Ceramic Oxides

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Al2O3 (Aluminum Oxide, Alumina)

COLE - Co-efficient of Linear Expansion 0.063


Alumina exists is two forms: In glazes as unmelted crystalline particles floating in the glaze melt (because alumina is very refractory) or chemically combined with SiO2 and fluxes in the glass. In bodies it will almost always exist as unmelted particles, although some may dissolve into the inter-particle glass. Al2O3 in kaolin is chemically combined whereas the Al2O3 in alumina hydrate is a crystalline solid. Generally, when we are talking about the chemically combined form, we refer to it as Al2O3. However, the term 'alumina' is pretty universal so you need to determine what is being referred to by the context. When we talk about ceramic chemistry we are always referring to the Al2O3 form.

-While alumina has a reputation of 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 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.

-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 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.



If your glaze can handle more silica and melt just as well then add it!

The cone 6 G1214M glaze on the left melts well. Can it benefit from a silica addition? Yes. The right adds 20% yet still melts as well, covers better, is more glossy, more resistant to leaching, harder and has a lower thermal expansion.

Which one contains more SiO2?

These cone 04 glazes both have 50% Gerstley Borate. The other 50% in the one on the left is PV Clay, a very low melting plastic feldspar. On the right, the other 50% is silica and kaolin, both very refractory materials. Yet the glaze on the right is melting far better. How is that possible? Likely because the silica and kaolin are supplying Al2O3 and SiO2, exactly the oxides that Gerstley Borate needs to form a good glass.

These two frits have one difference in the chemistry. What?

These two boron frits (Ferro 3124 left, 3134 right) have almost the same chemistry. But there is one difference: The one on the right has no Al2O3, the one on the left has 10%. Alumina plays an important role (as an oxide that builds the glass) in stiffening the melt, giving it body and lowering its thermal expansion, you can see that in the way these flow when melting at 1800F. The frit on the right is invaluable where the glaze needs clay to suspend it (because the clay can supply the Al2O3). The frit on the left is better when the glaze already has plenty of clay, so it supplies the Al2O3. Of course, you need to be able to do the chemistry to figure out how to substitute these for each other because it involves changing the silica and kaolin amounts in the recipe also.

Ceramic Oxide Periodic Table in SVG Format

The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.

What happens when glazes lack Al2O3?

This happens. They are glossy, but lack thickness and body. They are also prone to boron blue clouding (micro crystallization that occurs because low alumina melts crystallize much more readily on cooling). Another problem is lack of resistance to wear and to leaching (sufficient Al2O3 in the chemistry is essential to producing a strong and durable glass). This is a good example of the need to see a glaze not just as a recipe but as a chemical formula of oxides. The latter view enables us to compare it with other common recipes and the very low Al2O3 is immediately evident. Another problem: Low clay content (this has only 7.5% kaolin) creates a slurry that is difficult to use and quickly settles hard in the bucket.

Out Bound Links

  • (URLs) More technical info from Azom
  • (Materials - Material source) Kaolin - Al2O3.2SiO2 or Al2Si2O5(OH)4 - Hydrated alumina silicate, Pure clay mineral

    China Clay

  • (Materials - Material source) Feldspar
  • (Materials - Closest material equivalent) Calcined Alumina - Al2O3 - Aluminum oxide

    Alumina Calcined, Calcnd Alum, Ground Alumina, Corundum

  • (Materials - Material source) Alumina Hydrate - Al2O3.3H2O

    Alum Hydrate, Alumina Trihydrate, Hydrated Alumina, Alpha Aluminum Trihydroxide, Gibbsite

  • (Materials - Material source) Ball Clay - Highly Plastic Fine Particle Clay
  • (Oxides - Related) SiO2 - Silicon Dioxide, Silica

In Bound Links

By Tony Hansen

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