Monday, June 4, 2012

Piecing together the shards ...

OK, maybe one of my blog-readers can help with this. I had had multiple explosions in the bisque kiln when I opened it last night to check on the dryness of the pots. The greenware was reasonably dry when it went into the kiln and I had planned on leaving it there at 160 F for about 20 hours, to insure they were fully dry. That's my routine and it works 99 percent of the time. After the pots are fully dry, I turn up the kiln and they bisque to cone 07. I rarely lose any pots this way.
The first photo you see is the bottom shelf of my bisque kiln as I found it this morning. Those thousands of shards were once two 10-inch slab plates, made into slabs by pounding them out with my fist on the wheelhead. They were about a half-inch thick and, I thought, pretty thoroughly dry.
The other photo is from the top shelf of the bisque kiln, former baking dishes built from thrown bottomless walls and attached to similarly pounded and rib-flattened slab bottoms, about 3/8 of an inch thick. You can see that the baking dishes blew apart at the joint, which makes sense to me and points to the likelihood that they were not thoroughly dry when they went into the bisque kiln. The joint was thicker than the walls or the floor of the pot. Somewhere between ambient temperature and 160 degrees F they blew apart.
I can understand that. My mistake. I've made those mistakes before and it just reinforces the lesson of starting with completely dry greenware. What I don't understand is the other pots (and there were several more than I'm showing you) and their thorough disintegration at that low temperature. Were they still too wet? Does the pounding of the clay set up multiple stresses that result in these explosions? Was the phase of the moon to blame?
I'm kind of at a loss. Any ideas?






10 comments:

Dennis Allen said...

You may recall a similar picture of my kiln a couple of weeks ago.My thoughts (coming from my mistakes) 1. If you stack two 1/2 inch thick slabs together, you are 1" thick that takes a long time to dry but at 160 you would not have generated any steam. I usually do this at 190 and have never lost anything in this pre heat stage.2. new batch of clay? 3. Did you have a relay temporarily stick closed and take you above the boiling point then release. 4. When you felt the earth move last night it was not the throes of passion, it was an earthquake.

cookingwithgas said...

Can I laugh at Dennis- the guy does crack me up because my first thought was that you and Dee cause all kinds earth movement on the Cape.

My first thought is that there was still too much moisture, or possible trapped air, in the slabs.
When I make tiles that are about 3/4 to 1 inch thick I will set them aside for weeks- at least 3 covered to dry slow.
I then let them dry another week or so- I know painfully slow. Then about week 4 I preheat to 150 for 5-6 hours and then bisque.
We have noticed that when we blow up pots it seems to be at the 700-900 degree range. So we program a preheat and hold of about 3 hours even with the preheat the day before.
Other than that-I would go with the moon phase- seems about right to me. Nothing we did the last few days, other than drink, worked out well for us.
And like the fools we are we are firing today- why the hell not, what could possible go wrong?
WE can both go work at Wally world... I hear they have benefits.

Pauline said...

I make things with slabs, not a wheel thrower, but if there is one tip I use that works consistently for me and others who have used it, is bath towels (not plastic) for drying. You just can't beat the wicking action that pulls the moisture evenly through the clay from inside travelling to outside. just remmove/replace slightly dampened towels, or turn them over periodically and you will "dare I say never?" have this problem again. Also, the towels dry the pieces faster I think. My opinion is that you had uneven moisture, dry on outside, damp in the middle and when you heated the outside, it trapped the moisture inside somehow and cause it to blow. Use towels see if it works for you!

Julia said...

I seem to have problems with candling to dry in my electric kiln when we have unusual humidity out here in the desert during the spring & summer. Maybe it was the weather?!?

I like Pauline's suggestion of using towels when drying slabwork.

Dan Finnegan said...

I'm certain that it's moisture rather than your pounding method. I usually wait a much longer time for things like that to dry. It's always sad to see.

Hollis Engley said...

Well, you know ... shit happens. I can make more pots. Thanks, all for your ideas. I like Pauline's wicking towels ... never heard that one before. But I guess I lean toward the residual moisture idea, in spite of the fact that they dried longer than usual (and not stacked together, by the way) and appeared as dry as any pots that go into that kiln. I think the thickness must have held some moisture. Though the blowing happening at 160 or so just seems too low a temp. Time to try again.

madpotter1 said...

Ugh..... Hate when this happens. Trapped moisture is my bet too.

Hollis, a trick I use when in a hurry is wrapping a shelf in cleaner bag plastic and setting it in the sun. When I see beads of condensation collecting I turn the bag over. Dry plastic against the pots. I can do this 4 or 5 times on a hot sunny day. When the bag is no longer collecting condensation it's ready to bisque. Have never lost a pot doing this in 30 years. This goes for thick pots and joints. You can throw a pot and bisque the end of the next day if you are in a hurry.
Good pots and hot kilns to you!

Michèle Hastings said...

I think Dennis maybe right about a possible relay problem and that your kiln got too hot at one point. Like you, we fire things that aren't totally dry, keeping it under 200 degrees for a few hours the night before firing with little or no problems. We have an old manual kiln which means we have to watch it closely.

flowers said...

Hi Hollis,
Sorry to see what happened to all your pots. Do you think it was the joints? Or, was it they weren't dry enough? How does one know when they are thoroughly dry?

Vanessa

littlewrenpottery.co.uk said...

I've had pots explode on me before theres only a few reasons for it - the piece was too thick and started cracking, it wasnt dry enough and water was escaping too quickly or there were air bubbles in the clay. If you've had any foreign body in the clay as well like plaster that can also cause it to explode even a very small amount.