|Co-efficient of Linear Expansion
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).
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.
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 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.
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).
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).
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.
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).
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.
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.
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.
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.
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.
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.
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.
G1214Z Cone 6 matte 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.
More technical info from Azom
The purest of all clays in nature. Kaolins are used in porcelains and stonewares to impart whiteness, in glazes to supply Al2O3 and to suspend slurries.
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.
A fine particled highly plastic secondary clay used mainly to impart plasticity to clay and porcelain bodies and to suspend glaze, slips and engobe slurries.
A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Boron blue is a glaze fault involving the crystallization of calcium, boron and silicate compounds. It can be solved using ceramic chemistry.
A densification process occurring within a ceramic kiln. With increasing temperatures particles pack tighter and tighter together, bonding more and more into a stronger and stronger matrix.
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
|SiO2 - Silicon Dioxide, Silica
Predicting Glaze Durability by Chemistry in Insight-Live
How to spot out-of-balance indicators in the chemistry of ceramic glazes that suggest susceptibility to scratching or cutlery marking.
Runny Ceramic Glazes
Glazes of high melt fluidity are likely to run if applied to thickly or have not catcher glaze
|Alumina is used in combination with chrome, manganese, and cobalt to achieve pink colors.
|Cobalt depends on the presence of alumina or it will fire pinkish. Chrome reds like alumina also.
|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.
|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.
|By Tony Hansen
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