SiO2 (Silicon Dioxide, Silica)
NotesIn 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.
-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 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.
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.
Severely 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.
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.
Same glaze. One crystallizes and the other does not. Why?
This is cone 6 oxidation high iron (9%) high boron glossy glaze slow cooled (right) and free-fall cooled (left). Tthe iron silicate crystals invade the surface when they get the opportunity. That opportunity is time and the melt fluidity of the glaze. Each glaze has a temperature at which crystals form the best and that temperature can be quite a bit lower than you might expect (hundreds of degrees down from the firing cone). Many experimental firings are needed to best find and exploit it (or avoid slow-cooling through it).
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 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.
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