose themselves as fluxes, actively participating in the softening process.
These common Ferro frits have distinct uses in traditional ceramics
I used Veegum to form 10 gram GBMF test balls and fired them at cone 08 (1700F). Frits melt really well, they do have an LOI like raw materials. These contain boron (B2O3), it is a low expansion super-melter that raw materials don’t have. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al2O3 super feldspar. Its high-sodium makes it high thermal expansion. It works really well in bodies and is great to make glazes that craze. The high-MgO Frit 3249 (made for the abrasives industry) has a very-low expansion, it is great for fixing crazing glazes. Frit 3134 is similar to 3124 but without Al2O3. Use it where the glaze does not need more Al2O3 (e.g. already has enough clay). It is no accident that these are used by potters in North America, they complement each other well (equivalents are made around the world by others). The Gerstley Borate is a natural source of boron (with issues frits do not have).
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!
This chart compares the decompositional off-gassing (Loss on Ignition) behavior of six materials used in ceramic glazes as they are heated through the range 500-1700F. It is amazing that some can lose 40%, or even 50% of their weight on firing. For example, 100 grams of calcium carbonate will generate 45 grams of CO2! This chart is a reminder that some late gassers overlap early melters. That is a problem. The LOI (% weight loss) of these materials can affect glazes (causing bubbles, blisters, pinholes, crawling). Notice talc: It is not finished gassing until 1650F, yet many glazes have already begun melting by then (especially fritted ones). Even Gerstley Borate, a raw material, is beginning to melt while talc is barely finished gassing. And, there are lots of others that also create gases as they decompose during glaze melting (e.g. clays, carbonates, dioxides).
Frit shards from the smelting furnace
Frits are made by melting mixes of raw materials in a special furnace, then pouring the molten mix into water. That produces these shards of glass. These are ground to the white powder we use to make glazes.
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