Common: Al2O3, B2O3, BaO, C, CaO, CO2, CoO, Cr2O3, Cu2O, CuO, Fe2O3, FeO, H2O, K2O, Li2O, LOI, MgO, MnO, MnO2, Na2O, NiO, O, Organics, P2O5, PbO, SiO2, SnO2, SO3, SO4, SrO, TiO2, V2O5, ZnO, ZrO, ZrO2, Others

Al₂O₃ (Aluminum Oxide, Alumina)

Family:	Intermediate
Weight:	102.000
Expansion:	0.063
Fusion:	2040C

(Sources: Kaolin, Clays, Feldspar, Calcined Alumina, Alumina Hydrate)

-Alumina has a very high melting temperature and alumina ceramics can maintain up to 90% of their strength above 1100C. They are thus employed in many refractory materials (i.e. Calcium Aluminate Cements have PCEs above cone 35) and used to make parts that must withstand high temperature.

-Alumina controls the flow of the glaze melt, preventing it from running off the ware. It is thus called an intermediate oxide because it helps build strong chemical links between fluxes and silica.

-Fired alumina ceramic parts can be harder than tungsten carbide or zircon, two to four times as strong as electrical porcelain, and very resistant to abrasion. Alumina is thus used in grinding media, cutting tools, high temperature bearings, and a wide variety of mechanical parts.

-Alumina is second in importance to silica and combines with silica and basic fluxing oxides to prevent crystallization and give body and chemical stability to a glaze.

-It is the prime source of durability in glazes. It increases melting temperature, improves tensile strength, lowers expansion, and adds hardness and resistance to chemical attack. If a glaze contains too much alumina, then it may not melt enough but will likely be more hard and durable if firing temperature is increased. If a glaze has inadequate alumina, then it is likely that it will lack hardness and strength at any temperature.

-Increasing Al2O3 stiffens the melt and gives it stability over a wider range of temperatures (although excessive amounts may tend to cause crawling, pinholes, rough surfaces). The addition of alumina prevents devitrification (crystallization) of glazes during cooling because the stiffer melt resists free movement of molecules to form crystalline structures. Thus crystalline glazes tend to have less than .1 molar equivalents of Al2O3. The addition of small amounts of CaO will help reduce the viscosity of a melt and make it flow more freely.

-Calcined alumina does not work well in glazes or enamels as a source of Al2O3; however, the hydrated form can be effective to matte a glaze if it has a very fine particle size. If possible, kaolin or feldspar (and nepheline syenite) are the best sources. Kaolin especially is ideal because it is so important to other physical slurry properties (i.e. suspension, adhesion, and shrinkage control). If glaze batches are being calculated from a source formula, it is normal to supply all possible alumina from feldspar and kaolin until the alkali targets are met, then furnish any additional alumina requirements with Bayer process alumina hydrate. Sometimes Bayer alumina is added where exceptional freedom from iron is needed.

-In most cases, the addition of alumina raises the melting temperature of a glaze or glass. However, in some soda lime formulations, a small alumina addition can decrease melting temperature.

-In glass, small amounts can reduce the coefficient of expansion, increase tensile strength and surface tension, improve luster, lengthen working range, decrease devitrification, increase resistance to acid attack. When substituting for silica, alumina makes the glass more ductile and elastic.

-Alumina and boric acid are important constituents in all types of low expansion glasses for chemical ware, cooking, and thermometers.

-Alumina (preferably in the calcined form) can be used in clay bodies as an aggregate and filler in place of flint. This can increase the firing range, decrease quartz inversion firing problems, and increase hardness and whiteness in the fired body. However, alumina is much more expensive than flint.

-Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.

Properties

Glaze Color - Pink
Alumina is used in combination with chrome, manganese, and cobalt to achieve pink colors.
Glaze Color - Colbalt Blue
Cobalt depends on the presence of alumina or it will fire pinkish. Chrome reds like alumina also.
Surface Modifier - Crystalization
Since Alumina stiffens the glaze melt, it will prevent the growth of crystals during cooling because it is more difficult for the specific oxides needed to form the crystal, to travel to the site of formation. Thus most highly crystalline glazes have very little alumina.
Surface Modifier - Matte
The ratio of silica to alumina is mainly responsible for the degree of matteness in glazes. In the absence of boron, ratios of less than 5:1 are generally quite matte; ratios of greater than 8:1 are usually glossy in the absence of high titania, zinc, magnesia, or calcia (which cause volatile melting or crystallization during freezing). Ratios of 1:18 are possible, but certainly not typical. If a glaze remains matte when fired higher, it is a true alumina matte.

Linked Articles

G1214Z Cone 6 Matte Base Glaze
This glaze was developed using the 1214W glossy as a starting point. This article overviews the types of matte glazes and rationalizes the method used to make this one.