The glaze besides where it has cratered doesn't look bad. Makes me think it is the clay, not too complicated to try a different bisque firing or even rebisque your pots and glaze to see if that improves anything.
I am always a bit too lax with the electrical juice out the wall. Ran a 32amp kiln on 30amp breaker for 3 years although it was fired infrequently. Had relays get stuck on for 12 hours and wires melting during firings.
Took a knife to a wire still plugged in and turned on, learnt my lesson.
I don't think you understand what is happening with the currie method. It gives you a whole range of silica and alumina values so some of them have to be within the limits. Except it would not work very well with this glaze at all given the recipe.
I still can't see anything outside the limits in the original glaze recipe just the alumina looking too high. Are you taking the lower limits as values you can use in a glaze or that you need to be somewhere in the middle/top to have a food safe glaze? What is so bad about the lower end of the limits?
If it's not within the limits that doesn't mean it unsafe. Only a clue that it might be. Same the other way, being in the limits is only a clue that it is food safe glaze or a durable glass.
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The crackle is caused by a mismatch between the clay and glaze as they expand and contract during heating and cooling (CoE). You have a size 6 pot in a size 5 glaze. You are actually looking for this difference in the glaze and clay whereas a lot of people try to solve this 'problem' Any clay with a much lower expansion than glaze will make the glaze crackle.
I am sure your supplier will have a potash feldspar or be easily able to find it. Will the 10% kaolin not be enough for application onto bisque?
I am missing a step in the jump from mixtures of molecules to elements and their ionisation energies. We are not working with K but K2O or K2O.Al2O3.... and so on. If I did melt and boil a feldspar is it giving me elemental gas or molecular gas, if that question makes sense. We seem to have mixtures that work together to find the lowest melting point and nowhere near the boiling point. I understand maybe a tiny fraction may get enough energy to break bonds with the mix but it seems strange that I could lose a lot. Isn't my alumina and silica holding onto everything. That is why potash melts at 700 but feldspar 1200.
Why are you lowering the calcium? I am calcium bias as it is great but couldn't we up the calcium instead and drop some sodium. I haven't been able to take a good look at the chemistry as my insight account ran out but wouldn't that keep the green and solve expansion issues with a bit of alumina for less running.
Had a read of frank hamer on KNa and he talks about potassium not being volatile at cone10 but sodium is anywhere over 1200c (2192f). There is also mention that high sodium turns coppers blue.
Going to have a day watching youtube ionization videos sometimes I wish I went to my advanced chemistry lessons instead of playing card games.