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Post by Brimgiest on Mar 8, 2020 8:26:02 GMT -6
What are the preferrred tactics to avoid crazing within the classical crystal region of Stull? When one doesn't want to alter one's clay body or adjust Al/Si levels like this: www.youtube.com/watch?v=7-h6GlJrDDk |
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Post by Brimgiest on Mar 8, 2020 10:03:21 GMT -6
So I should rather find one of the FB forums?  |
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Post by evan cornish-keefe on Mar 9, 2020 8:33:15 GMT -6
Avoiding crazing using Stull's map is almost entirely about the silica and alumina amounts, so increasing those is the best way.
Alumina is not as detrimental to crystal growth as some people claim, large well defined crystals can form in a glaze with .2 Alumina (with about 25% EPK).
I've found glazes with .1 Al are extremely runny, at .2 Al glaze catchers aren't necessary if glaze application is the right thickness, and glaze thickness may be much more temperamental for good results.
Don't use the technique of keeping the Si/Al ratio the same as you increase Si and Al, many low alumina formulas have a 30:1 ratio, yet are very low in Silica still.
The silica level is important for nice looking crystals, I usually stay in the range of 2 to 2.2 Silica regardless of the amount of Alumina.
Lowering Silica to around 1.8 gives significantly larger crystals but more crazing and visually the crystals don't have as much "chatoyancy" in my observation.
As far as not altering clay body or Si:Al - my only other thought would be to raise the total RO to alter the crazed region of the map.
Say if you start with a flux ratio of .3-.7 and alter it to .2-.8 that should help with crazing a bit, the Zinc level would need to remain the same (around .5 ish) then add Whiting or another secondary flux.
That would make the crazed region of Stull's map a little smaller but I beleive altering Si and Al amounts would still be necessary to get rid of crazing.
One last thought on aesthetics: I prefer no crazing to a crazed glaze, but definitely prefer heavy crazing (lots of fine lines) to a slightly crazed glaze with the occasional large spaced out craze lines.
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Post by Brimgiest on Mar 11, 2020 5:35:35 GMT -6
Hi Evan,
Exactly, which is why I want to avoid it.
The glazes I'm after don't look the same when Al:Si is altered.
I'll try changing the ratio as you suggest, though I don't understand the mechanism at work. Is it related to how much it melts?
Even though it is crystalline it is not a zinc based glaze I'm working with. I'm at 2.11 Si and 0.24 Al. C11 at the top.
Crazing in a crystalline glaze makes me go crazy, there's already so much happening.
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Post by evan cornish-keefe on Mar 11, 2020 21:19:28 GMT -6
Good to know it's not a Zinc crystal glaze, or I'd probably be confusing the situation more.
Do you know what type of crystal is forming? Seeing the unity formula would be helpful info if you're open to sharing that.
As for changing the flux ratio, it isn't related to how much the glaze melts, rather the CTE of the glaze.
Stull's map is .3 K2O - .7 CaO and calcium has lower expansion than potassium so increasing the RO there would lower the CTE of the glaze.
Higher Expansion - Na2O, K20, CaO, SrO, BaO, SiO2 (crystalline quartz), Li2O, ZnO, MgO, Al2O3, SiO2 (melted glaze), B2O3 - Lower Expansion
My understanding is that there may not be a direct correlation to the thermal expansion of the glaze and the CTE of each ingredient used, but these give a general indication of how materials influence a glazes thermal expansion.
Tweaking Si and Al is nice because they aren't as drastically changing the glaze, whereas changing fluxes can significantly affect color and opacity.
For a while I started using Lithium Carb instead of frit 3110 in a few glazes to try and stop crazing, it wasn't that effective, but also it's solubility was an issue so I switched back to 3110.
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