Majolica, tin glaze
earthenwareLow 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.Matte GlazeA 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
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Wollastonite - CaSiO3 - Calcium Silicate
Wolastonite
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Zinc Oxide - ZnO - Pure Source Of Zinc
ZnO, Zincite
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Light Magnesium Carbonate - Mg5(CO3)4(OH)2.4H2O
Hydrated Magesium Carbonate Mineral, Hydromagnesite
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MatureA 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.Medium Temperature or Mid-Fire GlazeIn 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.Out Bound Links
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Melting TemperatureUnlike 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.Out Bound Links
Pictures
Example of how a frit softens over a wide temperature range
Metallic or Bronze GlazesMetallic glazes can be produced by using very high percentages of metal oxides (like copper carbonate, manganese dioxide) along with a fluid melter (e.g. a lead bisilicate frit). Up to 80% metal oxide is sometimes used. To encourage the development of the metallic crystals it can be helpful to add a catalyst (e.g. barium carbonate). These glazes can be very toxic to fire (metal fumes can be very dangerous). They are also completely unsuitable for use on functional ware.Out Bound Links
MineralCeramic 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
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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 glazesMocha 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).Monocottura, MonoporosaThe 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). Out Bound Links
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MottledSee Variegated. |
