Silicon dioxide


Co-efficient of Linear ExpansionFrit Softening PointDry M.O.R. (50% Silica)
1720C (From The Oxide Handbook)

In ceramics, SiO2 comes up when technicians talk about gtia laze chemistry. It is an oxide contributed by many ceramic materials: all clays, feldspars và 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 & KNaO.Thus when technicians talk about silica they might be speaking of the mineral or the oxide. Silica, as a mineral, is composed of silinhỏ dioxide (SiO2). In bodies SiO2 (as quartz mineral) will almost always exist as unmelted particles embedded in the fired matrix (although finer ones dissolve sầu inkhổng lồ the inter-particle glass). But in gLaser chemistry we are talking about silica, the oxide. All glazes that melt completely & re-solidify contain SiO2, the oxide. Many can be 70% or more. Materials yield their SiO2 lớn the gtia laze melt as kiln temperatures increase. Different materials dissolve inkhổng lồ the melt at different temperatures. The particle form 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 inkhổng lồ the glass structure.-SiO2 is the principle, and often only glass forming oxide in glaze. Normally comprises more than 60% of most glazes & 70% of clays. Special purpose formulations which laông chồng SiO2 often compromise structural stability and strength. Floating and container glass are more than 70% SiO2.-Adjust this in relation lớn fluxes lớn regulate melting temperature and gloss. Silica is refractory, it melts at high temperatures, but it is readily fluxed to lớn melt lower. So its percentage regulates the glazes melting range.-High SiO2 in relation to lớn Al2O3 produces a glossy gLaser (& vice versa). This is called the silica:alumimãng cầu ratio.-Increase it at the expense of B2O3 to make glaze harder, more durable and brilliant. Boric oxide and silica can be interchanged lớn adjust gtia laze melting temperature.-Decreasing SiO2 increases the melt fluidity; increasing it raises the melting temperature, increases acid resistance, lowers expansion, increases hardness and gloss, và increases devitrification.-It is normal lớn use as much as possible in any gLaser khổng lồ keep expansion low, lớn prevent crazing, increase durability & resistance to lớn leaching and enhance body/gLaser fired strength. chú ý, however, that in certain boracic and feldspathic compositions it can actually increase crazing so that other low expansion oxides may be needed lớn reduce gLaser expansion.-With boron và alumimãng cầu, it has the lowest expansion of all oxides.-In clay bodies, quartz mineral particles act as a filler và 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 kích thước 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.

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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

Severely cutlery marking in a gtia laze 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 lượt thích this when they laông chồng Al2O3 (& SiO2). The SiO2 is the glass builder và the Al2O3 gives the melt body toàn thân and stability. More important, Al2O3 imparts hardness and durability khổng lồ 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 và it forms networks with all the other oxides.

Crystalline & vitreous silica molecular structure


Several things are needed for high silica glazes lớn crystallize as they cool. First a sufficiently fluid melt in which molecules can be di động enough to assume their preferred connections. Second, cooling slowly enough khổng lồ give sầu 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 lớn prsự kiện crystallization. But Al2O3, & other oxides, disrupt the silicate hexagonal structure, making the gLaser more resistant khổng lồ crystallization.

Low expansion glazes craze less, but they can shiver


Example of serious gtia laze shivering using G1215U low expansion glaze on a high silica body toàn thân at cone 6. Be careful to lớn bởi vì a thermal bít tất tay 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 lớn work well. However it did not survive sầu a 300F oven-to-icewater IWCT demo 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 lớn accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock thử nghiệm & still fires absolutely crystal clear.

A settling, running gtia laze recipe gets a makeover


The original cone 6 recipe, WCB, fires lớn a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing và 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. Adjustment 1 (column 3) eliminates the feldspar và sources Al2O3 from kaolin và KNaO from Frit 3110. The chemistry of the new chemistry is very close. To make that happen the amounts of other materials had to be juggled (you can cliông xã on any material khổng lồ see what oxides it contributes). But the fired kiểm tra reveals that this one, although very similar, is melting more (because the frit releases its oxide more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition (lớn supply more SiO2). SiO2 is the glass former, the more a gLaser will accept, the better. Silica is refractory so the gtia laze will run less. It will also fire more durable & be more resistant to leaching.

Low fire gtia laze with the Al2O3 & SiO2 of a cone 6 glaze


Look at how fluid G3879 is at cone 06 even though it has the Al2O3 & 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 & 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 lớn tolerate large additions of kaolin. Each addition sacrifices some melt fluidity but the gLaser stays glossy & gets more durable (because of the increased Al2O3 & 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.

Silica plus modifiers matrix


How much silica can some glazes accept?


G2922B is a cone 6 clear gLaser that started as a well-known recipe "Perkins Studio Clear". We substituted Gerstley Borate with a frit (while maintaining the chemistry) & then noted that the gtia laze 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) & 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 gLaser, it hardens, stabilizes và lowers expansion.

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 khung. 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 sầu completely inlớn the melt lớn release those oxides). Each of these oxides contributes specific properties khổng lồ the glass. So, you can look at a formula and make a good prediction of the properties of the fired gtia laze. And know what specific oxide to increase or decrease lớn 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).

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If your glaze can handle more silica & 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 khổng lồ leaching, harder & has a lower thermal expansion.

Which one contains more SiO2?


These cone 04 glazes both have 1/2 Gerstley Borate. The other một nửa in the one on the left is PV Clay, a very low melting plastic feldspar. On the right, the other 1/2 is silica & kaolin, both very refractory materials. Yet the glaze on the right is melting far better. How is that possible? Likely because the silica và kaolin are supplying Al2O3 & SiO2, exactly the oxides that Gerstley Borate needs to khung a good glass.


Glass Sand
Nepheline Syenite
Ceramic OxideIn gLaser chemistry, the oxide is the basic unit of formulas và analyses. Knowledge of what materials supply an oxide và of how it affects the fired glass or glaze is a key khổng lồ control.
Limit FormulaA 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 khổng lồ effectively using these guidelines.
Al2O3 - Aluminum Oxide, Alumina
Predicting Glaze Durability by Chemistry in Insight-LiveHow to lớn spot out-of-balance indicators in the chemistry of ceramic glazes that suggest susceptibility khổng lồ scratching or cutlery marking.
How I Improved a Popular Cone 6 Clear Glaze Using Insight-LiveHow I found a ceramic gtia laze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed & got a fabulous more durable cone 6 clear.
GLaser is excessively runny on firing


Glaze Matteness
Low silica high alumimãng cầu glazes produce matte effects. The silica:alumina molar ratio is considered a good indicator of this type of matteness. A ratio of 5:1 is matte; 10:1 is glossy. The high alumina stiffens the gLaser melt preventing it from solidifying lớn a flat surface.

By Tony Hansen


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