•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
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 (search youtube for interesting videos). Huge quantities and varieties of frits are manufactured for the ceramic industry every year (especially for tile) by dozens of different companies.
While frits can be a bit of a mystery to smaller operations and potters who often use raw glazes, learning when to take advantage of frits can potentially solve many problems and improve products. Of course, frits are more expensive than raw materials, but the advantages often out-weigh the costs (e.g. they reduce costs in other stages of production). Many of the reasons for employing frits over raw materials parallel those for using stains over raw metal oxides. In general, higher percentages of frit enable better fired quality, lower melting temperature, fewer defects, better clarity, smoother surface, brighter colors, faster firing, lower thermal expansion. While glazes having 85% frit (plus 15% clay for suspension and hardening) might seem much too expensive, when the extra cost is balanced against the other benefits it can easily to worth it for smaller operations.
Frits are often described in accordance with the purpose of original formulation (e.g. "for producing clear glazes at 1050-1100C"). However, frits are just sources of oxides, so although one might have been intended for a specific purpose it can be used to source its oxides to any type of glaze. Frits may also be objectively described according to the relative amounts of oxides present. For example, a frit might be designated a "low boron high sodium leadless zinc flux". Typically the term "high" means greater than 10% (SiO2 is not usually mentioned as high since it is high in almost all frits), "medium" 2-10% and "low" less than 2%. So the above frit would be >10% Na2O, <2% B2O3 and unstated zinc (which would likely be medium).
When one learns to view frits as warehouses of oxides, almost any desired chemistry can be achieved by mixing them (and raw materials) in the correct proportions (of course this requires software to do the material juggling necessary to achieve a target chemistry). While frit data sheets will often recommend additions of a certain product to raise or lower melting temperature, raise or lower thermal expansion, increase durability, adjust mattness, etc., it is usually better to rationalize these things in terms of the chemistry. Technical references (and many pages on this site) document what oxides need to be added, removed, increased, decreased and how they interact to adjust temperature, expansion, gloss, resistance to leaching, hardness, color response, etc. By using glaze chemistry software the oxide formula of a glaze can often be moved in the needed direction by employing frits in stock (rather than having to add more materials to inventory). Amazingly very few technicians in pottery and ceramic industry have any idea that this can be done.
Here are some of the many reasons to use 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 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.
-Consistency and repeatability in production
Raw materials vary in physical properties and chemistry much more than frits. This also makes it possible to scale production of glaze effects that depend on a critical balance of chemistry that would be impossible to maintain with raw materials.
-To supply B2O3
Boron is the principle flux in most ceramic processes, but the raw forms are either soluble, inconsistent or have high LOI.
-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 (for example, there are no quartz grains to take into solution, they have already formed silicates). 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. An MgO frit, for example, enables its use at far lower temperatures than sourcing it from talc or dolomite.
Frits are also very 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 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.
-To avoid volatilization of gases during decomposition
Most raw ceramic materials contain sulfur or carbon compounds as well as H2O (some up to 50% by weight!). 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 lithium frits, for example, produce much better glazes than those made with the lithium and barium carbonate.
-To achieve homogeneity in the melt
Other than dissolution and very localized migration, melting raw glazes 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.
A frit can supply a specific chemistry that a raw material cannot (for example as a source of KNaO without much Al2O3 to enable getting more clay into a glaze while maintaining its chemistry; or to make a crystalline glaze which requires low Al2O3 and high KNaO). 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).
Frits make it possible to create chemistries that result in phase separations during cooling producing matteness, opacity or specific mechanical properties that the homogenous glass does not have. These effects are practically impossible with raw materials that do not melt enough, produce excessive gases of decomposition and do not cannot be combined to get the desired chemistry.
-Fast fire technology
Industry now measures firing time in minutes instead of hours. Frits can be formulated to melt quickly and evenly after body gases have been expelled, thus greatly reducing glaze imperfections. Fast firing also makes it economically feasible to go to higher temperatures. Defect free high strontium, barium and calcium glazes could never be made with raw materials for fast fire. In addition, fast fast makes it possible to break some traditional rules. For example, zinc-based glazes that are normally hostile to many stain types simply do not have time to subdue or alter the color.
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.
-Wide firing range
Many stains soften over a wide softening range as opposed to having a sudden melting temperature.
See the Frit master material record for more information (like provided below).
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.
A frit softens over a wide temperature range
This is unlike some raw materials which melt suddenly.
Frits melt so much better than raw materials
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. 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. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
Can you actually throw a Gerstley Borate glaze? Yes!
Worthington Clear is a popular low fire transparent glaze recipe. It has 55% Gerstley Borate plus 30% kaolin (Gerstley Borate melts at a very low temperature because it sources lots of boron). GB is also very plastic, like a clay. I have thrown a pot from this recipe! This explains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a plastic clay the shirinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries. Countless potters struggle with Gerstley Borate recipes. How could we fix this one? First, substitute all or part of the raw kaolin for calcined kaolin (using 10% less because it has zero LOI). Second: It is possible to calculate a recipe having the same chemistry but sourcing the magic melting oxide, boron, from a frit instead.
At 1550F Gerstley Borate suddenly shrinks! The melt fluidity ball tells us.
These balls were fired at 1550F and were the same size to start. The Gerstley Borate has suddenly shrunk dramatically in the last 40 degrees (and will melt down flat within the next 50). The talc is still refractory, the Ferro Frit 3124 slowly softens across a wide temperature range. The frit and Gerstley Borate are always fluxes, the talc is a flux under certain circumstances.
Frits do not dissolve in water, right? Wrong.
This is an example of two types of crystals that have formed on the surface of a fritted glaze after a long period of storage (Ferro Frit 3249 in this case). Frits are formulated to give chemistries that natural materials cannot supply. To do that they have to push the boundaries of stability (solubility). Any frit that has an inordinately high amount (compared to natural sources) of a specific oxide (in this case MgO) or lacks Al2O3 (like Frit 3134) are suspect.
Glaze chemistry works best when changing material amounts, not material types
Two glazes, same chemistry, different materials. The glaze on the left is sourcing CaO from wollastonite, the one on the right from calcium carbonate. Thus the oxide chemistry of the two is the same but the recipe of materials sourcing that chemistry is different. The difference in the melt flow you see here is an expression of how choosing different mineral sources to source an oxide can produce melting patterns that go outside what the chemistry suggests. The difference here is not extreme, but it can be much more. Glaze chemistry is relative, not absolute. It works best when you are changing material amounts, not material types. When you do introduce a very different mineral then you have a different system which has its own relative chemistry.
G2931F Ulexite-based transparent bubbles, G2931K frit-based version does not
I melted these two 9 gram balls on tiles to compare their melting (the chemistry of these is identical, the recipes are different). The Ulexite in the G2931F (left) drives the LOI to more than 14%. That means the while the ulexite is decomposing during melting it is creating gases that are creating bubbles in the glass. Notice the size of the F is greater (because it is full of bubbles). While this seems like a serious problem, in practice the F fires crystal-clear on most ware.
The data sheet of a frit having a proprietary chemistry
Some frit companies publish the chemistry of their frits, others do not. Some publish some of their products. Some published in the past but do not do so now. When frit data sheets do not provide an oxide analysis they become an impediment to use in glaze chemistry.
Do you know the purpose of these common Ferro frits?
I used a binder to form 10 gram balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chemistries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. 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 in bodies and is great to incorporate into glazes that shiver. The high-MgO Frit 3249 (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. it already has enough clay). It is no accident that these are used by potters in North America, they complement each other well. The Gerstley Borate is a natural source of boron (with issues frits do not have).
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.
Glaze melt fluidity comparison between G2931F and fritted G2931K show the effect of LOI
These two glazes have the same chemistry but different recipes. The F gets its boron from Ulexite, and Ulexite has a high LOI (it generates gases during firing, notice that these gases have affected the downward flow during melting). The frit-based version on the right flows cleanly and contains almost no bubbles. At high and medium temperatures potters seldom have bubble issues with glazes. This is not because they do not occur, it is because the appearance of typical glaze types are not affected by bubbles (and infact are often enhanced by them). But at low temperatures potters usually want to achieve good clarity in transparents and brilliance in a colors, so they find themselves in the same territory as the ceramic industry. An important way to do this is by using more frits (and the right firing schedules).
Glazes of the same chemistry: The fritted one melts better
It seems logical (and convenient) to just say that the kiln does not care what materials source the oxides in a glaze melt. Li2O, CaO, Al2O3, SiO2 are oxides (there are about ten common ones). The kiln just melts everything and constructs the glaze from the ones available. Right? Wrong! Things get more complicated when frits are introduced. Frits are man-made glasses, they melt much more readily than raw materials like feldspar. Raw materials are often crystalline. Crystals put up a fuss when asked to melt, often holding on as long as they can and then suddenly melting. Frits soften over a range and they start melting early. To illustrate: These two glazes have the same chemistry. But the one on the left sources sodium and alumina (Na2O3, Al2O3) from the 48% feldspar present. The other sources these from a frit (only 30% is needed for the same amount of Na2O3). The remainder of the recipe has been juggled to match the other oxides. The frit version is crystallizing on cooling (further testament to how fluid the melt is). What has happened here is great. Why? First, the chemistry has not changed (fewer firing differences). The frit has no Al2O3, it is being sourced from kaolin instead, now the slurry does not settle like a rock. Even better, silica can be added until the melt flow matches (might be up to 20%). That will drop the thermal expansion and reduce crazing. The added SiO2 will add resistance leaching and add durability. Frits are great! But you need to know how to incorporate them into a recipe using a little glaze chemistry.
A settling, running glaze recipe gets a makeover
The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay. Adjustment 1 (column 3) eliminates the feldspar and sources Al2O3 from kaolin and KNaO from Frit 3110. The chemistry of the new chemistry is very close. To make that happen the amounts of other materials had to be juggled (you can click on any material to see what oxides it contributes). But the fired test reveals that this one, although very similar, is melting more (because the frit releases its oxide more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition (to supply more SiO2). SiO2 is the glass former, the more a glaze will accept, the better. Silica is refractory so the glaze will run less. It will also fire more durable and be more resistant to leaching.
Out Bound Links
Frit - Frit master
The Chemistry, Physics and Manufacturing of Glaze Frits
A detailed discussion of the oxides and their purposes, crystallization, phase separation, thermal expansion, solubility, opacity, matteness, batching, melting.
Glaze chemistry is learning what each oxide does in a fired glaze and the relative advantages and disadvantages of each material supplying it. The chemistry of a glaze is expressed in a manner similar to its recipe, except that the items are oxides and the amounts can be by weight (an analysis) or n...
In Bound Links
A silicate is an SiO2-centric solid (crystalline or glass). A borosilicate simply is a silicate with boron. The term 'borosilicate' is synonymous with medium and low fire glazes because boron is not employed at high temperatures (CaO, Na2O, MgO, etc flux silica and bond with it to form crystalline o...
1: FRT - [label]
The number of different frits in the world can be intimidating, there are thousands. However, unlike stains, their are a wide range of standard formulations that have been made for many years. We are ...
On the theoretical glaze chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of materials. Fluxes are interactors (they often melt poorly on their own but react strongly with high melting materials where Al2O3/SiO2 predominate). There are less than ten commo...
GSPT - Frit Softening Point
GTTM - Glass Transition Temperature
A base glaze is one having no opacifiers, variegators or colorants. Thus it should be transparent if glossy and translucent if matte. Developing or adapting a base glaze for your ware is a very important first step in developing a manufacturing process that produces good quality. In fact, from a qua...
Conceptually we consider fired ceramic glazes as being composed of 'oxides' (materials contribute these). The ten major oxides likely make up 99% of all base glazes (and materials we use). The oxide formula of a glaze "explains" many details about the way the glaze fires (provided all the materials ...
By Tony Hansen