Al₂O₃ (Aluminum Oxide, Alumina)

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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).

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: Al2O3.

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

All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).

A functional matte cone 6 glaze should melt as well as a glossy

True functional mattes have fluid melts, like glossy glazes. They need this in order to develop a hard, non-scratching durable glass. The mechanism of the G1214Z1 matte on the right is high Al₂O₃, it is actually melting more than the glossy glaze on the left (G1214W).

Phase diagram of a SiO2:Al2O3:CaO:KNaO System

Phase diagram and stull chart showing the SiO₂-Al₂O₃-(0.7CaO+0.3KNaO) system. Courtesy of Matthew Katz, Alfred University

What happens when ceramic 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.

Cutlery marking here is directly related to the chemistry of the glaze

This is an example of cutlery marking in a cone 10 silky matte glaze lacking Al₂O₃, SiO₂ and having too much MgO. Al₂O₃-deficient glazes often have high melt fluidity and run during firing, this freezes to a glass that lacks durability and hardness. But sufficient MgO levels can stabilize the melt and produce a glaze that appears stable but is not. Glazes need sufficient Al₂O₃ (and SiO₂) to develop hardness and durability. Only after viewing the chemistry of this glaze did the cause for the marking become evident. This is an excellent demonstration of how imbalance in chemistry has real consequences. It is certainly possible to make a dolomite matte high temperature glaze that will not do this (G2571A is an example, it has lower MgO and higher Al₂O₃ and produces the same pleasant matte surface).

Crawling sanitary ware glaze sourcing Al2O3 from only feldspar

The original recipe had a very low clay content, sourcing almost all of its Al₂O₃ from feldspar instead. Although the glaze slurry was maintained at 1.78 specific gravity (an incredibly high value) and thus would have had very low shrinkage, it did not stick and harden well enough to the ware. Why? Lack of clay content in the glaze. The fix was to source much more of the Al₂O₃ from kaolin instead of feldspar. The reduction in feldspar shorted the glaze on KNaO and SiO₂ so these were sourced from a frit and pure silica instead (the calculations to do this were done in Insight-live.com). The change also provided opportunity to substitute some of the KNaO with lower expansion CaO. This reduced the thermal expansion and reduced crazing issues.

Example of a whole rock chemical analysis lab report

Powdered samples were sent to the lab. The numbers shown on this report are in percentage-by-weight. That means, for example, that 15.21% of the weight of the dry powder of Alberta Slip is Al₂O₃. Insight-live knows material chemistries in this way (whereas desktop Insight needs them as formulas). Some non-oxide elements are quantified as parts-per-million (these amounts are not normally high enough to take into account for traditional ceramic purposes). The LOI column shows how much mechanically and chemically bound water are gassed off during firing of the sample. The total is not exactly 100 because of inherent error in the method and compounds not included in the report.

Al2O3 in glazes make them durable and wear resistant

The cone 6 glazes on the left have double the boron of those on the right so they should be melting much more. But they flow less because they have much higher Al₂O₃ and SiO₂ contents. This effect renders them milky white vs. the transparent of those on the right. Why? Because G and H are trapping micro-bubbles because of the increased viscosity of the melt. In spite of this, the two on the left do fire almost transparent when applied to ware, they have enough fluidity to shed most of the bubbles when in a thin layer. The ones on the right are too fluid, they will run excessively on ware unless applied thinly. The sweet-spot is a little more fluidity than those on the left. But there is another very important factor: Durability. The increased Al₂O₃ in G and H make them fire harder, more resistant to abrasion. The added SiO₂ adds resistance to leaching.

Severe cutlery marking in a glaze lacking sufficient Al2O3

The glaze is cutlery marking (therefore lacking hardness). Why? Notice how severely it runs on a flow tester (even melting out holes in a firebrick). Yet it does not run on the cups when fired at the same temperature (cone 10)! Glazes run like this when they lack Al₂O₃ (and SiO₂). The SiO₂ is the glass builder and the Al₂O₃ gives the melt body and stability. More important, Al₂O₃ imparts hardness and durability to the fired glass. No wonder it is cutlery marking. Will it also leach? Very likely. That is why adequate silica is very important, it makes up more than 60% of most glazes. SiO₂ is the key glass builder and it forms networks with all the other oxides.

Same glaze, same kiln, same clay: The right one crystallized. Why?

Well, actually they are not exactly the same. This is 80% Alberta Slip and 20% frit. But the frit on the left is Ferro 3195 and on the right is 3134. By comparing the calculated chemistry for these two we can say that the likely reason for the difference is the Al₂O₃ content. Frit 3134 has almost none whereas 3195 has 12%. Al₂O₃ stiffens the glaze melt, that impedes crystal growth. And it stabilizes the melt against running during firing. Frit 3195 is thus much more "like a glaze" than is 3134, it is what Alberta Slip needs to melt as a transparent glass under normally cooling in the kiln.

Low fire glaze with the Al2O3 and SiO2 of a cone 6 glaze

Look at how fluid G3879 is at cone 06 even though it has the Al₂O₃ and SiO₂ of a cone 6 (or even cone 10 glaze)! It have found that glazes with lots of boron can tolerate amazingly high levels of Al₂O₃ and SiO₂ and still melt very well. And they create many options to lower thermal expansion that would not otherwise be available. The G3806N recipe has the amazing ability to tolerate large additions of kaolin. Each addition sacrifices some melt fluidity but the glaze stays glossy and gets more durable (because of the increased Al₂O₃ and SiO₂). And the thermal expansion drops even more. A highly melt fluid, super gloss with low thermal expansion is super difficult at cone 6, but here it is. The secret is high boron. From frits.

Why does the inside glaze crystallize with one frit and not the other?

It is about the oxide chemistry, as shown calculated below the recipes in my account at insight-live.com. These glazes are fired at cone 6 using the C6DHSC schedule (we are focussing on the amber glossy glaze on the insides of the mugs). Most oxides want to form silicate crystals (combine with SiO₂) as the glaze cools (if the cooling ramp is slow enough), iron oxide is not the least of these. Alumina (Al₂O₃) stabilizes the melt, that means it helps the melt to solidify as a glassy solid, not a crystalline one (thus, it does not devitrify). Notice the two Al₂O₃ values (black-on-red numbers): The glaze on the right has much less. That is because Ferro Frit 3134 contains almost no Al₂O₃ (notice in the blue panel, only 2%). The alumina in the glaze on the left, sourced more abundantly by Frit 3195, readily releases in the melt, ready to take on its job: Stiffen it and impede the formation of iron silicate crystals during cooling to create a better glass.

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