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  • Majolica, tin glaze earthenware

    Low fired pottery employing a red-burning clay covered with a soft opaque white glaze. Most majolica also has colored brushwork designs which are painted over the dried glaze. The Majolica process is exacting and requires careful technique and good technical understanding to make it successful. Metallic colors are brightest at low temperatures and stiff-melt white glazes provide an ideal canvas for them.
    The Magic of Fire book has more information on the Majolica process.

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    • (Glossary) Earthenware

      A clay fired at low temperatures (cone 010-02) whe...

  • 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 and clean well. The mechanism of typical matte glazes is a micro crystalline surface (high CaO glazes, for example, form crystals when cooling) or a wavy (non flat) surface that scatters light (high Al2O3 is the classic way to produce this effect, the alumina stiffens the melt preventing level-out). High temperature talc and dolomite glazes also create this effect because the MgO creates multiple phases in the melt that have different fluidity and refractive indexes). The latter is sometimes called a 'silky matte' and is more pleasant to the touch. At middle and low temperatures matte glazes are more difficult to formulate than gloss glazes, there is often a narrow window of chemistries (and particular firing methods are sometimes needed). Some materials act as matting agents and can work in both of the above ways. They may stiffen the glaze melt and prevent it from leveling completely during cooling (e.g. alumina or magnesium carbonate or even calcium carbonate at lower temperatures can do this). Other materials, especially those with high melting temperatures, can seed crystals (giving them a place to start). A good example is the formation of a calcium silicate matte with the addition of wollastonite (calcium silicate). Zircon materials and tin are other examples. Other materials will crystallize well if oversupplied (e.g. ZnO).

    Out Bound Links

    • (Library) Creating a Matte Glaze

      This chapter in the book (and matching video at di...

    • (Glossary) Cutlery Marking

      In glazes with this fault rubbing a metal knife or...

    • (Materials) Dolomite - CaCO3.MgCO3 or CaMg(CO3)2 - Double carbonate of magnesia/calcia

      Calcium Magnesium Carbonate, Raw Limestone

    • (Materials) Wollastonite - CaSiO3 - Calcium Silicate

      Wolastonite

    • (Materials) Zinc Oxide - ZnO - Pure Source Of Zinc

      ZnO, Zincite

    • (Materials) Light Magnesium Carbonate - Mg5(CO3)4(OH)2.4H2O

      Hydrated Magnesium Carbonate Mineral, Hydromagnesite, Magnesium Carbonate Light

    In Bound Links

  • Mature

    A term referring to the degree to which a clay or glaze has vitrified or melted in the kiln. A 'mature' stoneware or porcelain clay is normally one that is dense and strong, a 'mature' glaze flows well and heals imperfections to provide a good covering. Like the term 'vitrification' mature needs to be taken in context. A mature sintered refractory, for example is quite porous and would be considered immature for other uses.
    Pictures
    Fired test bars of a terra cotta clay showing varying levels of maturity or vitrification, DFAC disk showing solubles on an iron stoneware


  • 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 really have a melting temperature, they generally soften over a range. The reason they soften can be two fold. First, raw glazes contain particles of many types, each having its own melting behavior. Fluxes melt first, perhaps suddenly, then they dissolve other particles slowly until the entire mass is melted. Most fritted glazes normally melt more slowly simply because frits are pre-melted, quenched in water and ground (by definition, glasses soften or melt slowly).

    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. This is especially the case with melting temperature prediction.

    However ceramic calculations work well as a relative science. INSIGHT's dual recipe functionality makes it a natural for studying one recipe in relation to another with respect to maturing temperature, expansion, etc. Technicians change the chemistry of a recipe according to a knowledge of what direction the change should take the desired property. Then they relate fired results back to the chemical change and build understanding to use for subsequent fine tuning. It is common to develop prediction skills within specific 'oxide systems'. We teach people the interpretation skills they need to do this. Digitalfire is very hesitant to build temperature prediction into INSIGHT for fear it would make us appear in any way naive about this.

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    Pictures
    Example of how a frit softens over a wide temperature range


  • 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.

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    Pictures
    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% black stain and 7% iron.


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


  • Microwave Safe

    This is a term relating to the ability of a ceramic to resist fracture and super-heating during exposure to the micro waves. Porous bodies that can absorb water into the matrix which cannot quickly escape as steam are an obvious danger, especially where the ceramic is glazed (crazed glazes, unglazed footrings can provide channels for gradual water logging of a piece). Another obvious factor is the avoidance of body materials containing particles of iron (or high iron minerals) or red burning bodies simply having a high iron content. Of course, the same goes for other metallics. Glazes of high iron oxide powder content (or other metal oxide) could also be an issue. In addition, ware should be of even cross section and not overly thick. Simple common sense and testing will suffice to prove the 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.

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    • (Articles)

      Is Your Fired Ware Safe?

      Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it ...

  • Mineral

    Ceramic minerals have a highly ordered atomic structure and a specific range of crystalline manifestations. 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.

    More comprehensive 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.

    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 applying slips to ware so that one bleeds or diffuses into another. Typically oxides are mixed with tobacco juice and vinegar (e.g. apple cider works well) and a brush of the mix is touched to the surface of a coat of WET and freshly applied slip (i.e. Universal white slip).

  • Mole%

    The Mole Percent (Mole%) calculation type has become popular because it provides room to rationalize oxide identity, interplay, concentration, and firing temperature. The Seger unity model does not work as well at lower temperatures. Some oxides that are powerful fluxes at high temperatures are refractory in low fire. Dynamic reassignment of oxides to the Seger groups by temperature is not practical at this time. Oxides have a much more individual presence than the Seger method tends to recognize. Their contributions to particular properties often are not linear according to concentration. Thus a more complex understanding of concentration vs. effect is needed. Oxide interplay producing characteristics attributable to the group is not recognized by the Seger system. Boron is both a glass and a flux and the logic for its employment at various temperature ranges differs. It does not 'plug into' a Seger formula very well. Mole% is simply a calculation of the percentage of oxide molecules by number (as opposed to an analysis which compares their weights). Following is an example of how to convert a raw formula to a Mole% formula.
               Raw                   Percent
    
Oxides     Formula               Analysis
    
-----------------------------------------
    
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
    
-----------------------------------------


    Mole% ignores LOI as do formulas, it just looks at the oxides that makeup the fired glass. The INSIGHT Advisor dialog contains a few examples of target formulas from Richard Eppler and references are based on Mole%. These will give you a feel for how the system is used.

    Out Bound Links

    • (Glossary) Unity Formula

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

    In Bound Links

    • (Glossary) Analysis

      An analysis (or percentage analysis) is typically ...

    • (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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  • Mottled

    See Variegated.


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