Glaze flinging itself off of piece ?

[Jeanetta]

Hi!!

opened my glaze firing kiln today to mostly successes but with a couple funny new quandaries. (cone 6 clay and glazes)

had most the blue glazed pots turn out just fine , but this happened to two of them (see pictures).  When there are bare patches I’ve never had the glaze land on the shelf a little distance forward in front of the piece.

to be clear I dip glazed the entire thing and there wasn’t  any accidental wax on the rim.

A possible variable: maybe these had cream glaze on the rim (I can’t remember for certain)…the cream is fine on the other pieces but maybe was problematic here…can two glazes repel each other sometimes?

but again I’m not certain it wasn’t just all blue.

also (2nd question) in comparison to my first loads the blue glaze has become much duller in hue. I’m diligently mixing the exact same recipe. Maybe the cobalt carbonate supply is different??? Could this be why?

Would appreciate any thoughts or feedback

with gratitude, Jenna

 

[Min]

For the pots with the glaze jumping off I would be looking at a couple possible reasons. First, how soon after glazing did you start the firing and how quickly did it heat up? Another possible reason is if there were layered glazes and the first glaze was too dry when the second was applied the moisture from the second one can soak through the base glaze and loosen the bond with the clay. Layered dipping glazes should go on as soon as the pot can be handled without marring the base glaze.

Re cobalt and different colour of 2 glaze batches like in your last image, looks like you have rutile in your glaze, is the duller blue from a change in rutile? Also, was target cone confirmed with a cone? Could just be underfired as I'm not seeing the streaking in the duller glaze. For sure cobalt supplies can vary, if you have any of your old supply left I'ld test both in a clear glaze.  Put a small amount, like 0.50% of both supplies of cobalt in 2 small test batches of glaze and fire and see if you notice a difference.

[LinR]

l once used a cobalt blue glaze with rutile  that would spit off if any other glaze was put over it.  It could go over other glazes but not under.  Never knew why.  Lin

[Jeanetta]

Thank you Min for your thoughts knowledge  and reply!

Great info about the glaze drying differences- that’s super helpful I will remember that. I did all my glazing at the same time and let it dry over night indoors (live in cold climate)- BUT I have to truck my stuff up to my kiln site day of firing and occasionally dab on touch ups before if there are little dings. So this may be what happened - perhaps this was a touched up piece…..

the cones showed it fired true, & I don’t believe heating up was too drastic… ran my normal skutt 1027 “medium” program.. 

Interesting about the glaze. No the recipe doesn’t have any rutile in it. Thanks for the test suggestions!