Friday, September 4, 2009

*sigh*

So, I realize that I am new to the world of making "bigger" pots... and with that there is certainly some loss expected. However, I have had three bisque firings with these big shallow bowls in them and about 50%-60% of them have ended up just like this. I found this poor pot all blown to bits this morning. Now, to the naked eye, this one was 100% bone dry when it went in to the kiln. It had been on my shelf for over a week drying. I also did a 3 hour rise and preheat under 180 degrees. What am I doing wrong? Could they still be wet? Do you have to preheat longer for bigger pots? Could it be the form itself? Ar there others out there that make big forms that could assist? Any help would be greatly appreciated! Thanks!

15 comments:

cynthia said...

Nothing is any dryer than the atmosphere ..

Charles The Potter said...

I candle the kiln at 190 degrees F for a minimum of 8 hours, more likely 14 hours. If there is big work, perhaps as long as 20 hours. I rarely have things blow up anymore. Rushing is the enemy of all craftspeople, most especially potters.

June Perry said...

From the photo, it looks like you have a fairly thick cross section. The thicker the pot, the slower and longer you have to bisque. You can get explosions in too fast a bisque firing because the skin of pot gets too dry, in relation to the interior which still retains moisture.

Carolanne said...

I'm with Charles. 8 hours on low (212F) at the minimum.

traceybroome@mindspring.com said...

I have put just made kids pieces, thick and chunky in the kiln candled for 8 hours and never an explosion. The minimum I have ever candled is 6 hours and I was worried the whole time. We candle for 6-8 hours at Claymakers as well. Bigger is fun though, I'm going down that path myself right now!

John Britt said...

Lindsay,

I agree with Charles, 8 hours on low (212F) at a minimum. Lot of mass in big pots.

Lindsay Rogers said...

Wow! 8 hours! I guess I was undercutting it with only 3 hours. Huh. I was thinking they were totally dry and was trying not to be wasteful... but I'm guessing that's not the case. Well, I'm about to load up some more. I'll try it this time. Thanks so much everyone!!

Clay Club said...

I feel your pain. It's so frustrating to have new work crack or blow. Been experiencing it a lot myself :(
Let form a support group.

mikpo said...

Large and small pots we place on the (elect.) kiln lid during either a glaze or bisque firing. If in a rush, put pots in your elect. oven for an hour or so with door ajar. Set temp under 200. Pot should be too hot to handle when ready to be bisqued.

Beats the 8 hr program, and we've never had a blow up. We did have a blow up in the oven; we put the dial at 200 and pow! Very exciting. The pot was small too.

When living in NM we often had Indian women coming to our studio complaining that many of their pots would blow up in the pit firings. We tried to persuade them to preheat in a kitchen oven but I think the blow ups were part of the mystique. Made the pots more expensive. So many tragic events!

FetishGhost said...

I had a similar problem last year. The bottoms of 3 of my large bone dry pots were reduced to rubble. I pinned the probem down to a steam popping. I had kiln washed before loading and the weight of the pots trapped the steam under the bases. It might be just BS, but now I bisque all my large work on stilts and voila, no more problems.

Lindsay Rogers said...

I just talked to Mark Peters this morning about this issue as well. He said that he occasionally fires big pots upside down so the foot can dry out and not trap the moisture. I thought that was interesting. Thanks for all the suggestions! This has been really helpful. I have a bisque with three big bowls in it that is cooling now. I'm crossing my fingers!

John Britt said...

I use my kiln as an "oven" . Just set it to soak at 200F and vary the time depending on the thickness and wetness.

Andy"Stone Monkey" Pearson said...

I make large Bonsai pots and when bisque firing them I will also candle for about 6 h-8 hrs over night then continue the bisque for another 10-12hrs. I still have the odd loss but by and large they all survive now. I too feel your pain but it will all work out in the end
Regards
Andy

Lindsay Rogers said...

It worked! All three big bowls survived this last bisque. Thanks again!

June Perry said...

Glad to hear the good news!