|Monthly Tech-Tip |
|Co-efficient of Linear Expansion||0.035|
|Frit Softening Point||1720C (From The Oxide Handbook)|
|Dry M.O.R. (50% Silica)||932C|
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.
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.
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.
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.
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. Adjustment 1 (column 3) eliminates the feldspar and sources Al2O3 from kaolin and 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 click on any material to see what oxides it contributes). But the fired test 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 (to supply more SiO2). SiO2 is the glass former, the more a glaze will accept, the better. Silica is refractory so the glaze will run less. It will also fire more durable and be more resistant to leaching.
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.
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.
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).
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.
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.
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.
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
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.
How I Improved a Popular Cone 6 Clear Glaze Using Insight-Live
How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
Glaze is excessively runny on firing
|Glaze Matteness||Low silica high alumina 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 glaze melt preventing it from solidifying to a flat surface.|