Determining Optimum Cone For Glaze

[Tom Humphries]

I'm currently using my copy of the G1214W digital fire base at ^6, and am happy with the results. I'm currently thinking of firing to ^7 or even ^8 because of the absorption of my clay when fired.

Am I right in thinking the way to determine the melt at a certain cone is a comparison with reference formulas like Green and Cooper?

The G1214W seems to be within the limits of the Green and Cooper ^9 and ^6 formulas, apart from the CaO which is higher in both cases.

I have a number of glazes made up from my base formula so it would be nice if I could use them at higher cones, but obviously I don't want them to run all over the kiln...

If I do need to reformulate the glaze, how would I approach it in Insight? I have used some other online glaze calculators that give a specific cone number for a given glaze recipe. Its easy to see lowering the amount of frit would increase the cone, but is this a simplistic approach?

Thanks

Tom

[Norm Stuart]

Tom - This is my adjustment of your conversion to European materials. Remember that these limit models don't perfectly predict what will happen.

Ferro Frit melts at Cone 015, so with double the boron BPS Calcium Borate Frit will melt at least that low, and Ferro Frit 3110 melts at Cone 08. Yet without those two frits, it's likely none of this glaze will have melted by Cone 6 or Cone 8.

When I make a new Cone 6 glaze I fire one test tile at Cone 04 in our bisque, and the glaze on that tile is almost always a solid melt, but more opaque because not all of the material in that glass has melted. As a clear glaze like this, without special chemical reactions, fires hotter and longer it fully melts. This very likely occurs below Cone 6 and you're done with the "heat work".

As the kiln temperature rises higher a well-behaved glaze like this will remain largely unchanged until at some temperature it starts to run, indicating it needs either a little more alumina (kaolin) or a little less higher temperature flux like the potassium or calcium in the Wollastonite. Changing the the sodium level will mess with the COE.

So that's what I'd do. Make the converted glaze and fire it at Cone 04, Cone 6 and Cone 8 and look at how the glaze has changed at each temperature. It may not need any adjustment.

You'll notice the UK glaze has the same chemistry with both more alumina and silica, but I'm trying to solve for the fluxes, the Silica to Alumina ratio, and the Silica+Boron to Alumina ratio  - that's what matters. Just a hint more Potassium Oxide which will make any colors you add imperceptibly brighter.

[Tom Humphries]

Thanks for the adjustments Norm. I will do some tests and see what happens!