what happened to my piece??

[hlipper]

Hi folks - I've never had this happen before so figured I'd throw it out to the group so hopefully I can learn from my mistakes!

I made a large (14in x 12in diameter) planter using coiled flat slabs (3/8in thick). The bottom is a solid slab, same thickness. The build went perfectly well. It dried for a long time and came out of the bisque (^04)  looking beautiful, no cracking or anything. After the glaze firing at ^6 it had a major split on the bottom and cracking around the sides. I spent a lot of times compressing the clay, scoring and slipping, etc. Any thoughts? 

Thanks in advance for any input. This was a custom request and I want to make sure I get it right next time given the amount of time and clay!

[Callie Beller Diesel]

It can be frustrating figuring out how to fire bigger pieces for sure. It does’t sound like you did anything wrong in the construction. The thickness sounds about right for this size of piece, and even drying is a good thing, so I don’t think you have to change that part at least. The only thing I might suggest for construction is to reinforce the join between floor and wall with a coil on the inside, but otherwise it looks pretty good. 

I think this may be a matter of the piece either heating or cooling too quickly in the glaze. When you have a large piece like this with a flat bottom that has a lot of surface in contact with the kiln shelf, the shelf acts as a heat sink. Unless you’ve got the vertical parts  surrounded with other pieces, or even kiln posts, it can create a situation where the top part of the pot cools/heats slower than the base. The image is a bit fuzzy, so I can’t tell if the cracks are sharp or softened, so I don’t know if it failed on the way up or the way down, but he effect can be more pronounced if you’re using a fast or medium firing speed. 

Another thing that can happen is that the weight of the pot makes it difficult to expand and contract on the horizontal, also leading to cracking. If you fire your pot on a bed of something refractory (grog, powdered alumina, silica sand), or set it on a waster slab, the piece can shift on that surface. Or if you’re using a waster slab, the slab takes the pressure and cracks instead of your piece. 

So suggested fixes are:

1. If you fired fast, slow your cycle down.

2. Surround your next piece with other work or extra kiln posts in the glaze firing so that heating and cooling happen more evenly.

3. Set your piece on some grog so it can shift. 

[Pres]

I would add a few pointers also

• make certain bottom clay is from same batch as side strips, 
• Try firing upside down
• use a salt/herb shaker to shake grog onto the kiln shelf

best,

Pres

[PeterH]

8 minutes ago, Pres said:

use a salt/herb shaker to shake grog onto the kiln shelf

I found this a helpful way of thinking about it

 

[Chilly]

+1 to all the advice above.

And, 

If you don't want free silica sand/grog/? in your kiln, you can make 5-10 coils, about 5-10mm thick, and each half the length of the diameter of the bottom of the pot.

Arrange them like the spokes of a wheel (without touching at the centre) and sit your pot on them for the final firing.  This will allow the air underneath the pot to move around, and the pot can shift too if it needs to .

[hlipper]

Thank you all so so much for this amazing feedback. I feel a lot more confident going into my second build of this piece. Really appreciate you all!!

[Christine_M]

I am also about to fire a large planter with a 45cm base. All your tips will be a great help. 

I don't have any full shelves for my kiln, only half shelves. The work would span across the 2 halves (which I know is not ideal).

Would a possible work around be to:
stack 2 layers of shelves on top of each other, rotating the top layer 90 degrees,  covering the  gap between the halves?

Thanks

[Pres]

@Christine_MThat would not be a good idea, as it may make things worse. Make certain your shelves are on clean stilts setting evenly on the floor. leave only a slight gap between the two half shelves, use grog or rollers to support the piece and allow for expansion /contraction.

 

best,

Pres

[Christine_M]

Thanks @Pres
I'll give that a go.