Clear glaze shivering

[Hannahquigleyceramics]

Hi

I’ve had a problem on some pots recently with shivering on the lip of some mugs.

I use stoneware clay bought from potterycrafts and use their own made clear stoneware glaze on them.

it has only happened to a few of my pieces and was just wondering what factors I can change to help minimise this happening again?

I have read leaving the pot after glazing it to dry properly before firing can help and to make sure the kiln is completely cool before unloading also.

any other factors I can control to keep this from happening?!

 

thanks

 

hannah quigley

[Callie Beller Diesel]

This is the second time this month someone in the UK has mentioned this problem. The other poster didn’t mention whether they were using potterycrafts or not, but the question you describe is so similar, I’m wondering if there’s a batch issue somewhere. I would start by calling them and seeing if they’ve been getting reports of this problem.

Shivering usually happens when the clay and the glaze have a COE or CTE mismatch (Coefficient of Thermal Expansion). Shivering results in harsh cracking, and in the extreme the glaze can pop off the pot after it’s cooled, usually in shards. It’s more common for it to happen on a porcelain body, or a white stoneware with a lot of free silica that forms crystobalite. If your pieces are thrown very thinly, this will exacerbate any tendencies for it to happen. It can also happen sometimes with a glaze that’s got too much lithium.

 Spitting can happen if your pots are really wet and you start your firing really fast, or if there’s a glaze ingredient that contains a lot of material that burns off rapidly, and causes some of the glaze to come off the pot. I hear uncalcined colemanite is a culprit for this one. Spitting usually results in a lot of glaze all over the shelf, and it’s all melted and a mess, because it happens early in the firing.