Three breaks in Bisque firing

[PierreM]

Hey all,

I am still a freshman into ceramics world, and still figuring things from modeling, to firings and glazing.

I recently met the same problem through the bisque firing, with three différent pieces, so I figured I might ask around ... in case the problem was frequent, and avoided with a extra technique.

For all 3, the foot detaches from the main part (beeing a vase, or a cup) almost at the same spot.

My first though was that the round shape of the main piece and the flat/rounded shape of the foot were not perfectly touching, thus creating a void, or a bubble.

For the last one, (whose foot also broke in 2) I discovered a different color inside the bisqued clay, kind of brown-ish. And wondered if that 's normal? (or maybe over/under fired?)

FIY, I used paperclay for assembling parts,

And the first 2 one I've been able to fix with paper clay - re-bisque - which seems to have worked (althou it's not pretty- and might not live though the 3rd firing)

But I intend to re-do them, whitout the foot failure.

Would you have any comment / Suggestion to prevent this problem?

This is a Earthenware - all 3 had been drying for minimum 1 month - were completely boned dryed - and none of them was showing cracks after drying process -

the schedule looked like this : 100°C/hr > 600°C > Hold 10min // 150°C/hr > 1040°C > Hold 10min

 

This forum has been such a helpful ressource,  I cant wait to have your feedback and exchange with you

Just added few pictures to illustrate -

Thank you so much for your interest and Help

Pierre

 

 

[Callie Beller Diesel]

I think the images have been uploaded in the reverse of how you talk about them. To confirm, I’m reading the items that appear at the bottom of your post as what you refer to as 1 and 2, which are a bowl on a small pedestal and a vase with a lobed neck. The one at the top (5 photos) is what you refer to as the third piece, which resembles a cup with a stem. (They’re very cool, btw!)

I don’t think the sharp right angle is contributing to the cracking in these pieces, although it can lead to surface cracks in those areas. The images of the break on #3 gives some good insight on your working methods. Because you can see the score marks where you made the attachment, that means the pieces weren’t properly adhered to each other. Even with paper clay, you still need to wiggle the pieces against each other until they stop sliding to ensure a good join. Especially when joining major structural elements that hold a lot of weight. One of the pieces might have also been too dry for the other.   

Because the clay is discoloured in the center of the goblet piece, it would indicate that organics didn’t have enough time to burn all the way out. Unless your kiln was a gas kiln and not vented properly, the early half of the firing was likely too fast, especially if there was no other work in the kiln. The easiest way to fix this is to build the base hollow, so that the piece can fire more evenly. 

However, if you need the extra weight for stability, you may have to slow down the first part of your bisque cycle to 80* C/hour to 550, and possibly add another 5-10 min to the first hold.  Firing the piece on a waster slab, or some kind of refractory sand (silica or alumina, or even grog/chamotte) will help relieve the stress on it as it expands and contracts through quartz inversion. 

 

Edited September 12 by Callie Beller Diesel
Removed some redundant language

[Pres]

After reading @Callie Beller Diesel's excellent review, there are a few questions I would ask.

• First, referring to bone dry. That term is really quite relative especially if you live in a high humidity climate, or if the drying occured in Summer/Fall  months where things are usually more humid. If very humid, I would always extend my candle time. I used to fire my kiln without a setter and would use a simple test to see if the kiln is ready to move out one heat range to another. Place your dry open  hand over the open peep hole to check for moisture. If your hand comes away slightly damp then the pots are still too damp. Wait until you no longer feel moisture on your hand. Then check on the next level turning up your kiln until you have no moisture feel on your hand.
• On assembly, you say you use paper clay to assemble. Have you ever tried Magic Water? This simple material is made up of  1 gal. of water, 3 tbsp of liquid sodium silicate & 1 1/2 tsp. of Soda Ash.  It works very well to  join pieces of clay that are in the regular working stages of cheese hard to leather hard together. Far superior to any slip, and a much cleaner join. I would really try it to see if can help you with your joins.

best,

Pres

[Mark C.]

From what I can see they are most all connection issues and also to wet and to thick issues for the speed of the firing cycle.

More slip and scoring when connecting (more pressure and both pieces need to be same moisture) less thick walls will help. get the work dryer and slow firing is best especially for thick work.

[Kelly in AK]

Different cracks have different reasons. @Mark C. has narrowed in on some principles behind your troubles. I see in the first group of photos a bisque that was too fast in its earliest stages along with pieces joined that are too dissimilar in their drying states. The work in this first group of photos is large and thick at the base. I see evidence of explosion from steam, it was not dry enough. Candling the kiln (around 200°F for 4-8 hours) prevents steam explosions. Clay contains moisture from atmospheric humidity, even after being dried for an extended period of time. 

The other photos also suggest attachments made of clay that is too different in drying stages to be confidently joined without cracking. Perhaps also the relative thickness of each section created tension that manifested as cracks. 

Paper clay and magic water/slip have their limits. Some principles of clay craftsmanship: Attaching clay to clay is best accomplished when separate pieces are at the same state of dryness/wetness. The dryer the clay is, the more difficult it is to join pieces without cracks forming. The larger the work, the more difficult everything is. Consistency of thickness is important to avoid stresses building up due to uneven drying.

It may not be relevant here, but for good measure I’ll mention this in relation to managing the stages of clay drying: rewetting a too dry piece does not revert it to leather hard. This practice tends to introduce more complications than it eliminates. I have seen many pieces ruined by the misuse of a spray bottle. 

Your work is beautiful. The flaws are manageable and relatively small, you will overcome them. Bravo, freshman.