SiO2 (Silicon Dioxide, Silica)
NotesSilica exists is two forms: In glazes as unmelted crystalline particles floating in the glaze melt (can happen if the particle size is not fine enough) or chemically combined with Al2O3 and fluxes in the glass. In bodies it will almost always exist as unmelted quartz particles, although finer ones dissolve into the inter-particle glass. SiO2 in feldspar is chemically combined whereas the SiO2 in quartz is a crystalline solid. Generally, when we are talking about the chemically combined form, we refer to it as SiO2. However, the term 'silica' 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 SiO2.
-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. High SiO2 in relation to Al2O3 produces a glossy glaze (and vice versa).
-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, 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.
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