Alumina
(Al2O3 , Molecular Weight 102, Expansion: 0.063, Fusion: 2040° C)
Alumina Characteristics:
Critical in glaze formulation (second only to silica), alumina stiffens the melted glaze, stabilizes and hardens the fired glaze, and lends chemical durability to fired surface. Although very refractory (heat resistant), alumina helps form strong chemical links between the fluxes and silica, improving the melt. That is to say, increasing the alumina in a glaze can actually lower the melting point of the glaze. For this reason, alumina is known as an intermediate oxide.
The amount of alumina helps determine maturing range of glaze, and the ratio of alumina to silica is primarily responsible for the degree of matteness or glossiness in glazes. In glazes without boron, silica to alumina ratios lower than 5:1 are dry mattes, while ratios higher than 8:1 tend to be glossy. Over-supply of alumina in a glaze can cause crawling. Large amounts of alumina give rise to micro crystals growing in the cooling magma and matte glazes result. Interestingly, alumina also retards crystallization, preventing the molecules in the glaze from rearranging themselves into crystals upon cooling, earning it the term stabilizer oxide. Alumina Hydrate added to a glaze can act as an opacifier by producing gas bubbles and by remaining as unmelted particles.
Color response:
Alumina has little effect on most colors. Small amounts can produce a pink with chrome manganese and cobalt, and higher amounts discourage copper red. Cobalt, in the absence of alumina, can fire to a pinkish hue.
Sources of Alumina:
Alumina Hydrate
Al2O3•2H2O , Molecular weight 156
Alumina hydrate (bauxite), the raw ore from which the metal aluminum is smelted, is an expensive source of pure alumina for glazes. It is less easily melted than the alumina in either feldspars or kaolin, and thus must be used as a very fine powder. It stays in suspension better than calcined alumina, and offers some advantages over that material, though gram for gram Alumina Hydrate supplies less alumina than the calcined form due to its included water.
Alumina, Calcined
Al2O3 , Molecular weight 102, Melting Point, 2040 C
Calcined alumina is made by heating alumina hydrate to form pure alumina oxide. This is the most dense form of the oxide, and thus is difficult to keep in suspension. However, 325 mesh calcined alumina may be substituted for flint as a filler in porcelain bodies. It reduces drying shrinkage, increases thixotropic qualities, provides firing strength in the kiln, strengthens fired work, and whitens the body. It also lowers thermal expansion in clay bodies, and so can increase crazing of glazes on bodies with alumina substituted for silica.
In a glaze, 400 mesh calcined alumina and flint may be substituted for some of the kaolin component in order to cut drying shrinkage and crawling. Calcining a portion of the kaolin will have the same effect.
Ambligonite
Li2O•AlF•PO4 , molecular weight 296
Ambligonite is most useful in glazes at higher temperatures due to its high alumina content, and it can be found in some Shino-type glazes for this reason (Shino glazes are high in alumina). Fluorine and phosphorous out-gassing can cause blistering in reduction firings if the glaze melts early (a problem in Shino-type glazes!).
Cryolite
Na3•AlF6 , Molecular weight 140
Infrequently used except for special effect glazes due to fluorine out-gassing during the firing, cryolite can be a valuable source of sodium and alumina.
Kaolin
Ideally Al2O3•2SiO2•2H2O, Molecular Weight 258
Kaolin is an excellent source of both alumina and silica in glazes and is typically supplied by EPK or OM-4 ball clay. Kaolin, especially EPK, helps suspend the other glaze materials in the bucket.
Pyrophillite (Al2O3•4SiO2•H2O, Molecular weight 218)
Pyrophillite carries no advantage over kaolin in a glaze, but substituted for half of the flint in a porcelain body, pyrophillite lowers drying shrinkage, increases hot strength, lowers thermal expansion, and increases whiteness. It also lowers the thermal expansion of the body and thus can cause glazes to craze more.
Feldspars (various formulae, see feldspar articles)
Feldspars all are excellent sources of silica, alumina, and different flux oxides and are the basis for most high temperature glazes.
Bibliography
This article taken wholly from
Grimmer, Stephen. “A Short Course on Glaze Chemistry for Ceramic Artists.” Winnipeg: 2009.
References Cited
Currie, Ian. Revealing Glazes Using the Grid Method. Maryvale, Queensland, Australia: Bootstrap Press, 2000.
Currie, Ian. Stoneware Glazes : A Systematic Approach. Maryvale, Queensland, Australia: Bootstrap Press, 1986.
Hamer, Frank and Hamer, Janet. The Potter's Dictionary of Materials and Techniques. Philadelphia: University of Pennsylvania Press, 19??.
Hansen, Tony. The Magic of Fire Reference. Medicine Hat, Alberta, Canada: Digital Fire Corporation, 1998.
Rhodes, Daniel. Clay and Glazes for the Potter. New York: Chiltons, 1973.
