|Monthly Tech-Tip |
Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
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. Silica is an excellent example. Quartz is the common mineral-form of silica. It is one of the least thermal-expansion-tolerant minerals (1.5% at 2000F). So, how it is that adding quartz powder to glazes does the opposite, reducing their thermal expansion? It is because it dissolves in the melt to become a glass, it is no longer crystalline. Silica glass has an exceptionally low thermal expansion. Fused silica, for example, is made by melting quartz and suddenly cooling it so quickly that it does not get a chance to crystallize. This produces one of the lowest thermal expansion materials available (0.2% at 2000F). Some industries 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. But they have a severe limitation: At plant shut-downs they must be replaced. Why? Because as they cool they recrystallize, turning back into quartz, completely losing their low thermal expansion! The point here is that quartz and silica glass are the same chemically, SiO2. The cooled slabs look the same. But mineralogically there are very different.
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.
10 grams GBMF test 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).
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.
List of Minerals at Wikipedia
Polarized Light Microscopy Gallery
Mineral SEM gallery page at The Mineralogical Society website
Mineral at Wikipedia
Minerals at Wikipedia
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Water in Ceramics
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