Applying a very thin layer of clear glaze over painted porcelain

[Min]

28 minutes ago, Bill Kielb said:

If I look up the calculated COE of the original glaze recipe it is 6.5 which puts it pretty much in the middle of your tested  range and it did not craze.  I also see several references to successful use along with pictures that appear reasonably clear. I wonder if something is being overlooked here.

Change the materials (like the spar or nepheline syenite etc) and you will get a different COE that fits or doesn't. COE figures work best when the materials don't change but the amounts of them can. 6.5 might fit for one glaze recipe but not another that uses different materials due to a number of factors such as oxide elasticity, excess oxides, oxides that don't expand in a linear fashion etc.

edit: @thiamant demonstrated this when they decreased silica in some of the tests:

"....and tried with different silica levels, decreasing silica levels really makes things worse ( I was hoping maybe less cloudiness by doing so, but all tests from 1 to 4 had crazing)"

[thiamant]

1 hour ago, Bill Kielb said:

It is interesting, If I look up the calculated COE of the original glaze recipe it is 6.5 which puts it pretty much in the middle of your tested  range and it did not craze.

What recipe are you referring to? Anyway, that's why I said that I don't blindly trust COE calculators. As @Min said you can't just compare COE of two different glazes and predict whether it's going to work... 

1 hour ago, Bill Kielb said:

I also see several references to successful use along with pictures that appear reasonably clear. I wonder if something is being overlooked here.

I didn't say they were all wrong. Actually test #9 looks pretty promising (as I said it was too thinly applied and it's a bit dry but now that I have a good candidate I can try different thickness). And test #5 seems to work on body 1, which is probably the one I will be using. So having found two candidates is good. I feel I'm almost there. 

BTW, I don't want to say this loud because prices might go even higher but I just fell in love with spodumene and lithium in general. I used it in another glaze (very different from this one) and it just gives me a perfect melt, 0 blisters, really, that extra kick on fluxing power is a real thing, plus how you can reduce CTE so easily... 

[Bill Kielb]

2 hours ago, Min said:

Change the materials (like the spar or nepheline syenite etc) and you will get a different COE that fits or doesn't. COE figures work best when the materials don't change but the amounts of them can. 6.5 might fit for one glaze recipe but not another that uses different materials due to a number of factors such as oxide elasticity, excess oxides, oxides that don't expand in a linear fashion etc.

Yes, I see, I assume his COE included his reduced silica formulations  so a range of 6-6.75 seems not very relational when components change. I am not much of a calculated COE guy but I guess it’s useful, just maybe  less relational across compositions than I realized.

54 minutes ago, thiamant said:

didn't say they were all wrong. Actually test #9 looks pretty promising (as I said it was too thinly applied and it's a bit dry but now that I have a good candidate I can try different thickness). And test #5 seems to work on body 1, which is probably the one I will be using. So having found two candidates is good. I feel I'm almost there. 

No worries, you are on your way, just noticed some results posted after the change to wollastonite which looked very clear.