Digitalfire Ceramic Glossary

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  • Majolica

    Pottery fired to a low temperature employing a red-burning terra cotta clay covered with a soft opaque white glaze. Historically, majolica glazes (or tin glazed earthenware) were opacified using Tin Oxide, but now Zirconium silicate is most often used. Most majolica also has colored brushwork designs that are painted over the dried glaze (the painting process is tricky because you are painting on a very absorbent surface, you get one brush stroke!). Metallic colorants are brightest at low temperatures and the zircon-stiffened white glaze provides an ideal canvas for them. Colorant formulations (mixtures of stain powders, melters, hardeners, fillers) need to be tuned to melt enough so that they become one with the glaze below, but not so much that they bleed excessively at the edge of brush strokes. Different families of stains have different melting behaviors and chemistry requirements for the host glaze and medium (the color development and degree of melting in the final firing product depend on this). You can buy premixed majolica colors and often these work well, but do not assume that just because it comes in a jar it is perfect or that you cannot formulate something better for your application.

    Ware made using the Majolica process is not strong, the clays generally have 10% porosity or more (fired at cone 06-04). This low temperature means that the body-glaze interface is much less developed than in stoneware (so the glaze is not stuck on nearly as well). It is thus important that the thermal expansion of the glaze be matched to the body to prevent crazing and shivering. Ware is not as durable in use and any bare sections will absorb water and tend to water-log the clay matrix over time. It is thus best to make a foot ring so that the entire piece can be glazed, leaving only a narrow channel of exposed body to touch the kiln shelf.


    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.

    A dried terra cotta mug on the left, bisque fired to cone 06 on the right

    These were fired to cone 06, about 1800F. Of course, there is normally some shrinkage so the bisque piece would be a little smaller. Even though the matrix is very porous and is under developed, the iron in the body is already beginning to impose its color.

    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).

    Out Bound Links

    • (Articles)

      The Majolica Earthenware Process

      Understanding the advantages and disadvantages of low fire ware and the chemistry of physics of the ...

    • (Articles)

      G1916M Cone 06-04 Base Glaze

      This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base fo...

    • (Glossary) Opacifier, Opacification

      A glaze additive that transforms an otherwise tran...

    • (Glossary) Terra cotta

      'Terra Cotta' (Italian for 'cooked earth') is red ...

    • (Glossary) Crazing

      Crazing refers to small hairline cracks in glazed ...

    • (Glossary) Shivering, peeling

      A defect in glazed ware where the glaze is compres...

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  • Marbling

    The practice of mixing porcelains of two or more different colors (the porcelains are typically colored using metal oxides or stains). To produce a quality fired product it is important that the clays are the same stiffness and have the same drying and fired shrinkages and the same thermal expansions.


    Marbling using a translucent porcelain

    A transparent glazed marbled bowl by Tony Hansen. It is a made from Plainsman Polar Ice (a New Zealand kaolin based porcelain) and fired to cone 6 with G2926B clear glaze. 5% Mason 6306 teal blue stain was added to the clay, then this was wedged only a few times. The piece was thrown, then trimmed on the outside at the leather hard stage and sanded on the inside when dry.

    Cone 6 porcelain marbled and thrown

    These bowls were made by Tony Hansen using a mixture of white and stained New-Zealand-kaolin-based porcelain (Plainsman Polar Ice) fired at cone 6. The body is not only white, but very translucent.

  • Material Expansion

    Ceramic materials exhibit thermal expansions just as fired bodies. Manufactured materials, like frits, have a consistent easily measured thermal expansion because they are completely melted during processing. They also calculate well, that is, the thermal expansion can be predicted. Raw materials, on the other hand are complex (expecially clays). Many raw materials are not homogeneous, thus when subjected to the heat of a kiln, different particles within the powder matrix will exhibit different thermal expansion characteristics (some of which can be multi-stage) and they interact in complex ways. For example, some may be refractory and resist melting, others may form a glass melt and take yet others into solution while still others may interact to create new crystalline species that survive for a period and then inturn interact for a time and finally go into solution or melt. If a material was fired alone the final thermal expansion would be a product of the extent to which all of this was allowed to proceed (according to the temperature the kiln reached). Thus, while it may be practical to publish the thermal expansion of a frit for calculation purposes, this is not practical with the vast majority of raw materials. It is better to calculate the expansion of the glaze into which they are incorporated based on the calculated quantities of each oxide. Of course, the reliability of the calculation is also based on whether the glaze completes melts or not.

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    • (Troubles) Glaze Crazing
      Ask the right questions to analyse the real cause ...
  • Material Substitution

    Material substitution is a constant and ongoing part of any ceramic enterprise that is using clay and/or glaze recipes. Often lack of availability, quality issues and price are the motivating factors. In addition, when recipes need to be used in another locality where the same raw material brands or even types are unavailable, substitutions must be done.

    In ceramics, glazes fire the way they do primarily because of their chemistry. The most basic substitution is to simply replace one material for another which has a chemistry similar enough that the fired properties will not be adversely affected. This is often possible with different brand names of the same mineral or refined material (e.g. Calcium carbonate, zinc oxide). The more complex the chemistry of the raw material, the more likely there is to be issues with changes to another (e.g. feldspar). In some cases substitution recipes are recommended where a mix of two or more other materials is said to be equivalent. Another problem can occur where, although the chemistry of the substitute is very similar, it's physical properties or particle size are different enough to affect the working properties of the glaze (e.g. a kaolin) or even it's fired appearance (a metal oxide).

    A given chemistry can be supplied by many different mixes of refined and raw material powders, each of these having advantages and disadvantages regarding price, ease of use in production, toxicity, etc. It is common to use ceramic chemistry to calculate how to juggle a recipe to substitute one material for another of slightly or even very different chemistry while maintaining the chemistry of the glaze as a whole. The calculcation is more complex where the substitute is bringing along other oxides not in the material being substituted (or fewer) or the original or substitute has a very different weight loss on firing (LOI). Frits especially can have complex chemistries and obviously it is more complciated if they are involved in the substitution calculation.

    When clay bodies and porcelains require material substitution, the issue is physical properties (which are often not directly related to chemistry). Thus testing must be done to see how maturity, drying properties, plastic and/or forming behavior, texture, firing color and character, thermal expansion and other properties are affected. A series of tests usually must be done to alter the recipe to accommodate the new material while maintaining the needed properties.

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  • Matte Glaze

    A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. It is common to see poor quality matte surfaces on name-brand table ware sold in major stores.

    The vast majority of random material mixes that melt well want to be glossy. Matteness can be a product of the physical or mineral form of a material used, the chemistry and selection of materials to source that chemistry and often the firing schedule. While some types of mattes are stable, with others it can be difficult to maintain the same fired texture through material and firing variations. The best mattes are those whose mechanism is understood and have an adjuster (a firing change or a material whose percentage can be raised or lowered to fine tune the degree or character of the matteness).

    The visual character of mattes, even those within the same mechanism, varies widely and is often difficult to characterize. Matteness is often part of a larger visual character that involves color and variegation.

    Mechanisms that produce matte glazes produce surfaces that scatter light:
    -Micro crystalline surfaces. High CaO glazes, for example, form minute calcium silicate crystals when cooling (at normal cooling rates). Wollastonite especially can do this, but also other sources of CaO. Another oxide that crystallizes well if oversupplied is ZnO, the size of the crystal being determined by the rate of cooling and level of ZnO.
    -Micro-wavy or rippled (non flat) surfaces can be produced multiple ways. High Al2O3 (if supplied in a form that can decompose to enable Al2O3 to enter the melt), for example, stiffens the melt preventing level-out during cooling. Glaze melts that contain multiple melt phases solidify in a non-homogeneous way to produce a glass that both scatters light from within and from its surface.
    -A special case of micro-rippled surfaces is MgO. It is a very effective matting agent at both high and middle temperatures. Talc and dolomite source the MgO to create this effect (although can differ in appearance). In higher temperatures the MgO creates multiple phases in the melt that have different fluidity and refractive indexes. These are sometimes called 'silky mattes' and are pleasant to the touch. Amazingly MgO also produces this effect at middle temperatures even though it is not an active melter there. Levels of 0.3 or higher will stiffen the melt without detrimentally affecting glass development and produce very pleasant matte surfaces. This works in boron fluxed glazes that have high or low Al2O3 and low or medium SiO2 levels, producing surfaces that do not cutlery mark and glazes that do not craze (because of the low expansion of MgO).
    -Crowbar method! Materials whose individual particles are so refractory that they simply do not dissolve in the melt, if added judiciously to the right base, can produce a workable matte. Magnesium carbonate is an example. Even calcium carbonate, if supplied in raw form, does not melt at lower temperatures and can thus matte a glaze. But the best example is calcined alumina, if used in sufficiently fine particle size, can matte a glaze even with a small addition. However, alumina hydrate, by contrast requires a much greater addition. Why? It enters the chemistry of the melt and imparts a true alumina matte, the latter just increases the melting temperature because it is so refractory.
    -Mechanisms that are not well understood. An example is barium mattes. Although they appear to be crystallized, some have found that no matter how fast they are cooled they still have the same degree of matteness. At the same time, fritted forms of the same amount of barium do not matte! In this system it appears the carbonate form supplies the BaO and seeds the crystals.

    Employing combinations of these mechanisms is normally not practical because they can conflict. For example, a crystal matte is based on a highly fluid, well melting glaze, whereas an alumina matte is the opposite. However an exception to this is magnesia mattes, they can occur where alumina is high and silica is low (the alumina matte mechanism, although MgO can matte glazes that also have low MgO).

    Functional matte glazes are more difficult to formulate (especially at middle and low temperatures) because they have a narrow window of chemistries or have recipes containing matting agents that are highly active (resulting in large changes in the degree matteness for small variations in the recipe or process). For crystal mattes, specific firing methods are also needed (e.g. slower cooling). Also, the degree to which mattes do not level out completely on cooling determines how easy-to-clean the surface of the glass will be.


    The reflection of light on a matte glaze

    A refined-material cone 10R dolomite matte (left) vs. one made using Ravenscrag Slip

    GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a buff stoneware at cone 10R. Surfaces feel identical, the slightly darker color is due to iron content in the Ravenscrag. The former was formulated to mimic the latter using as much Ravenscrag Slip as possible yet still maintain the same chemistry.

    Look at recipes before wasting time and money on them.

    Are they serious? This is a cone 6 melt flow comparison between a matte recipe, found online at a respected website, and a well-fluxed glossy glaze we use often. Yes, it is matte. But why? Because it is not melted! Matte glazes used on functional surfaces need to melt well, they should flow like a glossy glaze. How does that happen? This recipe has 40% nepheline syenite. Plus lots of dolomite and calcium carbonate. These are powerful fluxes, but at cone 10, not cone 6! To melt a cone 6 glaze boron, zinc or lithia are needed. Boron is by far the most common and best general purpose melter for potters (it comes in frits and gerstley borate, colemanite or ulexite; industry uses more boron, zinc and lithia frits). The lesson: Look at recipes before trying them.

    A functional matte cone 6 glaze should melt as well as a glossy

    True functional mattes have fluid melts, like glossy glazes. They need this in order to develop a hard, non-scratching durable glass. The mechanism of the matte on the right is high Al2O3 (G1214Z), it is actually melting more than the glossy glaze on the left (G1214W).

    How to matte Ravenscrag Slip at cone 10 by adding talc

    2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.

    2, 5, 10 and 15% calcined alumina added to Ravenscrag Slip

    The Ravenscag:Alumina mix was applied to a buff stoneware fired at cone 10R (by Kat Valenzuela). Matting begins at only 5% producing a very dry surface by 15%. The matte is simply a product of the refractory nature of the alumina as a material, it does not disassociate in the melt to yield its Al2O3 as an oxide (as would a feldspar, frit or clay). The same test using alumina hydrate demonstrates that it disassociates better (although not completely).

    2, 5, 10, 15% dolomite added to Ravenscrag Slip at cone 10R

    This is a buff stoneware clay. Crystal development toward a dolomite matte begins at 15%. By Kat Valenzuela.

    2, 5, 10 and 15% alumina hydrate added to Ravenscrag Slip

    Pure Ravenscrag Slip is glaze-like by itself (thus tolerating the alumina addition while still melting as a glaze). It was applied on a buff stoneware which was then fired at cone 10R (by Kat Valenzuela). This same test was done using equal additions of calcined alumina. The results demonstrated that the hydrated version much more readily decomposes to yield its Al2O3, as an oxide, to the glaze melt. By 15% it is matting and producing a silky surface. However crazing also starts at 10%. The more Al2O3 added the lower the glaze expansion should be, so why is this happening? It appears that the disassociation is not complete, some of the raw material remains to impose its high expansion.

    How to turn a dolomite matte white glaze into a bamboo matte

    Make cone 10R bamboo colors using the GR10-J Ravenscrag silky matte base recipe (right) and adding 1% iron (left), (0.5% centre). These samples are porcelain. This iron addition also works using the G2571A matte base recipe.

    Compare two glazes having different mechanisms for their matteness

    These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived and 300F to ice water plunge without cracking). This looks like it could be a great liner glaze.

    Ravenscrag dolomite matte

    GR10-J Ravenscrag dolomite matte base glaze at cone 10R on Plainsman H443 iron speckled clay. This recipe was created by starting with the popular G2571 base recipe (googleable) and calculating a mix of materials having the maximum possible Ravenscrag Slip percentage. The resultant glaze has the same excellent surface properties (resistance to staining and cutlery marking) but has even better application and working properties. It is a little more tan in color because of the iron content of Ravenscrag Slip (see

    A matte that is matte because it is not melting completely

    Left: This specimen of VC71 cone 6 matte glaze was felt-marked and cleaned with acetone. A closeup of the ink specks reveals they are held in micro-bubbles breaking at the surface. This specimen has also been thermally stressed in a 300F/icewater test (causing the crazing pattern, which curiously, only shows up on part of the surface). Right: An adjustment to VC71 that adds more boron and Al2O3/SiO2 (while preserving the Si:Al ratio). It is much glossier, confirming that, even though the VC71 matte surface feels functional to the touch, it is a product of improper melting.

    A good matte glaze. A bad matte glaze.

    A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.

    Tuning the degree of gloss in a colored matte glaze

    Matte glazes have a fragile mechanism. That means the same recipe will be more matte for some people, more glossy for others (due to material, process and firing differences). In addition, certain colors will matte the base more and others will gloss it more. It is therefore critical for matte glaze recipes to have adjustability (a way to change the degree of gloss), both for circumstances and colors. This recipe is Plainsman G2934 base matte with 6% Mason 6600 black stain added. It has been formulated to be on the more matte side of the scale so that for most people a simple addition of G2926B (M370 transparent ultra clear base recipe) will increase the gloss. That means users need to be prepared to adjust each color of the matte to fine tune its degree of gloss. Here you can see 5, 10, 15 and 20% additions of the gloss recipe.

    A true matte is still matte when you over fire it

    The top glaze is VC71, a popular matte cone 6 glaze used by potters. Bottom is G2934 matte, a public domain recipe produced by Plainsman Clays. The latter is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. VC71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher. This suggests that the cone 6 matteness is due to incomplete melting. For the same reason, it is whiter in color (as soon as it begins to melt and have depth the color darkens).

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  • Mature

    A term referring to the degree to which a clay or glaze has vitrified or sintered during the firing. A 'mature' stoneware or porcelain clay is normally one that is dense and strong. Mature clays used for functional ware are dense enough to resist soaking up water. Firing commercial clays to lower temperature than what they are intended for will result in weaker pieces that become waterlogged over time continually weakening. Every clay has an ideal range (often quite narrow) to which it should be fired to develop the properties needed. Some manufacturers list wider-than-practical firing ranges for certain clay bodies, leaving customers to discover the best temperature. Others recommend a vitrification temperature that is too high. Often clay bodies are purposely used at temperatures less than mature to get more stability in the kiln, more margin or error, or to achieve a color that is lost if fired higher.

    Glazes can also be described using this term. A 'mature' glaze has been fired high enough and held at that temperature long enough such that its melt flows well, heals imperfections and provides a good covering. It cools to a hard, durable surface that is resistant to leaching. Firing that same glaze to a low temperature would compromise these properties.

    Like the term 'vitrified', the term 'mature' needs to be taken in context. A mature sintered refractory, for example, can be quite porous, yet it can still be very strong.


    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.

    Cone 2: Where we see the real difference between terra cottas and white bodies

    The terra cotta (red earthenware) body on the upper left is melting, it is way past zero porosity, past vitrified. The red one below it and third one down on the right have 1% porosity (like a stoneware), they are still fairly stable at cone 2. The two at the bottom have higher iron contents and are also 1% porosity. By contrast the buff and white bodies have 10%+ porosities. Terra cotta bodies do not just have high iron content to fire them red, they also have high flux content (e.g. sodium and potassium bearing minerals) that vitrifies them at low temperatures. White burning bodies are white because they are more pure (not only lacking the iron but also the fluxes). The upper right? Barnard slip. It has really high iron but has less fluxes than the terra cottas (having about 3% porosity).

    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.

    These two pieces will not mature to the same degree in a firing

    Soak the firing 30 minutes to mature the mug and the planter will not mature. Soak 2 hours for the planter and the glaze may melt too much and the clay be too vitreous. This is a troublesome issue with electric kilns. Furthermore, they employ radiant heat. That means that sections of ware on the shady side (or the under side) will never reach the temperature of those on the element side no matter how long you soak.

    How much porcelain flux is too much?

    A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.

    Cone 6 kaolin porcelain verses ball clay porcelain.

    Typical porcelains are made using clay (for workability), feldspar (for fired maturity) and silica (for structural integrity and glaze fit). These cone 6 test bars demonstrate the fired color difference between using kaolin (top) and ball clay (bottom). The top one employs #6 Tile super plastic kaolin, but even with this it still needs a 3% bentonite addition for plasticity. The bottom one uses Old Hickory #5 and M23, these are very clean ball clays but still nowhere near the whiteness of kaolins. Plus, 1% bentonite was still needed to get adequate plasticity for throwing. Which is better? For workability and drying, the bottom one is much better. For fired appearance, the top one.

    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.

    Particle size and LOI determine behaviour of over-fired bodies

    These are four terra cotta body disks that have been fired to cone 10 reduction. The fluxing action of the iron has assisted to take them well along in melting. Notice that one is hardly bubbling at all, it is Redart clay that has been ground to 200 mesh (the lower right one is a body mix of 200 mesh materials also containing it). The upper left one is bubbling alot more. Why? Not just because it is melted more (in fact, the one on the lower left is the most melted). It is a body made from clays that have been ground to 42 mesh. Among the particles are larger ones that generate gases as they decompose. Yes, the particles in the others do the same, but their smaller size enables earlier decomposition and expulsion of smaller gas amounts distributed at many more vents. Some bodies cannot be fired to a point of zero porosity, they will bubble before they get there.

    Out Bound Links

    • (Glossary) Vitrification

      Vitrification is the solidification of a melt into...

    In Bound Links

    • (Glossary) Porcelain

      Traditional utilitarian porcelains are comparative...

    • (Glossary) Functional

      A functional clay body is one that produces a cera...

    • (Glossary) Warping

      Normally refers to a body firing problem where ves...

  • MDT

    MDT is an acronym for Materials Definition Table. It is the materials database of Digitalfire Insight glaze chemistry software. It is called a table because Insight reads it into memory from an XML file at program startup and forms and internal data structure of material rows and oxide columns. It references these when calculating the oxide chemistry of recipes. The MDT stores material chemistry as formulas and formula weights. It does not matter whether the formulas are unified or not as stored in the table, Insight just needs to know the proportions of oxides numbers.

    The Digitalfire Reference library has the ability to export a subset of its materials as an MDT file. This enables users to login and start with a general list (e.g. North America, Europe) and then add specific materials as needed. They can then download the collection as an MDT file and put that file in the Insight folder in the documents folder on their computer (where desktop Insight will see it the next time it starts up).

    The XML text format of the MDT file is the product of evolution over the years. Earlier formats listed materials and oxides/amounts on separate lines without identifying tags. XML is a very flexible format that can embody relational and hierarchical structures along with attributes (you can open an MDT file using your text editor or word processor to see what the format looks like). Since most people do not need more than a couple of hundred materials in their library, materials never needed to be stored in a database, the text format was faster and more flexible.


    Digitalfire Insight materials dialog window

    Desktop Insight remembers materials (in its database) as formulas and their formula weights. From this it can calculate the LOI. Materials can have alternate names so they are more likely to be found in calculating recipes. This dialog provides tools for adding, editing, deleting, importing and exporting materials.

    Desktop INSIGHT MDT dialog showing kaolin LOI

    The LOI appears below the material name and alternative names (beside the weight). The formula that goes with that LOI is the bold numbers in the blanks beside the oxide names on the right.

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  • Medium Temperature or Mid-Fire Glaze

    In functional ceramics this term generally refers to glazes that mature from cone 4 to 7. At these temperatures it is difficult to compound glazes that will melt well without the need for powerful melters like zinc and boron. Thus a medium temperature glaze contains mostly the same kinds of ingredients as a high temperature one, but additionally it needs a source of zinc or boron (boron is by far more popular and less troublesome). Typically frits are employed to supply the B2O3. Historically Gerstley Borate and Colemanite were very common sources also. Boron has a low thermal expansion and thus is an ideal additive since it reduces the tendency of glazes to craze. Since there are no practical insoluble sources of pure boron, ceramic chemistry is normally needed to determine how to best incorporate boron-sourcing materials.

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  • Melting Temperature

    Unlike crystalline minerals, glazes do not have a specific melting temperature, they soften over a range of temperatures. And after they have been melted they become increasingly fluid and homogeneous. The softening process is not a linear one, this is especially so because raw glaze powders are a mix of many different mineral (and possibly man-made) particles, each of which has its own melting behavior. Flux particles melt first but not just because they are fluxes. The mere proximity of other particles with which they like to interact creates a union that melts readily at temperatures much lower than either individual one would melt on its own. These interactions are also greatly enhanced when the particles are smaller and when groups of different particles react together (the more different types and sizes there are the more complex is their interactions). And then, as viscous glass melts evolve, they develop their own changing dynamics that can accelerate or slow the dissolution of remaining larger and more refractory unmelted ones. When sufficient time is available, the entire mass is melted. However as the last particle dissolves the melt exists as a discontinuity of chemistries and viscosities (phases). Message: OnLine INSIGHTAs yet more time passes the homogeneity of the mix increases. The higher the proportion of fritted materials the more predictable the process is.

    That means that an individual glaze can actually serve well across a range of temperatures and can exhibit itself in a variety of ways. At one temperature it might be a matte, at another a glossy, at another a crystalline or reactive. Various firing schedules within a temperature will increase the number of personalities even more. It is thus better to see glazes as more dynamic and less easily pigeon-holed and classified.

    Again, the complexities of oxide interactions and firing methods along with the wide range of physical and mineralogical properties of materials supplying oxides make the prediction of absolute values for fired properties an inexact and highly system-specific science at best. Modelling this in an absolute sense (predicting exact viscosities at exact temperatures for any mix of materials) using chemistry and mathematics is not possible. Notwithstanding that, the single biggest factor affecting the degree to which we can predict what a melt will do (in a comparative way) is its chemistry. This is especially so when we keep all other factors equal. For example, if we can measure the melt fluidity at a specific temperature then we can predict whether it will melt more or less if specific changes are made in the chemistry. The certainly of that prediction will be greater when we source the chemistry from all the same materials and it will be less when we introduce new ones.

    Digitalfire Insight's ability to show side-by-side recipe chemistry and physics makes it a natural for studying one in relation to another with respect to melting and softening behavior. Technicians build a knowledge of what types of chemistry changes are most effective in altering melting temperature without unduly affecting other properties and how this relates to the materials that are sourcing that chemistry. With experience you will develop the ability to predict the range in which a glaze might be useful in your circumstances better than any algorithm could do.

    For these reasons Digitalfire has been very hesitant to build temperature prediction into our software.


    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.

    A frit softens over a wide temperature range

    This is unlike some raw materials which melt suddenly.

    Frits melt so much better than raw materials

    Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.

    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%).

    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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  • Metallic or Bronze Glazes

    Glassy iridescent metallic glazes can most easily be produced in oxidation using very high a percentage of manganese dioxide (the metal fumes of which can be very dangerous) in a borax or lead based frit or glaze. Manganese is an active melter, so 50% or it and a borax frit will produce a very fluid glaze at cone 6. Other metal oxides like copper and cobalt are also active fluxes and melt even better than manganese, but they want to form crystals during cooling (the micro-crystals of copper completely matte the surface). To utilize copper and cobalt a frit base of high alumina is required to make the melt stiff enough to resist crystal formation.

    Up to 80% metal oxide is sometimes used. If crystals are desired, their development can be encouraged by adding a catalyst (e.g. barium carbonate). As noted, these glazes can be very toxic to fire because of the danger of the metallic fumes. They are completely unsuitable for use on functional surfaces.

    In reduction firing it is obviously easier to produce metallic surfaces, thus much lower amounts are needed. A key reason for this is that iron, while refractory in oxidation, is an active flux in reduction. In addition, iron oxide is inexpensive whereas the other metal oxides useful for this purpose are very, very expensive. Bronze-like surfaces can also be made by the addition of rutile.


    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.

    This is a metallic silky crystal black, it is Alberta Slip plus 5% Mason 6600 black stain, 5% Mason 6666 black and 7% iron.

    Metallic deep purple by firing pure alberta slip at cone 10R, then refiring at cone 6 oxidation.

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  • Microwave Safe

    The most obvious problem almost everyone has seen is the failure of gold decorated bone china in the oven. This is because conducting metals arc and spark. Notwithstanding this, any non-porcelain has a certain percentage of iron in the clay. Iron red stonewares, for example, can contain 4-5% iron. However this metal is not in the metallic form, it exists as part of the silica crystal matrix of the ceramic. Notwithstanding this, bodies of higher iron content do heat up so common sense is needed. Of course body materials containing particles of pure metal (for specking) would be bad. Glazes also are an issue. Those of of high iron oxide content (or other metal oxide) will clearly be an issue (e.g. tenmokus).

    However ware exposed to the micro waves can also super-heat it for another reason. Ceramic bodies often fire with porosity, especially earthenwares and high temperature iron reds. These porous bodies that can absorb water into the matrix. Obviously this water is going to turn into steam inside the clay matrix. That steam will either super-heat the body or the pressure will fracture it. This is an obvious danger in the microwave, especially where the ceramic is glazed preventing quick water escape (crazed glazes, bare patches or unglazed footrings can also provide entry channels for gradual water logging of a piece).

    A common sense issue is cross section of the ware. If it is uneven (very thin in some places and thick in others) then the warming food in the container will heat it unevenly resulting in possible cracking.

    Simple common sense and testing should suffice to prove the microwave suitability of a ceramic. To test, just put a little water in a piece and try it in a microwave for 30 seconds, if it feels a lot hotter than the water then there is a problem. If that is OK, try it for a minute.


    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.

    A cone 10 reduction tenmoku glaze with about 10% iron oxide

    Fired on a porcelain in a gas kiln.

    Out Bound Links

    In Bound Links

    • (Glossary) Food Safe

      In recent years potters and small manufacturers ha...

  • Mineralogy

    In contrast to man-made materials (like frits), ceramic minerals have a highly ordered atomic structure and a specific range of crystalline manifestations. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry when recipes are mixtures of minerals and man-made materials.

    Minerals are complex, their properties are a product of their crystal structure (even the tiny particles in the powdered form are crystalline). A given chemistry can exhibit itself in more than one mineral form, each having its own crystalline structure and physical properties. Minerals can have phases or different crystalline forms and these can be converted one to another by the application of specific heating and cooling curves and exist between specific temperatures (thus certain mineral may only exist during a firing, you will never be able to hold them in your hand). The most common mineral is quartz, it can exist in a variety of forms (e.g. tridymite, cristobalite). Mica and mullite are good examples of materials used in ceramics exclusively for their mineralogy, not their chemistry. Many ceramic minerals are silicates. Minerals have specific melting temperatures and well defined events in their thermal decomposition history. Materials are mixtures of minerals and material powders are mixtures of microscopic mineral particles.

    Understanding that quartz mineral and silica glass have vastly different physical properties is often the beginnings of understanding the relationship between the mineralogy of the materials we use and their chemistry. Fused silica, for example, is one of the lowest thermal expansion materials available (0.2% at 2000F). Some industries, for example, use fused silica slabs weighing more than a ton as valves in large pipes where temperatures are not only high but suddenly change, yet these slabs do not crack. These slabs operate continuously at high temperatures, however at plant shut down when they are cooled they crystallize and must be discarded! Quartz, on the other hand, is one of the least thermal-expansion-tolerant minerals (1.5% at 2000F) and even thin sections crack very easily on sudden temperature changes. Yet both have the same SiO2 chemistry.

    Understanding minerals also involves understanding how CO2 and H2O incorporate into the crystal structure of so many minerals and how to adapt a firing process withstand expulsion or how to process the mineral to take these out and store it to keep them out.

    A good example of where a potter needs to consider mineralogy is when he is formulating a clay based engobe to apply over earthenware or low fire stoneware. The amount of quartz mineral in the body and slip needs to match fairly closely to minimize chances of the slip-body bond being compromised as the piece is cooled through quartz inversion. He also can utilize a mix of calcined and raw kaolins (two different mineral forms of the same material) to control the shrinkage properties of the slip while maintaining the fired character.

    More technical definition from Richard Willis: The crystallized aggregates of atomic elements, morphologically distinguishable by 32 possible geometrical shapes (symmetry elements and their combinations) which in turn can be grouped into six crystal systems according to the complexity of their symmetries: isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. The aggregates (elements combined forming a given mineral) are determined by chemical bonding, which can occur electrostatically, electron-sharing, metallicazation, or residualization. Bonding effects hardness, density, solubility, melting point, tenacity, specific gravity, magnetism, structural properties, colors, etc. Subsequently, minerals can be classified into 11 groups according to chemical and physical properties: native elements, sulfides, sulfosalts, oxides and hydroxides, halides, carbonates, nitrates, borates, sulfates, phosphates, and silicates.


    When both mineralogy and chemistry are shown on a data sheet

    Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.

    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).

    Out Bound Links

    In Bound Links

    • (Glossary) Glass vs. Crystalline

      In ceramic technology the term 'glass' is contrast...

    • (Glossary) Water

      There is a need to discuss water in ceramic produc...

    • (Project) Ceramic Minerals Overview

      The materials we use are powders and we assess the...

    • (Tests) TRMN - Trace Minerals
  • Mocha glazes

    Mocha diffusion is a technique of layering slips onto ware so that the edges of the upper one bleed outward into the lower. An alkali/acid mechanism is employed. The lower layer is a typical water based slip (usually white or cream) that is gelled enough to stay stable on the ware and wet long enough to apply the upper layer. Plastic slips (those high in bentonite or ball clay) work the best. The upper layer is acid-based and stained and very runny, people use a wide range acidic solutions (like tobacco or lemon juice, vinegar). When the stained acid is painted over the wet base its edges bleed outward in accordance with the fluidities and degree of difference in pH. The effect survives firing well.


    Mocca mugs by Victor Duffhues

    M370 mug using M370 clear glaze by Victor Duffhues.

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  • Mole%

    Mole% is a way of expressing the oxide formula of a fired glaze or glass (technicians can extrapolate fired properties like melting temperature, thermal expansion, hardness, resistance to leaching, etc. by examining the chemistry of a glaze). Mole% is preferred over the Seger unity formula by many technicians who use glaze or glass chemistry. Mole% is one of the calculation types in Digitalfire Insight software and

    Mole% expresses the percentage of oxide molecules by number (as opposed to an analysis which compares their weights). The Seger Unity formula and the Mole% formula are both simply retotals of a raw formula. Consider a raw formula as simply a tabulation of the total number of molecules of each oxide type in a sample. The formulas of batch recipes of different totals therefore also have different totals. Mole% is an expression of the percentage of each oxide type. A Seger Unity formula retotals such that all the fluxes total one. Thus Seger formulas have small numbers, all are less than one except for the SiO2.

    When one works with unity formulas in glaze chemistry, changes to a recipe often change only some of the numbers (or only one) in the formula. For example, if the silica weight is adjusted, only the SiO2 amount in the formula will change. However with Mole% all of the numbers change with any change in the recipe. Thus Mole% is more about seeing the concentrations of oxides in the whole than about comparing their concentrations.

    Following is an example of how to convert a raw formula to a Mole% formula. The formula is simply totaled and each number then divided by that total times 100.

               Raw                   Mole
    Oxides Formula Percent
    K2O 0.6 / 12.1 x 100 = 5.0%
    CaO 1.3 / 12.1 x 100 = 10.7
    MgO 0.2 / 12.1 x 100 = 1.7
    ZnO 0.1 / 12.1 x 100 = 0.8
    Al2O3 0.9 / 12.1 x 100 = 7.4
    SiO2 9.0 / 12.1 x 100 = 74.3
    ----- -----
    Total 12.1 100.0


    A Limitation of the Seger Unity Formula

    Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide a different system with its own relative chemistry is entered. An extreme example of this would be to source Al2O3 to a glaze using calcined alumina instead of kaolin. Although the formula may be exactly the same, the fired result would be completely different because very little of the alumina would dissolve into the glaze melt. At the opposite extreme, a different frit could be used to supply a set of oxides (while maintaining the overall chemistry of the glaze) and the fired result would be much more chemically predictable. Why? Because the readily and release their oxides the the melt.

    Out Bound Links

    • (Glossary) Unity Formula

      A "unity formula" is just a formula that has been ...

    In Bound Links

    • (Glossary) Analysis

      Conceptually we consider fired glazes as being com...

    • (Glossary) LOI

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

  • Monocottura, Monoporosa

    The single-firing process (as opposed to Bicottura which fires twice or more times) of making tile from terra cotta clay and firing it high enough to achieve a strong and dense product. Normally and engobe layer is applied over the body and glaze over that. The technique requires considerable expertise to develop a strong fired product (by virtue of crystal development in the matrix) and deal with the physical and thermal expansion matching of engobe and glaze to the body (and each other).

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  • Mosaic Tile

    More information coming on automation techniques to make it practical for potters and ceramic artists or entrepreneurs to take on projects and contracts to make architectural mosaics.


    Mosaics on the wall in the New York Subway

    White tile sections have been cut out and the mosaic inserted.

    Mosaic by Sikiu Perez

    Low temperature white talc body with bright glazes.

    Mosaic by Sikiu Perez

    Made using low fire white earthenware and glazes

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  • Mottled

    See Variegated.
  • Mullite Crystals

    These grow in the porcelain matrix during firing and are a big reason why vitrified porcelain is so strong (the fired porcelain matrix is not just a bunch of silica particles glued together with feldspar glass, it has a complex structure, mullite is a key part of that). Mullite crystals are converted from kaolin particles and are long, producing a fibrous mesh.

    In Bound Links

    • (Glossary) Fired Strength

      The fired strength of clays can be measured. The t...

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