RAW MATERIALS AND OXIDES
RAW MATERIALS AND OXIDES
RAW MATERIALS AND OXIDES

REFSAN ALUMINIUM OXIDE (5407)

Brand : REFSAN
Price : 6,05TL
ÇOKALAZÖDE BOYA

Aluminum Oxide EFT 180 :  53-90 mic

Chemical Analysis 

Al2O3 : 99,81 %

SiO2 : 0,02 %

Fe2O3 : 0,035 %

TiO2 : 0,004 %

Na2O : 0,11 %

CaO+MgO : 0,020 %

Physical Properties

Density : 3,94 g/cm3

Hardness : 9 Mohs

Melting Point : 1950°C

ALUMINUM OXIDE

In ceramics, Al2O3 comes up when technicians talk about glaze chemistry. It is an oxide mostly contributed by clays, feldspars and frits. As glazes melt oxides are liberated from materials and they form a glass structure. Al2O3 is very important in that structure, mainly imparting stability to the melt and durability to the fired glass. Almost all glazes have significant Al2O3 (second only to SiO2).

Al2O3 in kaolin or feldspar is chemically combined with SiO2 and is readily dissolved into glaze melts. However the Al2O3 in alumina hydrate or calcined alumina is a crystalline solid (these materials are very refractory and sintered into a multitude of hi-tech ceramic products). Thus, alumina, as a material, is not a good source of Al2O3 to glaze melts, it does not readily melt and yield the oxides. In bodies it will almost always exist as unmelted particles (although some very small particles could dissolve into the inter-particle feldspar glass).

Thus, when we refer to alumina, the context must be considered to determine if the reference is to Al2O3, the oxide, or alumina, the material.

-Strangely, window and container glasses have only tiny percentages of Al2O3. A durable glass forms having the simple chemistry 10% CaO, 13% Na2O and 75% SiO2. The way in which the glass is manufactured into products allows for the low Al2O3. But if glass cullet (powdered glass) is attempted as a ceramic glaze it runs and crazes very badly.

-While alumina has a reputation for being super refractory, other pure oxides like CaO and MgO actually melt much higher! But the difference is that when alumina particles are combined with those of other oxides it maintains its refractory character while the others interact and become fluxes.

-Al2O3 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 SiO2. When Al2O3 bonds with SiO2 (via a shared oxygen atom) it becomes an integral part of the silicon matrix (and thereby does not affect the transparency of a glass).

-Al2O3 is second in importance to silica and combines with SiO2 and basic fluxing oxides to prevent crystallization (provided CaO is not too high) and give body to the glaze melt and chemical stability to the frozen glass.

-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 Al2O3 , then it may not melt enough (but will likely be more hard and durable if firing temperature is increased). If a glaze has inadequate Al2O3 , 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 Al2O3 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.

-As noted, calcined alumina powder does not work well in glazes or enamels as a source of Al2O3, it just does not dissolve into the melt unless exceedingly fine and in low percentages. However, the hydrated form can be effective to matte a glaze if (it has a very fine particle size). If possible, kaolin, pyrophyllite or feldspar (and nepheline syenite) are the best sources of Al2O3 for glass building. Kaolin especially is ideal as a source 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 until the alkali targets are met, then topped up with kaolin. If there are any additional Al2O3 requirements Bayer process alumina hydrate can be employed (but this is very rarely needed). Sometimes Bayer alumina is added in preference to kaolin where exceptional freedom from iron is needed.

-In most cases, the addition of Al2O3, as an oxide in the chemistry, raises the melting temperature of a glaze or glass. However, in some soda lime formulations, a small Al2O3 addition can actually 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.

-The ratio of SiO2 to Al2O3 is often referred to as an indicator of glaze matteness (low ratios are more matte). However if there are any other glasses (like B2O3) these have to be rationalized into the prediction. There is an assumption that the glaze is well melted for this to be applicable. Often the ratio must be quite low (glazes glazes generally want to be glossy if well melted and not slow cooled).

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

-The presence of alumina in silicate glass reduces phase separation.

-There is a case where higher alumina content can actually encourage crystal growth (Anorthite CaO.Al2O3.2SiO2). In high gloss, fast fire glazes (where CaO is often abundant), alumina content must be optimized: high enough to prevent phase separation and impart its other beneficial properties, but low enough to prevent the growth of the crystals (see article on gloss glazes).