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 and blister. 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.


    Your boron glaze might melt alot earlier than you think

    The porcelain mug on the left is fired to cone 6 with G2926B clear glossy glaze. This recipe only contains 25% boron frit (0.33 molar of B2O3). Yet the mug on the right (the same clay and glaze) is only fired to cone 02 yet the glaze is already well melted! What does this mean? Industry avoids high boron glazes (they consider 0.33 high boron) because this early melting behavior means gases cannot clear before the glaze starts to melt (causing surface defects). For this reason, fast fire glazes melt much later. Yet many middle temperature reactive glazes in use by potters have double the amount of B2O3 that this glaze has!

    Why fast fire glazes flux using zinc

    We are comparing the degree of melt fluidity (10 gram balls melted down onto a tile) between two base clear glazes fired to cone 6 (top) and cone 1 (bottom). Left: G2926B clear boron-fluxed (0.33 molar) clear base glaze sold by Plainsman Clays. Right: G3814 zinc-fluxed (0.19 molar) clear base. Two things are clear: Zinc is a powerful flux (it only takes 5% in the recipe to yield the 0.19 molar). Zinc melts late: Notice that the boron-fluxed glaze is already flowing well at cone 1, whereas the zinc one has not even started. This is very good for fast fire because the unmelted glaze will pass more gases of decomposition from the body before it melts, producing fewer glaze defects.

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  • Feldspar Glazes

    Quite simply, feldspar glazes are high in feldspar. Feldspar by itself melts well at high temperatures, however to be a balanced glaze (durable, well fitted to the body, non-leachable, etc) it needs additions of other fluxes and silica. It is very educational to work through the process of comparing the chemistry of a feldspar to a target formula for a typical medium or high temperature glaze. Instructors commonly show students how to add materials to a feldspar to bring the chemistry into line with the type of glaze being sought.

    Since feldspar melts so well, it is common to find glazes that contain high percentages, even up to 70%. In two ways high feldspar glaze cause alot of misery in ceramics and pottery. Anything above around 40% is usually trouble.

    1) High feldspar glazes settle in the bucket. Why? Almost all glazes need significant Al2O3 (for durability and to thicken the melt). Typically it is sourced mainly from clay, especially kaolin, and secondarily from feldspar. But when feldspar percentages are high kaolin must be reduced, or Al2O3 is oversupplied. That accounts for the poor application and slurry properties (e.g. settling, dusting, drips and running). These situations can be fixed using glaze chemistry to source Al2O3 more from kaolin or ball clay and less from feldspar (the Na2O/K2O can come from a much lower alumina material, like a frit (e.g. Ferro Frit 3110).

    2) High feldspar glazes have high amounts of Na2O and K2O. Yes, these are good melters (fluxes) but they have by far the highest thermal expansions of any oxide. This means crazing. High feldspar glazes almost always craze. Glaze chemistry is again needed. The solution is to trade some of the K2O and Na2O (KNaO) for lower expansion fluxes (preferably MgO, but also CaO, SrO, Li2O; any other flux because they all have much lower expansion that KNaO). When feldspar is reduced in the recipe Al2O3 and SiO2 are lost but these can be easily made up by kaolin and silica.

    High feldspar glazes are often the product of a line or triaxial blending project. But the problem with this approach is that glazes are selected based too much on the visual appeal of a fired sample. When the chemistry is not considered the out-of-balance recipe gets into production and later slips into the destructive trade in undocumented unsuitable glaze recipes. Thousands of recipes too-high-in-feldspar are in common use. Not only is crazing an issue, but their tendency to have an unbalanced chemistry impacts their leachability and durability.


    Flow tester used to compare feldspars

    Feldspars, the primary high temperature flux, melt less than you think.

    A cone 8 comparative flow tests of Custer, G-200 and i-minerals high soda and high potassium feldspars. Notice how little the pure materials are moving (bottom), even though they are fired to cone 11. In addition, the sodium feldspars move better than the potassium ones. But feldspars do their real fluxing work when they can interact with other materials. Notice how well they flow with only 10% frit added (top), even though they are being fired three cones lower.

    Feldspar melts by itself to be a glaze? Hold on!

    Pure MinSpar feldspar fired at cone 6 on Plainsman M370 porcelain. Although it is melting, the crazing is extreme! And expected. Feldspars contain a high percentage of K2O and Na2O (KNaO), these two oxides have the highest thermal expansion of any other oxide. Thus, glazes high in feldspar (e.g. 50%) are likely to craze. Using a little glaze chemistry, it is often possible to substitute some of the KNaO for another fluxing oxide having a lower thermal expansion.

    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.

    A soda feldspar cone 4-7

    Pure soda feldspar (Minspar 200) fired like-a-glaze at cone 4, 5, 6 and 7 on porcelainous stoneware samples. The bottom samples are balls that have melted down at cone 7 and 8. Notice there is no melting at all at cone 4. Also, serious crazing is highlighted on the cone 6 sample (it is also happening at cone 5 and 7).

    Do your functional glazes do this? Fix them. Now.

    These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.

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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. Although they might appear similar, fire bricks are much more expensive than structural bricks and can withstand temperatures that would melt most common bricks. Firebrick types are categorized for their heat duty and the types of materials and atmospheres they must come into contact with. A wide variety of refractory shapes are also produced.

    Users of firebricks look for technical specifications that include things like thermal expansion, thermal conductivity, resistance to spalling. Bricks are also classified according to chemical and material composition, method of manufacture and the application to which they will be put.

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    • (Glossary) Refractory

      Refractory, as a noun, refers to a material that d...

    • (Materials) Mulcoa 70 Mullite - Refractory Aggregate, Mulgrain 70, Mulcoa 70 Calcine

      Mulcoa 70 Calcined Grog, Mulcoa 70 Calcined Mullite

    • (Glossary) Fireclay

      A refractory naturally occurring secondary clay. F...

  • Fireclay

    A refractory naturally occurring secondary clay. Fireclays are refractory because they contain high concentrations of SiO2 and Al2O3 and low concentrations of fluxes (like Na2O, K2O, CaO, MgO). Kaolins actually qualify as a super-duty fireclay because they contain almost no fluxing oxides (and are thus very refractory). However they are not used for other reasons, not the least of which is that they are highly refined, in comparison, and therefore much more expensive. In addition, fireclays are typically quite plastic (which kaolins are not). This is actually an advantage because they can support the addition of grog and still function well in the forming process. Fireclays often contain particulate impurities (that need to be ground down) and enough iron to stain them somewhat when fired.

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


    Is Lincoln 60 really a fireclay? Simple physical testing says...

    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.

    Lignite can be big trouble

    Example of the lignite particles in a fireclay (Pine Lake) that have been exposed on the rim of a vessel after sponging. This is a coarse clay, but if it were incorporated into a recipe of a stoneware, glaze pinholing would be likey.

    Pine Lake fireclay lab test bars fired to cone 10R (top) and 7,8,9,10 oxidation (from bottom to top).

    An iron fireclay? Yes.

    The natural Plainsman St. Rose Red clay before it is ground. This has about 6% iron oxide and is used to color high temperature throwing and sculpture bodies. It is quite refractory, very unusual for a clay this high in iron. It is from St. Rose, Manitoba.

    Jordan Fireclay fired bars. Cone 6 to 10 oxidation (top to bottom).

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    • (Glossary) Refractory

      Refractory, as a noun, refers to a material that d...

    • (Glossary) Secondary Clay

      Clays that have been transported by water from the...

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    • (Glossary) Firebrick

      A brick capable of withstanding high temperatures ...

  • Fired Strength

    The fired strength of clays can be measured. The test is sometimes called M.O.R. or modulus of rupture, recognizing the fact that brittle ceramics fail suddenly (as opposed to others that fail after some plastic deformation). This is also known simply as tensile strength (because the point of failure is always where the sample is under most tension). Ceramics perform much better under compressive strength testing than they do when stressed flexurally, compressive testing is more common in the structural ceramics industry.

    Common sense suggests 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 (particle packing, vitrified vs. sintered, shape and surface properties) and products fired at lower temperature can rival the strength of high fire.

    For glazeless vitrified ceramics, maturity is a key factor in achieving optimal fired strength. Testing is required since optimal strength may produce a body with more fired warping than desired. Strength may also drop off less than expected at lower levels of vitrification. 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 glaze under too much compression as this could produce shivering and contribute to spectacular failures for certain types of ware.

    People accustomed to working only with vitrified bodies are often surprised at how strong sintered ones can be. Even though the latter lack the glass to cement particles together and do not develop crystalline mesh matrices their particle size distributions, density and the much higher temperatures to which they are fired produce surprising strength.


    How much does clay shrink when bisque fired?

    Not much. These mugs were exactly the same height before a bisque firing to cone 06. The clay is a porcelain made from kaolin, feldspar and silica.

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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.


    Firing schedules at

    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.

    The difficulty of vitrifying the base of heavy stoneware

    This 1 gallon heavy crock was fired to cone 6 (at 108F/hr during the final 200 degrees) and soaked 20 minutes (in a electric kiln). The bare clay base should be the color of the top test bar (which has gone to cone 6). Yet, it is the color of the bottom bar (which has gone to cone 4)! That means the base only made it to cone 4. The vertical walls are the right color (so they made cone 6). It may seem that this problem could be solved by simply firing with a longer hold at cone 6. But electric kilns heat by radiation, that base will never reach the same temperature as the sidewalls!

    More carbon needs to burn out than you might think!

    Hard to believe, but this carbon is on ten-gram balls of low fire glazes having 85% frit. Yes, this is an extreme test because glazes are applied in thin layers, but glazes sit atop bodies much higher in carbon bearing materials. And the carbon is sticking around at temperatures much higher than it is supposed to (not yet burned away at 1500F)! The lower row is G1916J, the upper is G1916Q. These balls were fired to determine the point at which the glazes densify enough that they will not pass gases being burned from the body below (around 1450F). Our firings of these glazes now soak at 1400F (on the way up). Not surpisingly, industrial manufacturers seek low carbon content materials.

    Plainsman iron red clays with rutile blue Alberta Slip glaze

    Cone 6 mugs made from Plainsman M350 (left) and M390 dark burning cone 6 bodies. The outside glaze is Alberta-Slip-based GA6-C rutile blue and the inside is GA6-A base (20% frit 3134 and 80% Alberta Slip). That inside glaze is normally glossy, but crystallizes to a stunning silky matte when fired using the schedule needed for the rutile blue (cool 100F and soak, slow cool to 1400F).

    How to get more accurate firings time after time

    When I fire our two small lab test kilns I always include cones (I fire a dozen temperatures). I want the firing to finish when the cone is around 5-6 oclock. To make that happen I record observations on which to base the temperature I will program for the final step the next time. Where do I record these? In the schedules I maintain in our group account. I use this every day, it is very important because we need accurate firings.

    Why is the clay blistering on this figurine?

    This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.

    Manually programming a typical electric hobby kiln electronic controller

    Each prompt is two pictures here, they alternate between prompt and value every second (when not changing a value I just press Enter). I start by pressing the Enter Program button (under the Start/Stop button). It asks which program (I press Enter to accept 2). Then it asks how many segments (I accept the value of 3, I am only changing one number). After that it enables me to set up each step (shown as rows 2,3,4) by entering the degrees F/hr, the temperature to go to and the minutes to hold there. The last step is to set a temperature a subdued alarm should start sounding. In this program I am going to 240F at 300F/hr and holding 60 minutes, then 2095 at 400/hr and holding zero, then 2195 at 108/hr and holding 10 minutes. The 9999 alarm setting just means it will never sound. They key value I change almost every firing (based on observations of cones in the previous) is row 4 column 4, the final temperature.

    Why is the terra cotta glaze on the right crystal-clear while the other bisters and clouds?

    The answer is: Firing schedule. These are the same glaze, same thickness, Ulexite-based G2931B, fired to cone 03 on a terra cotta body. The one on the left is fired to cone 03, soaked half and hour and the kiln is turned off. The one on the right is fired to 100F below cone 03, soaked half an hour, then at 108F/hr to cone 03 and soaked 45 minutes, then control-cooled at 108F/hr to 1500F. The blisters likely heal on the slow cool. The micro-bubble-clouds likely dissipate on the first soak and gradual rise to temperature.

    Let me count the reasons this glossy white cone 6 glaze is pinholing

    First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gases must bubble up through. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.

    Out Bound Links

    In Bound Links

    • (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...

    • (Glossary) Boron, Borate

      The term 'boron' refers to the oxide B2O3. 'Borate...

  • Firing Shrinkage

    All clays shrink during drying. Most people that have anything to do with using plastic clay will note that the drying shrinkage increases as does plasticity, and with that increase comes more drying cracks. This happens because plastic clays have finer particle sizes and thus greater particle surface area and more inter-particle water holding things together. As that water is removed during drying, the resultant particle packing shrinks the entire mass more. Notwithstanding this, testing effort can reward you with sweet-spots in formulation (in mixes of ball clay, kaolin, feldspar, silica for example) where higher-than-expected plasticity can be achieved with lower-than-expected drying shrinkage.

    Drying shrinkage can easily be measured (see test procedures linked on this page), this enables you to compare one clay with another. From this value you can infer things about relative plasticity and particle size. The shrinkage measurement results are more meaningful when methods of sample preparation and water content or stiffness are controlled and when they are done as a matter of routine over time. It is not always practical to make shrinkage test bars, some clays shrink so much and dry so slowly (e.g. bentonite, ball clay) that the raw powder must be mixed with a calcined version of itself or with a non-plastic (like silica). Other clays can lack plasticity and make it difficult to roll and form test bars.

    Fired shrinkage (shrinkage from dry to fired) is a comparative indicator of the degree of vitrification. As a clay is fired higher it shrinks more and more to a point of maximum shrinkage (after which swelling occurs as a precursor to melting). If fired shrinkages are measured over a range of temperatures for a body it is possible to create a graph to get a visual representation of the body's maturing range. The shrinkage plotted against temperature produces a line that increases to a maximum, levels out and then drops off. As noted, fired shrinkages are relative within a system, there is no absolute of how much a clay should shrink when fired. It is common for whitewares to shrink 7-8% during firing, vitreous porcelains more than 10%. Again, these percentages are not total shrinkage pugged-to-fired, but dry-to-fired. Stonewares might shrink 5-6%, earthenwares 3-4% or less. Some special purpose sintered bodies have very low shrinkages (almost zero).

    It is very important to consider firing shrinkage when adopting an engobe to fit a clay body. If they do not shrink the same amount the engobe will be either excessively compressed for excessively stretched on to the body below. While some incompatibility can be tolerated an overgraze having an unmatched thermal expansion can be a cause of failure in the bond between engobe and body. The firing shrinkage is normally adjusted by changing the amount of frit or feldspar in the engobe recipe.

    Developing an efficient way to make, fire, measure, boil and weigh test bars is a key to being able to study fired shrinkage of your bodies and body materials. You can use an account at to learn how to do this and log and report your results.


    Scale, calipers and fired test bars to be measured for shrinkage

    The white one feels smoother, but it is actually far coarser. Why?

    Large particle kaolin (left) and small-particle ball clay (right) DFAC drying disks demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.

    Particle size drastically affects drying performance

    These are DFAC drying performance tests of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). This test dries a flat disk that has the center section covered to delay its progress in comparison to the outer section (thus setting up stresses). Finer particle sizes greatly increase shrinkage and this increases the number of cracks and the cracking pattern of this specimen. Notice it has also increased the amount of soluble salts that have concentrated between the two zones, more is dissolving because of the increased particle surface area.

    Stonewares dry better than porcelains

    The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.

    How to test drying and firing compatibility between engobe and body

    I have made bi-body strips for testing to tune a white slip for a terra cotta clay body (about 4 mm thick). They need to shrink a similar amount in drying and firing to be as compatible as possible. Here, the white body needs more plastic clay or a bentonite addition to shrink more. It also needs a little less frit or a coarser silica to shrink a little less on firing (pending porosity tests to match their fired density). Amazingly, the fired bars break much more easily one way that the other, because on one side the clay is being stretched (and ceramic does not do well under tension).

    It is very important to fit the engobe to the body

    This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there). Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way! Right: Test bars of the white slip and red body compare their drying and firing shrinkages. Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less). Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).

    When two clays are joined are they compatible?

    These bi-body strips are made by rolling two clays together in a thin sandwich. Three porcelains are being compared to a very plastic grogged sculpture body. After drying (top) they curl a little, two toward the sculpture body and one, the most plastic of the porcelains, toward the white. But on firing to cone 8 they curl dramatically toward the porcelain side (because it shrinks alot more). Now imagine one of these porcelains is being used as a slip on this body.

    Test bars of different terra cotta clays fired at different temperatures

    Bottom: cone 2, next up: cone 02, next up: cone 04. You can see varying levels of maturity (or vitrification). It is common for terra cotta clays to fire like this, from a light red at cone 06 and then darkening progressively as the temperature rises. Typical materials develop deep red color around cone 02 and then turn brown and begin to expand as the temperature continues to rise past that (the bottom bar appears stable but it has expanded alot, this is a precursor to looming rapid melting). The top disk is a cone 10R clay. It shares an attribute with the cone 02 terra cotta. Its variegated brown and red coloration actually depends on it not being mature, having a 4-5% porosity. If it were fired higher it would turn solid chocolate brown like the over-fired terra cotta at the bottom.

    Stacking of SHAB clay test bars for firing

    OM#4 ball clay test bars fired from cone 4-10 oxidation and cone 10 reduction. The yellow on bar 12 is iron stained soluble salts.

    Example of firing test bars stacked into an electric kiln for firing.

    What really is Barnard Slip?

    These are fired bars of Barnard Slip going from cone 06 (bottom) to cone 6 (top). By cone 5 it is beginning to expand and at cone 6 it has turned glossy and is starting to melt.

    How much does a porcelain piece shrink on firing?

    Left: Dry mug. Right: Glazed and fired to cone 6. This is Polar Ice porcelain from Plainsman Clays. It is very vitreous and has the highest fired shrinkage of any body they make (14-15% total). This is the highest firing shrinkage you should ever normally encounter with a pottery clay.

    Double-slip layer incised decoration: A challenge in slip-body fitting

    An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.

    Out Bound Links

    • (Glossary) Ultimate Particles

      Processed ceramic materials are typically ground t...

    • (Articles)

      The Physics of Clay Bodies

      Learn to test your clay bodies and recording the results in an organized way and understanding the p...

    • (Glossary) Porosity

      In ceramic testing this term generally refers to t...

    • (Glossary) Engobe

      A white or colored slip applied to clay as a coati...

    • (Tests) SHAB - Shrinkage/Absorption
    • (Glossary) Vitrification

      Vitrification is the solidification of a melt into...

    In Bound Links

    • (Project) Ceramic Tests Overview

      Every ceramic production facility should have some...

    • (Tests) FSHR - Firing Shrinkage
    • (Tests) DSHR - Drying Shrinkage
    • (Glossary) Drying Performance

      Refers to the ability of a clay to dry without cra...

  • 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.

    Out Bound Links

    In Bound Links

  • 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 or undesired property.

    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. It will thus have a higher shrinkage on drying and likely 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 or reflormulated such that the needed oxides are supplied by non-soluble materials.


    The same engobe. Same water content. What is the difference?

    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.

    Should you throw out the brown water on top of settled glazes?

    This is water from the top of a glaze that had been sitting for more than a year. Clearly, the solute contains iron. It is being dissolved out of one or more of the white powders making in the glaze recipe, so the iron at least is a contaminant. This should be thrown out and replaced with clean water. Why? We do not want anything dissolved in glaze slurries. It either migrates into the body with the water it absorbs during glazing or it migrates to the surface as the water evaporates. Both are bad. How much dissolved material would be lost? It would be measured in tenths or hundreds of a gram. Hypothetically then, if a bucket contains 1000 grams of the material, one ten-thousandth of it would be lost!

    Which clay contains more soluble salts?

    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.

    Would it be possible to glaze a stainless steel spoon?

    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.

    How to stop an slip from running

    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.

    When to use vinegar and when to use epsom salts to flocculate a slurry

    Slurries with more clay (like engobes, slips) generally respond better to epsom salts. However the extra clay also makes them more likely to go moldy, so you may need to add a few drops of Dettol to kill the bacteria. Vinegar works better for glaze surries, but only if they have sufficient specific gravity. Many people like to make an epsom salts solution and add that, but if you have a good mixer you may find it more intuitive to add the crystals and wait 30 seconds for the viscosity to respond.

    In Bound Links

  • 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'.


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

    A flow tester has revealed the problem with this glaze

    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.

    How can you make Ravenscrag Floating Blue dance more?

    Here it is fired to cone 8 where the melt obviously have much more fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course it is running alot more so caution will be needed.

    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.

    When glazes are highly fluid they can...

    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.

    The glaze broke the pot!

    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

    Stains having varying fluxing effects on a host glaze

    Plainsman M340 Transparent liner with various stains added (cone 6). These bubbles were fired on a bed of alumina powder, so they flattened more freely according to melt flow. You can see which stains flux the glaze more by which bubbles have flattened more. The deep blue and browns have flowed the most, the manganese alumina pink the least. This knowledge could be applied when mixing these glazes, compensating the degree of melt of the host accordingly.

    Melting range is mainly about boron content

    Fired at 1850. Notice that Frit 3195 is melting earlier. By 1950F, they appear much more similar. Melting earlier can be a disadvantage, it means that gases still escaping as materials in the body and glaze decompose get trapped in the glass matrix. But if the glaze melts later, these have more time to burn away. Glazes that have a lower B2O3 content will melt later, frit 3195 has 23% while Frit 3124 only has 14%).

    Melt fluidity: Cornwall Stone vs. Nepheline Syenite

    Three Cornwall Stone shipments fired at cone 8 in melt flow testers and compared to Nepheline Syenite. Each contains 10% Ferro Frit 3134.

    Lights go on with side-by-side fired samples and chemistry

    10 grams balls of these three glazes were fired to cone 6 on porcelain tiles. Notice the difference in the degree of melt? Why? You could just say glaze 2 has more frit and feldspar. But we can dig deeper. Compare the yellow and blue numbers: Glaze 2 and 3 have much more B2O3 (boron, the key flux for cone 6 glazes) and lower SiO2 (silica, it is refractory). That is a better explanation for the much greater melting. But notice that glaze 2 and 3 have the same chemistry, but 3 is melting more? Why? Because of the mineralogy of Gerstley Borate. It yields its boron earlier in the firing, getting the melting started sooner. Notice it also stains the glaze amber, it is not as pure as the frit. Notice the calculated thermal expansion: That greater melting came at a cost, the thermal expansion is alot higher so 2 and 3 glaze will be more likely to craze than G2926B (number 1).

    Will a cone or ball flow out better in a melt flow test?

    This is G2926B cone 6 transparent glaze. I am developing a simple test procedure to produce an absolute measurable value for glaze melt flow and it appeared it would be worthwhile to create a mold to make these cone-shaped samples. But I was wrong. Both specimens are exactly 10 grams, but the simple ball flows better. This is likely because of better early heat penetration because there is only a small area of contact with the tile.

    Comparing the melt fluidity of two shipments of Custer Feldspar

    Melt flow test comparing Custer Feldspar from Feb/2012 (right) with Mar/2011. Custer Feldspar does not melt like this by itself at cone 10. It was mixed 80:20 Feldspar:Ferro Frit 3134. This test demonstrates that the material has been very consistent between these two shipments.

    Checking new glazes using a melt fluidity test

    This is an example of how useful a flow tester can be to check new glaze recipes before putting them on ware and into your kiln. This was fired to only cone 4, yet that fritted glaze on the left is completely over-melted. The other one is not doing anything at all. These balls are easy to make, you only need weigh out a 50 gram batch of glaze, screen it, then pour it on a plaster bat until it is dewatered enough to be plastic enough to roll these 10 gram balls.

    Switching copper carbonate for copper oxide in a fluid glaze

    The top samples are 10 gram balls melted down onto porcelain tiles at cone 6 (this is a high melt fluidity glaze). These balls demonstrate melt mobility and susceptibility to bubbling but also color (notice how washed out the color is for thin layers on the bottom two tiles). Both have the same chemistry but recipe 2 has been altered to improve slurry properties. Left: Original recipe with high feldspar, low clay (poor suspending) using 1.75% copper carbonate. Right: New recipe with low feldspar, higher clay (good suspending) using 1% copper oxide. The copper oxide recipe is not bubbling any less even though copper oxide does not gas. The bubbles must be coming from the kaolin.

    A super glassy ultra-clear brilliantly glossy cone 6 clear base glaze? Yes!

    I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. In the end I will pick one or two, fix the issues and provide instructions.

    Two bases, 2% copper additions. Which is the better transparent?

    Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.

    Glaze bubbles behaving badly!

    These melted-down-ten-gram balls of glaze demonstrate the different ways in which tiny bubbles disrupt transparent glazes. These bubbles are generated during firing as particles in the body and glaze decompose. This test is a good way to compare bubble sizes and populations, they are a product of melt viscosity and surface tension. The glaze on the top left is the clearest but has the largest bubbles, these are the type that are most likely to leave surface defects (you can see dimples). At the same time its lack of micro-bubbles will make it the most transparent in thinner layers. The one on the bottom right has so many tiny bubbles that it has turned white. Even though it is not flowing as much it will have less surface defects. The one on the top right has both large bubbles and tinier ones but no clouds of micro-bubbles.

    Out Bound Links

    • (Articles)

      A Low Cost Tester of Glaze Melt Fluidity

      This device to measure glaze melt fluidity helps you better understand your glazes and materials and...

    • (Troubles) Glaze Blisters
      Questions and suggestions to help you reason out t...
    • (Tests) GMFA - Glaze Melt Fluidity Absolute
    • (Tests) GLFL - Glaze Melt Flow

    In Bound Links

    • (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) GSPT - Softening Point
  • 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.


    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).

    How do metal oxides compare in their degrees of melting?

    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.

    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.

    At 1550F Gerstley Borate suddenly shrinks!

    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.

    Stains having varying fluxing effects on a host glaze

    Plainsman M340 Transparent liner with various stains added (cone 6). These bubbles were fired on a bed of alumina powder, so they flattened more freely according to melt flow. You can see which stains flux the glaze more by which bubbles have flattened more. The deep blue and browns have flowed the most, the manganese alumina pink the least. This knowledge could be applied when mixing these glazes, compensating the degree of melt of the host accordingly.

    2% Copper carbonate in two different cone 6 copper-blues

    The top base glaze has just enough melt fluidity to produce a brilliant transparent (without colorant additions). However it does not have enough fluidity to pass the bubbles and heal over from the decomposition of this added copper carbonate! Why is lower glaze passing the bubbles? How can it melt better yet have 65% less boron? How can it not be crazing when the COE calculates to 7.7 (vs. 6.4)? First, it has 40% less Al2O3 and SiO2 (which normally stiffen the melt). Second, it has higher flux content that is more diversified (it adds two new ones: SrO, ZnO). That zinc is a key to why it is melting so well and why it starts melting later (enabling unimpeded gas escape until then). It also benefits from the mixed-oxide-effect, the diversity itself improves the melt. And the crazing? The ZnO obviously pushes the COE down disproportionately to its percentage.

    Out Bound Links

    • (Glossary) Refractory

      Refractory, as a noun, refers to a material that d...

    • (Glossary) Frit

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

    In Bound Links

  • 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.

    Out Bound Links

  • 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.


    Example of a whole rock chemical analysis lab report

    Powdered samples were sent to the lab. The numbers shown on this chemical analysis report are in percentage-by-weight. That means, for example, that 15.21% of the weight of the dry powder of Alberta Slip is Al2O3. Insight-live knows material chemistries in this way (whereas desktop Insight needs them as formulas). Some non-oxide elements are quantified as parts-per-million (these amounts are not normally high enough to take into account for traditional ceramic purposes). The LOI column shows how much mechanically and chemically bound water are gassed off during firing of the sample. The total is not exactly 100 because of inherent error in the method and compounds not included in the report.

    INSIGHT showing formula and analysis side-by-side

    Ceramic glaze oxides periodic table

    All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.

    Ceramic Oxide Periodic Table in SVG Format

    The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.

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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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  • 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).


    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

    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 (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.

    At 1550F Gerstley Borate suddenly shrinks!

    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.

    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...

    • (Tests) GSPT - Softening Point
    • (Tests) GTTM - Glass Transition Temperature
    • (Glossary) Base Glaze

      A base glaze is one having no opacifiers, variegat...

  • 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 e.g. because of water logging). 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.


    Water-logging happens when a clay is underfired

    The cone 6 glaze is well developed, it is not crazed. But the clay underneath is not developed, not vitreous. This crack happened when the mug was bumped (because of poor strength). It is barely visible. When the mug is filled with water, this happens. How fast? This picture was taken about 5 seconds later. If this was crazing, and this piece was in actual use, the clay would gradually become completely water logged. Then one day someone would put it in the microwave! Boom.

    Underfiring a clay is OK if the glaze fits? No it is not.

    Left: Plainsman M340 fired to cone 6 where it achieves about 1.5% porosity, good density and strength. Right: H550, a Plainsman body intended to mature at cone 10, but fired to cone 6 using the same glaze. Although the glaze melts well and the mug appears OK, it is not. It is porous and weak. In fact, it has cracked during use (the crack runs diagonally down from the rim). It was then dipped into water for a few moments and immediately the water penetrated the crack and began to soak into the body (you can see it spreading out from the crack). If this glaze were to craze the entire thing would be waterlogged in minutes.

    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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