Expert: Leisha Hiester - 7/24/2008

Question
QUESTION: A tall cone piece of pottery came out of the kiln, which was having some problems, supposedly fixed. It was tall twenty or some inches. It cracked at the top but most disturbing are all the bubbles around the lower four or 6 inches as I'm going to do all my new pots with this same new batch of engobe... Did I mix it wrong. This hasn't happened in years. I'm thinking of just sticking it back in the kiln and firing to cone six to see if they go away. This happened at a 06 bisque in a skut kiln using MEDIUM... And it only took 7 hours which I found very unusual as 11-14 hours seems to be the norm. Everone else is using holds. I'm thinking it cooled too fast. What do you think?— thanks

ANSWER: Scott,
Do you have a photo of it? If it's a cone shape and you were doing the bisque, the bottom may not have been dry enough. The moisture could've caused the bubbles. Are you mixing your own clay? Mixing your own engobe? Let me know some more info and I'll sure try to help you figure this out.
Leisha

---------- FOLLOW-UP ----------

QUESTION: I don't have a photo of it as it is in the guild I use in Toledo, OH. About an hour away. The cone is open at the bottom and I'm sure it was dry. I'm thinking that it cracked so badly at the top of the cone because maybe the bottom was so flat and the kiln shelves so new that heat needed to escape and that is why the crack is so extensive at the top. I'm not so concerned about what happened to this piece as I am about the engobe which is already on four large vases, currently drying and almost ready for bisquing. I'm also wondering why the bisque was only 7 hours. Everyone says use the ramp mode next time so that is what I will do. The cone engobe is rather shiny towards the top, as it should be, but where it blistered it is matt. The original recipe called for TIN, but I had pinking problems, so I substituted with Superpax, I'm thinking maybe I could use Zircopax. This is a tried and true recipe I've used so I'm thinking all sorts of things, not mixing enough.... too thick, too thin.... I've always used it pretty thick with no problems. We have a new kiln that I may have to use instead as now I'm hesitant about using the old one. And I'll only fire one vase to start. I swear this stuff keeps me up at night!!

Answer
Scott,

Hey, I grew up near Toledo. Glad to hear there's some active pottery going on there. Where is the guild located? I'd love to visit it next time I'm there.

What'd I'd do before you fire those other pieces is use the engobe on a small piece or test pot first. See if the same thing happens. If it does you've at least isolated the problem to the engobe or the firing. Also fire a test piece in the new kiln. Mark the tests clearly on the back of the clay so you know which is which and ensure they go into the right kilns yourself. It's difficult when you're working in a co-op environment. You sometimes have to babysit your own work. I keep a notebook on every firing I do so I can study mistakes and best results.

Do you use a hydrometer? See if someone has one you can borrow to mix your batch.

I understand about being kept up at night. I have a 'potter's yard,' I call it. It's sorta my graveyard. It's a base of river rocks. Atop the rocks are all my mistakes. I keep them in plain view where I can see them. After a while they become joys because I look at them & laugh & realize how far I've come & what I learned from that piece. Alot of great people stated, 'I've just found one mor way it won't work!" Keep at it, but do it wisely & patiently with lots of testing. Keep me posted. Leisha