SiO₂ (Silicon Dioxide, Silica)

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In ceramics, SiO₂ comes up when technicians talk about glaze chemistry. It is an oxide contributed by many ceramic materials: all clays, feldspars and frits. Quartz or silica powder is almost 100% SiO₂. But the SiO₂ in quartz is something completely different than SiO₂ in feldspar. In the latter it is chemically combined with Al₂O₃ and KNaO.

Thus when technicians talk about silica they might be speaking of the mineral or the oxide. Silica, as a mineral, is composed of silicon dioxide (SiO₂). In bodies SiO₂ (as quartz mineral) will almost always exist as unmelted particles embedded in the fired matrix (although finer ones dissolve into the inter-particle glass). But in glaze chemistry we are talking about silica, the oxide. All glazes that melt completely and re-solidify contain SiO₂, the oxide. Many can be 70% or more. Materials yield their SiO₂ to the glaze melt as kiln temperatures increase. Different materials dissolve into the melt at different temperatures. The particle size of materials affects the speed at which they dissolve in the melt. SiO₂ is the principle glass former in glazes. SiO₂ can bond with almost any other oxide and bring them into the glass structure.

-SiO₂ is the principle, and often only glass forming oxide in glaze. Normally comprises more than 60% of most glazes and 70% of clays. Special purpose formulations which lack SiO₂ often compromise structural stability and strength. Floating and container glass are more than 70% SiO₂.

-Adjust this in relation to fluxes to regulate melting temperature and gloss. Silica is refractory, it melts at high temperatures, but it is readily fluxed to melt lower. So its percentage regulates the glazes melting range.

-High SiO₂ in relation to Al₂O₃ produces a glossy glaze (and vice versa). This is called the silica:alumina ratio.

-Increase it at the expense of B₂O₃ to make glaze harder, more durable and brilliant. Boric oxide and silica can be interchanged to adjust glaze melting temperature.

-Decreasing SiO₂ increases the melt fluidity; increasing it raises the melting temperature, increases acid resistance, lowers expansion, increases hardness and gloss, and increases devitrification.

-It is normal to use as much as possible in any glaze to keep expansion low, to prevent crazing, increase durability and resistance to leaching and enhance body/glaze fired strength. Note, however, that in certain boracic and feldspathic compositions it can actually increase crazing so that other low expansion oxides may be needed to reduce glaze expansion.

-With boron and alumina, it has the lowest expansion of all oxides.

-In clay bodies, quartz mineral particles act as a filler and behave as an aggregate, while chemically combined SiO₂ in feldspar, kaolin, ball clay, etc., participates directly in the chemical reactions taking place to build silicate glasses. Thus the particle size of the parent material is often important in determining whether contributed silica affects the chemistry or participates simply as an aggregate in the fired matrix.

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.

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).

How much silica can some glazes accept?

G2922B is a cone 6 clear glaze that started as a well-known recipe "Perkins Studio Clear". We substituted Gerstley Borate with a frit (while maintaining the chemistry) and then noted that the glaze was highly fluid. Since I wanted to keep its thermal expansion as low as possible, I added 10% silica. 2926B shows that it is very well tolerated. Then I added 5% more (2926D) and 10% more (2926E which is still very glossy). That means that E represents a full 20% silica addition! SiO₂ has no real downsides in any well melted glossy glaze, it hardens, stabilizes and lowers expansion.

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

Low expansion glazes craze less, but they can shiver

Example of serious glaze shivering using G1215U low expansion glaze on a high silica body at cone 6. Be careful to do a thermal stress test before using a transparent glaze on functional ware.

An example where adding silica really helps a glaze

The flow on the left is an adjusted Perkins Frit Clear (we substituted frit for Gerstley Borate). It is a cone 6 transparent that appeared to work well. However it did not survive a 300F oven-to-icewater IWCT test without crazing on Plainsman M370. The amount of flow (which increases a little in the frit version) indicates that it is plenty fluid enough to accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock test and still fires absolutely crystal clear.

Silica plus modifiers matrix

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.

Crystalline and vitreous silica molecular structure

Several things are needed for high silica glazes to crystallize as they cool. First a sufficiently fluid melt in which molecules can be mobile enough to assume their preferred connections. Second, cooling slowly enough to give them time to do this. Third, the slow cooling needs to occur at the temperature at which this best happens. Silica is highly crystallizable, melts of pure silica must be cooled very quickly to prevent crystallization. But Al₂O₃, and other oxides, disrupt the silicate hexagonal structure, making the glaze more resistant to crystallization.

A high feldspar glaze is settling, running and crazing. What to do?

The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay in the recipe. Adjustment 1 (column 3 in the picture) eliminates the feldspar and sources Al₂O₃ from kaolin and KNaO from Frit 3110 (preserving the glaze's chemistry). To make that happen the amounts of other materials had to be juggled. But the fired test revealed that this one, although very similar, is melting more (because the frit releases its oxides more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition. SiO₂ is the glass former, the more a glaze will accept without losing the intended visual character, the better. The result is less running and more durability and resistance to leaching.

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.

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