Digitalfire Ceramic Glossary

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  • Fast Fire Glazes

    Fast fire glazes are used in most industries now and many can fire up and down in less than two hours. Traditional alkali and boron glazes melt too early and gases of decomposition from the body cause them to bubble. Fast fire glazes thus need to melt late and quickly. Fast fire glazes can also be formulated to form a crystal network early in the firing (from CaO or MgO) that is porous and stable to above 1000C (after which it collapses and melts quickly). Search for the term "fast fire" in the materials area to find frits intended for this purpose. This will help you to learn about the chemistry of fast fire glazes. Generally, they have much lower boron and sodium and higher zinc, magnesia, calcia and silica.

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  • Feldspar Glazes

    Quite simply, glazes high in feldspar. Feldspar by itself melts well at high temperatures but it needs additions of other fluxes and silica to produce a balanced glaze that does not leach. The process of comparing the chemistry of a feldspar to a target formula for a typical medium or high temperature glaze, and adding materials to bring it into line, is quite fascinating. Since feldspar melts so well, it is common to find reactive glazes (ones with interesting visual surfaces) that contain high percentages, even up to 70%. However, since feldspar contains so much alumina, these glazes typically have almost no clay (since its presence would add alumina and destroy the active melting nature). That means they have poor slurry properties (e.g. settling, dusting, flocculating, running). These situations can be fixed using ceramic chemistry by supplying the Na2O/K2O from a low alumina material (eg. a frit) thus enabling an increase in the amount of clay in the recipe.

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  • Firebrick

    A brick capable of withstanding high temperatures without deforming. 'Insulating firebricks' have the additional advantage of acting as good insulators due to the large pockets of air in the matrix of the brick. There are many different kinds of firebricks available, some very expensive. Types are categorized for their heat duty and the types of materials and atmospheres they must come into contact with.
  • Fireclay

    A refractory naturally occurring secondary clay. Fireclays are refractory because they contain high concentrations of SiO2 and Al2O3 (or both) and low concentrations of fluxes like Na2O, K2O, CaO, MgO. Kaolins are clays and are very refactory, however they are typically highly refined and much more expensive than a raw fireclay would be and are primary materials. Fireclays are typically quite plastic and often contain impurities that need to be ground down. They also often contain enough iron to stain them somewhat when fired.

    A fireclay with a PCE of 30 is said to be a super duty fireclay. Fireclays with have high porosities when fired to cone 10. It is not unusual for clays to be labelled as fireclays when they actually are not.

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    • (Glossary) Refractory

      Refractory, as a noun, refers to materials that do...


    Materials are not always what their name suggests. These are Lincoln Fireclay test bars fired from cone 6-11 oxidation and 10 reduction (top). The clay vitrifies progressively from cone 7 upward (3% porosity at cone 7 to 0.1% by cone 10 oxidation and reduction, bloating by cone 11). Is it is fireclay? No.

    Particles from each category in a particle size distribution test of Skagit Fireclay

    Skatgit Fireclay test bars fired from cone 8-11 and 10 reduction.

    Example of the lignite particles in a fireclay (Pine Lake) that have been exposed on the rim of a vessel after sponging.

  • Fired Strength

    The fired strength of clays can be measured. The test is sometimes call M.O.R. (modulus of rupture), or just tensile strength.
    Common sense of course says that the more vitrified a clay is, the stronger it will be. Likewise, we assume that higher temperatures produce stronger ware. The growth of mullite crystals in porcelain at high temperatures can contribute to alot of strength. However other factors also contribute to fired strength.

    For glazeless ceramics, maturity is a key factor in achieving optimal fired strength. Testing is required since optimal strength may produce a body with more warping (during firing) than desired or strength may be almost as good at lower levels of vitrification than you might think. Bodies that have been vitrified too much and have become glassy lose strength and become brittle. One reason is that over maturity can detrimentally affect the development of mullite crystals (pyrophyllite is often added to porcelains to encourage better development of a mesh of long mullite crystals within the matrix). Lower temperature clay bodies can develop considerable strength at much higher porosities that you might expect. Infact, one of the strongest bodies we have ever tested was fired at cone 1 with around 3-4% porosity (more than 10,000 psi). However, in industry, good strength is achieved at much higher porosities than this, especially when body materials are very fine and the process densifies the matrix well. Wollastonite suppliers claim that additions of their material can greatly improve the fired strength of non-vitreous bodies. Thus, the optimal fired strength of a body is a product of a number of compromises involved with firing, forming, materials, glazing and the needed thermal expansion.

    Ceramic is brittle, so any surface discontinuities (e.g. micro-tears made during forming from poor plasticity), large cavities or pores (e.g. from material burned away during firing) or aggregate particles (coarse grog particles are often surrounded by micro-cracks as a product of drying and firing) provide places for failures to propagate from. A body matrix can have coarser particles, but these must be complemented by a range of sizes that produce an overall matrix that has densified well during drying and firing.

    When ceramics are glazed and number of new factors must be considered. Glaze fit is very important. Crazing is a defect that produces micro-cracks that provide convenient sites for failure when stresses occur. We have measured a 300% difference in fired strength between a poorly fitted glaze and a well fitted one. A white stoneware, for example, measured about 2500 psi with a crazing glaze, while a well fitted one measured 8000 psi. Care must be exercised not to have glazed under too much compression and this could produce shivering and contribute to spectacular failures (when they occur).

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  • Firing

    At it most basic level, firing is process of heating a clay (or recipe of clays and minerals) to a temperature sufficient to fuse the particles together. However today, each type of ceramic has its not only its own firing temperature, but also schedule (control of the rate of rise and fall of the kiln). In addition the atmospheric pressure and atmosphere itself within the kiln are controlled for many types of firing, either by restricting the amount of oxygen in the chamber or replacing it entirely by another gas (like nitrogen). In addition kilns subject the load to drafts to help even out temperature and atmosphere and carry away water vapor and products of combustion and decomposition of bodies and glazes. Firing also varies in the types of fuel that are used (e.g. coal, gas, wood, sawdust, oil, electric) and the type of kiln (kilns vary widely in the way they deliver heat to the ware and channel it out).

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    • (Glossary) Water Smoking

      Refers to the period in firing where the last of t...

  • Firing Schedule

    In most electric periodic kilns firing schedules are programmed into electronic controllers to control the rate-of-rise, soaking time and often the cooling curve. In industry firings are very fast, optimization of every stage is absolutely critical, in hobby ceramics and small companies firings are much slower and the awareness of the need to plan and adhere to firing schedules is less. While many periodic gas kilns also have electronic controllers, it is common to manually oversee the rate-of-rise and atmosphere of the firing. The thermal history to which ware is exposed in a tunnel kilns is controlled by the speed of the ware through the kiln and control of the heat and draft in various parts of the tunnel.

    This is an often-overlooked aspect of the ceramic process and yet is very important, since it relates so directly to glaze quality and body maturity. The secret to the unique properties of many special purpose ceramic products (e.g. alumina ceramics, thermal expansion failure resistant ware, crystalline glazes, porcelains) and the consistency of many types of traditional ceramics lies in the firing curve. Engineers spend alot of time designing good firing schedules.

    Schedules must account for the needs of the ware, the kiln, the environment and the budget. These include slow early heat-up to enable water to escape, reaching the desired state of maturity without cracking or other firing defects, attention to temperatures where sudden changes in body or glaze materials occur (e.g. volume changes associated with quartz, cristobalite inversion), the ability of the kiln to follow and the need to save energy. If well designed, it should be possible to predict the end of a firing accurately. For example, a cone 6-10 electric hobby kiln should finish within 5-10 minutes of the projected. Industrial kilns, likewise, should finish within minutes of the target. The ability to predict the end is an indicator of the quality and practicality of the schedule.

    An account at provides an excellent environment to develop and maintain firing schedules as a part of a larger regimen of managing recipe, material and test data.

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    • (Articles)

      Electric Hobby Kilns: What You Need to Know

      Electric hobby kilns are certainly not up to the quality and capability of small industrial electric...

    • (Project) Ceramic Firing Schedules

      Fast firing is almost universal in most sectors of...

    • (Glossary) Crystalline glazes

      Crystals can form during cooling and solidificatio...

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    • (Glossary) Firing

      At it most basic level, firing is process of heati...

    • (Glossary) Water Smoking

      Refers to the period in firing where the last of t...


    A cone 11 oxidation firing schedule used at Plainsman Clays (maintained in our account at Using these schedules we can predict the end of a firing within 5-10 minutes at all temperatures. We can also link schedules to recipes and report a schedule so it can be taken to the kiln and used as a guide to enter the program.

    This crock was fired to cone 6 (at 108F/hr during the final 200 degrees) and soaked 20 minutes. Yet, it is the color of the cone 4 test bar (lower)! It should be the more vitreous color of the cone 6 bar. This underscores the need for more soaking.

  • Flameware

    Flameware is ceramic that can withstand sudden temperature changes without cracking (i.e. stove top burners). Ovenware is another class of ceramics, it is not as resistant to thermal shock as flameware. There is some confusion among clay manufacturers and retailers of flameware about this. Japanese donabe ware is advertised as flameware, but its ability to withstand higher temperatures is showcased (rather than its resistance to sudden temperature changes). One supplier talks about their flameware body's ability to withstand 800 degrees but does not mention thermal shock resistance either (any clay can easily withstand 800 degrees, kilns fire to more than double that).

    Ceramic is much more susceptible to thermal shock failure than most other materials because of its brittle nature, lack of elasticity and tendency to propagate cracks. Thus the creation of true flameware requires compromising things like plasticity and vitrification. Non-vitreous flameware bodies can be made using high a proportions of a low expansion material like kyanite, mullite, pyrophyllite or molochite (powder or grog) plastic-bonded with a small amount of clay or organic binder and fire-bonded with a glass producing flux. Of course, if the particles of these materials are altered or taken into solution in the glass bonder (e.g. feldspar) then the low expansion character of their natural state is lost.

    While large manufacturers may have the resources to have special low expansion frits formulated for glazing flameware, a potter would find it very difficult to make a glaze of low enough expansion not to craze.

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    • (Glossary) Ovenware

      Ovenware clay bodies have a lower thermal expansio...

  • Flashing

    A fired visual effect on bare clay surfaces in fuel burning kilns (especially wood). Clay surfaces that have been flashed have been subjected to a thermal history of variations in flame, ash, kiln atmosphere and even imposed vapors (like salt and soda). The degree to which these forces have varied determines the visual variation across the surface of the ceramic. Historical ceramics often had flashing simply as a consequence of the lack of control of the process of clay preparation, forming, drying and firing. In recent years there has been a focus on the reproduction of this rustic look, various methods seek to reproduce the process, others only the final product. A popular method is the application of slips having a makeup likely to react with the atmosphere or flame in the kiln. Slips of high alumina content, for example, are likely to react with an atmosphere containing ash (since the ash can be high in silica and soda). Likewise, a slip high in fine silica and alumina is likely to react with fumes of soda. Slips containing some iron will exhibit differing coloration where differing amounts of flame has touched.

    Flashing effect on a cone 10 wood fired sample.

  • Flocculate, flocculation, flocculant

    The opposite of deflocculation. Flocculation in a slurry can be a desired property or undesired.

    For the latter, a ceramic glaze or clay slurry that would otherwise be thin and runny can be made into a gel by the simple addition of a flocculant. This is typically done to improve suspension properties or enable application of engobes and slips (and sometimes glazes) in a thicker layer that does not run or drip. To achieve the gel the flocculation process normally requires a slurry of higher water content, thus it will have a higher shrinkage on drying and can take longer to dry completely. But technicians learn how to balance these issues to make the process successful. Common flocculants include calcium chloride, vinegar and epsom salts.

    Glazes can change their viscosity with storage, when they thicken they are said to 'flocculate'. In these cases slightly soluble materials in the mix (e.g. nepheline syenite, gerstley borate, boron frits, clays containing sulfates) can act to change the viscosity of the slurry. It can be difficult to deflocculate these slurries and make them usable again, thus such glazes are best used soon after they are made.

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    The engobe on the left, even though it has a fairly low water content, is running off the leather hard clay, dripping and drying slowly. The one on the right has been flocculated with epsom salts. Now there are no drips, there are no thin or thick sections. It gels after a few seconds and can be uprighted and set on the shelf for drying.

    Water from the top of a glaze that had been sitting for more than a year and settled. Clearly, iron containing soluble materials are in one or more of the materials in the glaze.

    Example of sedimentation test to compare soluble salts water extracts from suspended clay. This simple test also reveals ultimate particle size distribution differences in clays that a sieve analysis cannot do.

    This is a stainless steel spoon that has been dipped into a ceramic engobe that has been flocculated using epsom salts. Without the salts the slip completely runs off leaving only a film. But with the right amount it stays on the spoon in an even layer (as a gel), then hardens as it dewaters (left) and finally dries completely (right) with no cracks! It fired to cone 03 with no cracks. If this were fired high enough it would transform to a glaze. But it would craze. Special low expansion frits are available to make enamels for metals.

    The flocculated slip (left) hangs on, stays even and does not run. The normal slip (right) is thin and running on verticals and thinning at the rim.

  • Fluidity, Melt Fluidity

    Glazes become fluid when they melt, they are molten. The fluidity (or viscosity) of this melt needs to be considered, especially when troubleshooting problems. While two different fired glazes may appear to have melted a similar amount (even on a vertical surface), one may be radically more fluid than the other (this becomes evident in a fluidity tester or when the glaze is applied thicker). While it might seem logical that a matte glaze has a fairly stiff (viscous) melt, it might actually be highly fluid and runny (because the matteness is a product of crystallization on the surface during cooling). Melt flow testers are an ideal way to get a true picture of how fluid a glaze melt really is. In a well-designed melt flow tester a glaze with the correct degree of melt flow will travel half way down the runway.

    Glaze melt fluidity relates closely to a variety of problems like pinholing, crawling, gloss, blistering, crazing and even leaching. Logically, glazes for vertical surfaces will be more viscous than tile glazes, for example, which are applied to horizontal surfaces. Molten glaze viscosity can be understood in terms of molecular silicate chains (which also link across to other chains). The chemistry of the melt determines the rigidity of the structure and therefore the viscosity of the melt. Glazes high in powerful fluxes (like boron, lithium, sodium) melt and run more. In functional ware, for example, it is desirable to have enough melt to bring into solution all the material particles and produce a fired surface that has good gloss. However if too much flux is present the fired glaze is not as hard, it can have higher thermal expansion (if it contains high KNaO), may be more prone to blistering and is more likely to leach. Thus it is best to tune the ratio of fluxes to SiO2 and Al2O3 such that the melt has the right degree of movement and no more. Even special purpose reactive or matte glazes need to be tuned. In the case of the former, a compromise is needed between the high fluidity needed to produce the visual effect and a more stable and harder stiffer melt. For matte glazes, a less fluid type that relies more on high MgO rather than high Al2O3 only will have less cutlery marking of the fired glass.

    Blistering often occurs in glazes of high melt fluidity. This might appear illogical since it would seem that such melts would more readily pass gases of decomposition from the body. However, the problem often happens because these glazes begin to melt (and seal the body surface) at much lower temperatures than one might think. Then they just keep percolating the escaping gases as the kiln is soaked and even continue after the kiln is shut off. Fast dropping temperatures finally freeze these blisters into the glass at an even lower temperature than they first melted at. Employing a flux system that melts later or firing the kiln down to the freezing point and slowing the descent there might be the solution.

    The Potter's dictionary has a very good discussion with diagrams of this under the term 'viscosity'.

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    • (Glossary) Viscosity

      The term viscosity is used in ceramics most often ...

    • (Project) Ceramic Thermal Events

      Many ceramic problems relate to a lack of understa...

    • (Tests) MLPT - Melting Point (MP)


    Example of how iron turns to a flux in reduction firing and makes the glaze melt much more fluid.

    The glaze is cutlery marking (therefore lacking hardness). Why? Notice how severely it runs on a flow tester (even melting out holes in a firebrick). Yet it does not run on the cups when fired at the same temperature (cone 10)! Glazes run like this when they lack SiO2 and Al2O3. The SiO2 is the glass builder and the Al2O3 gives the melt body and stability. Al2O3 also imparts hardness to the fired glass. No wonder it is cutlery marking. Will it also leach? Very likely.

    The glaze on the left (90% Ravenscrag Slip and 10% iron oxide) is transformed into a highly fluid reduction tenmoku (right GR10-K1) with just 5% added calcium carbonate.

    An example of a highly fluid glaze melt that has pooled in the bottom of a bowl. The fluidity is partly a product of high KNaO, thus it is also crazed (because KNaO has a very high thermal expansion). While it may to decorative, this effect comes at a cost. The crazing weakens the piece, much more than you might think (200%+). Those cracks in that thick layer at the bottom are deep, they want to continue down into the body and will do so at the first opportunity (e.g. sudden temperature change, bump). Also, fluid glazes like these are more likely to leach.

    A example of a highly fluid cone 6 glaze that has pooled in the bottom of a mug (and crystallized). It has caused a crack all the way around that has separated the base. Glazes normally need to be under some compression to avoid crazing (by having a lower-than-the-body thermal expansion), but if they are thick like this the body does not have the strength to resist the extra outward pressure the glaze can be exerting. Conversely, if the glaze is under tension (having too high an expansion), the cracks that develop within it to relieve the tension are deep and wider and thus more likely to propagate into the body below.

    Flow tester used to compare feldspars

    Flow tester comparing the melt fluidity of Albany Slip vs. Alberta Slip at cone 10R

  • Flux

    On the theoretical chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of others. Fluxing oxides interact with others, sometimes their combinations flux much more than logic would expect given their individual performance. Normally, the more kinds of fluxes present in a mix the lower its melting temperature is (called the 'mixed oxide effect'). Fluxes interact with the surface molecular structure of other materials and pull them away (dissolve them) molecule-by-molecule.

    Examples of fluxing oxides for high temperature glazes are K2O, Na2O, CaO, SrO, Li2O, MgO, ZnO (CaO and MgO are not active at lower temperatures). In glaze chemistry, each of these oxides is an individual with its own optimal percentage and interaction with silica and alumina. Fluxing oxides make up a minor part of the glaze, they interact with the SiO2 glass former and Al2O3 (and other fluxes). If used in this way, CaO, for example, reacts strongly with stoneware and porcelain glazes to lower their melting temperature.

    Colorants can also be powerful fluxes. Copper, cobalt and manganese all melt very actively in oxidation and reduction. However iron, a refractory material in oxidation, is a strong flux in reduction.

    When the term flux is used on the material level, it is referring to the fact that the chemistry of the material contributes a significant amount of one or more of the fluxing oxides. Feldspar is an excellent example of a natural mix of refractory and fluxing oxides that, together, melt at a fairly low temperature. However, raw materials commonly viewed as fluxes, do not always melt well by themselves. Dolomite, like calcium carbonate, is a stoneware glaze fluxing material. But by itself it can be dead-burned and used as a heavy duty refractory for ladles and slag furnaces! Talc, in small percentages in middle temperature clay bodies, acts as a strong flux. However in large percentages, it is refractory also. Calcium carbonate is another example. While being a strong glaze flux at higher temperatures, it is refractory in a 75:25 mix with bentonite (where the conditions for interaction to produce a glass are not present).

    B2O3 is a very low melting oxide, the ceramic industry depends very heavily on it. But B2O3 is not a flux, it is a low melting glass (it does not depend on percentage and interaction to activate, it works across the entire temperature range used in traditional ceramics). Almost all frits contain at least some B2O3.

    Fluxing oxides in frits melt much better than in raw materials. MgO is an excellent example. Glazes that employ frit to supply the MgO melt much better than those employing dolomite or talc. SrO is a similar story.

    Understandably, predicting the effects of a flux addition to a glaze (e.g. melting temperature) is very complex (involving interactions, eutectics, proportions, premelting, atmostphere and the physical and mineralogical properties of the particles). For this reason, ceramic chemistry is applied much more in a relative sense than absolute to predict melting temperature.

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    • (Glossary) Refractory

      Refractory, as a noun, refers to materials that do...

    • (Glossary) Frit

      A ceramic glass that has been premixed from raw po...

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    The glaze on the left (90% Ravenscrag Slip and 10% iron oxide) is transformed into a highly fluid reduction tenmoku (right GR10-K1) with just 5% added calcium carbonate.

    Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and dry shrinkage vary widely. Materials normally acting as fluxes are refractory.

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

    Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.

    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.

    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.

  • Foot Ring

    Footrings, as opposed to flat bottomed containers, lift the piece off the table and enable glazing all of the bottom. While foot rings add extra effort to the finishing stage at fabrication, they also make it easier to glaze the ware (articles can be dipped and quickly sponged to remove the glaze). Only shallow foot rings are possible in machine made items whereas hand made pieces can distinguish themselves with much deeper rings.

    An example of a foot ring in a cone 10 reduction mug (it was tooled and sponged at the leather hard stage). It has channels to drain water in the dish washer.

    An example of an unfinished foot ring (on a salt glazed mug). This technique is popular with many potters.

  • Forming Method

    Refers to the method by which a ceramic component or object is created or manufactured. Common traditional ceramics forming methods include dusting/die pressing, jiggering/jolleying, slip casting, extrusion, ram pressing, throwing, etc. Forming methods in advanced ceramics also include isostatic pressing, tape casting, injection molding, green machining, hot pressing, hot isostatic pressing, diamond grinding. Choosing an appropriate forming method for a specific object is a big factor in achieving low costs coupled with high quality.

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  • Formula

    Conceptually we consider fired ceramic glazes as being composed of 'oxides'. But materials are also. The ten major oxides likely make up 98% of all base glazes (and materials we use). The oxide formula of a glaze "explains" many details about the way the glaze fires. That means we can predict what will happen during firing and we can propose changes that have a high likelihood of fixing a problem or moving a glaze property in a certain direction. The chemistry of a glazes are normally expressed as formulas (sometimes people refer to "glaze formulas" when what they actually mean is "glaze recipes").

    A formula expresses an oxide mix according to the relative numbers of molecule types. A formula is suited to analyzing and predicting properties of a fired glaze or glass. It gives us a picture of the molecular structure that is responsible for fired behavior. Since the kiln fires build these oxide molecules one-by-one into a structure, it follows that one will never really 'understand' why a glaze fires the way it does without seeing its oxide formula.

    Formulas are flexible. We can arbitrarily retotal one without affecting the relative numbers of oxide molecules. In fact, this retotaling of a formula is standard procedure to produce a 'Seger unity formula'. With a formula, you need not worry whether there is 1 gram, 1 ton, or one billion molecules, only relative numbers matter. This is why it is allowable to express a formula showing molecule parts (e.g. 0.4 MgO; in reality this would not occur, but on paper a formula helps us compare relative numbers of oxide molecules in a ratio).

    An example of a raw formula



    Glass Formers







    Al2O3 0.9 SiO2 9.0

    Notice in the above that oxides are grouped into three columns: the bases, acids, and amphoterics (or simply as the RO, R2O3 , and RO2 oxides; where "R" is the element combining with oxygen). Actually, the ratio of R to O is significant. The right column has the greatest oxygen component, the left has the least. Simplistically, we can view these three groups as the silica:alumina:fluxes system. This latter convention is not really correct because there are more glass builders than SiO2 , other intermediates besides Al2O3, and the RO's do more than just flux. But because this method evokes immediate recognition, let's use it anyway. Ancient potters referred to these three as the blood, flesh, and bones of a glaze (not a bad way to think of it).

    Any formula has a formula weight, that is, the total calculated weight for that mix of molecules. Atomic weights are published in any ceramic text so it is easy to calculate the weight of each oxide. Calculating the weight of the whole is just a matter of simple addition and then increasing that weight to account to LOI.

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    • (Library) Feldspar Teaches Us About Formulas, LOI, Unity - INSIGHT Video Tutorials 1B, 1C

      Deals with the basics of oxides, formulas and anal...

    • (Glossary) Analysis

      Conceptually we consider fired glazes as being com...

    • (Glossary) Oxide

      An oxide is a molecule like K2O, Al2O3. They are t...

    • (Glossary) Unity Formula

      Conceptually we consider fired ceramic glazes as b...

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  • Formula Weight

    Quite simply, the weight of a formula. Typically, in glaze chemistry, when we refer to formula weight it is assumed we are talking about the weight of the fired formula of a glaze (without LOI and volatiles). However is is possible to also talk about the formula weight of a material (although materials are normally evaluated as analyses). In this case, the weight specified includes the volatiles (e.g. CO2, carbon, CO, H2O, etc) that burn away during firing.

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    • (Glossary) Formula

      Conceptually we consider fired ceramic glazes as b...

    • (Glossary) LOI

      Simplistically, LOI is the amount of weight a mate...

  • 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. 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 problems and improve products. Of course, frits are more expensive than raw materials, but the advantages often out-weight the costs or 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.

    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.

    -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).

    -Special effects
    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.

    -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.

    -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).

    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...


    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

    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.

    Worthington Clear is a popular low fire transparent glaze recipe. It has 55% Gerstley Borate (which is quite plastic) plus 30% kaolin. That means you can actually throw it as if it were a clay, in fact it has excellent plasticity! This explains why it gels almost immediately on slurry mixing, dewaters extremely slowly and shrinks and cracks during drying on the ware. Yet countless potters struggle with this recipe. Frits frits are a better source of the B2O3. It is common to see both clay and Gerstley Borate in recipes, often they impart way too much shrinkage and dry very slowly. A quick fix is to substitute all or part of the raw kaolin for calcined kaolin.

    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.

  • Functional

    A functional clay body is one that produces a ceramic that is durable. However there are a number of caveats with this. First, the item must maintain that strength and durability in service (degradation is common). Of course, the body has to be fired sufficiently high to vitrify enough to have strength considered to be suitable for the application. Second, if glazed, it needs to fit the glaze (or engobe); if the glaze is under compression or tension this can greatly weaken the body both immediately after firing and progressively over time as micro-crack networks grow and water penetrates.

    There is no specific absorption rate that indicates functionality. Absorption is a product of porosity in the body. Obviously a body having micro-cracks as porosity is not going to be as strong as one having normal inter-particle pore space. Porosity is typically a product of the nature of the pore space which in turn is a product of the nature of the matrix that hosts it. Pore shape and inter-connections and the micro-porosity of the ceramic matrix determine the degree to which water can penetrate to fill it. Thus a body may have a much higher porosity than a standard porosity test indicates if the pore-interconnects are lacking.

    One body having a higher porosity can be stronger than another having a lower porosity, this can be the case for several reasons. First, the nature of the matrix that hosts the pore network can be a greater determiner of overall strength than the simple existence of pores. A matrix having large silica particles with cracks radiating outward (because of quartz inversion during firing) will obviously not be strong. Also, a well fitted, impermeable glaze can greatly strengthen a body. Many glazed ceramic tiles, for example, are remarkably strong, yet they can have a high porosity. Also, one body of higher porosity may have a matrix that better terminates the growth of micro-cracks than another, and thus maintain its strength over time better.

    Out Bound Links

    • (Glossary) Vitrification

      Vitrification is the solidification of a melt into...

    • (Glossary) Mature

      A term referring to the degree to which a clay or ...

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