slab plates center warping during glaze fire

[chris123]

I'm attempting to transition from fully thrown plates to partially slab constructed to save time and effort. The first iteration of the plates are simple with a flat bottom, no foot, and a short turned up (nearly vertical) side about 3/4-1".

The problem I have is that after the glaze fire the center of the plate has risen up and there's a hump that goes nearly from edge to edge. Before the glaze firing, the greenware and bisqueware plates are flat and look fine.

My process:

1. Roll the slabs out to about 3/8" thickness
2. Cut into rough circles by hand
3. Toss on the wheel and flatten down on a bat
4. Cut to a perfect circle
5. While on the wheel, turn the edge up with a wooden tool and my hand

Once it's dry, remove from bat and cleanup any edges with a sponge. Bisque fire flat on kiln shelves, glaze and high fire the same way.

[Kelly in AK]

That’s a very cool way to make a plate! 

Are the bottoms glazed? If not, it could be the glaze causing it. Glaze with a tightly compressive fit will bow a flat test tile glazed on one side. It would be magnified on a thin plate. 

[Roberta12]

@chris123 When I dry my slab plates and trays, I put a bag of rice in the middle of them while they are drying. I do this with porcelain and stoneware.    It's just one of the small things a potter can do to try to insure a measure of success.  Along with maybe changing glazing or how you fire or or or....

What clay are you using and  what temp do you bisque and fire to?

Also, just a thought, "toss it on the wheel" going with clay has a memory situation, maybe carefully put the clay slab on the bat?

 

 

 

 

[Piedmont Pottery]

For me it would be faster to just throw the plates on the wheel from the start rather than using the slab roller.   However, I do find that a foot ring adds stability to plates during firing.  Have you considered using a slump mold instead of throwing the slabs?

[Mark C.]

For me throwing the whokle thing on the wheel is faster by far. The time is trimming a foot.The wares are solid and do not warp. The compression is good and no memory issues with the clay .

[Rae Reich]

Plates take up a lot of kiln space! The skills to make consistent, flat, even-bottomed thrown plates need as much repetition as the other forms, so as many failures as need to be expected can be ‘time and kiln space’ expensive.

I like hump-molded slab plates with a foot ring of a coil added . Foot ring can be thrown on if the mold is attached to a bat. Remove from mold immediately before clay begins to shrink by flipping it over onto another bat. I put a few sheets of newspaper (please support your local newspaper!) or dispenser-type heavy paper towel on the drying bat so that the plate can shrink/dry without sticking to the bat.

Placement of the foot ring can help or hinder the flatness of the finished plate. Look at many examples carefully to make the best choices. 

[Pres]

Tried this years ago, and found it was indeed easier to just throw them. I tried mine throwing the flattened slab on a plaster form with a flat bottom and the sides shaped for a plate. while shaping it I threw the foot ring. Alas 2-3 out of 10 would warp. I decided it was because of the shaping of the form while on the wheel would cause particles to become circular in motion as opposed to the center being non aligned from the rolling out. Just my thinking at the time, but now I throw much faster plates using large slightly curved ribs.

best,

Pres

[neilestrick]

Not everyone wants to or is able to throw dinner plates on the wheel. I slab build my plates, and it is much faster than throwing and I don't have problems with humping in the middle.

@chris123 I think the issue is either with glaze tension as mentioned above, or it could be from uneven heating/cooling between the edges and the center or the top and bottom.

1. Does it happen with all of your glazes or just one?

2. What size kiln do you have, and what firing schedule are you using?

[Mark C.]

I make square slab plates on a plaster slump molds . they are not footed (no coils added). They pop off the plaster well and have a curve to them which lessens in the glazze fire. They have a square area that is flat in about 1/3 of the form with the lip angled up. I have to sizes and have way better luck with the smaller 8-9 inch than the larger 14-12 inch which loves to lay down to much in a glaze fire

[Min]

On 3/2/2024 at 1:01 PM, chris123 said:

I'm attempting to transition from fully thrown plates to partially slab constructed to save time and effort. The first iteration of the plates are simple with a flat bottom, no foot, and a short turned up (nearly vertical) side about 3/4-1".

I'm seeing a lot of plates made like this these days, it's a good way to make a plate with this type of design. I've got one bisque fired as a test but haven't got it glazed yet, it was really fast to make and I also like that it doesn't require any trimming. That nearly vertical edge is going to help prevent sagging/warping during the glaze fire compared to a flatter more horizontal rim.

I would suggest putting one of yours in a glaze firing with no glaze on it, if it doesn't warp then it would suggest the glaze fit could be the problem.

Welcome to the Forum.

Edited March 9 by Min

[Callie Beller Diesel]

I’ve seen this form a bunch, and yeah, it’s tricky to get right. One solution for the warping I saw another potter online do was to do all the minimal trimming and cleanup as you describe, but they then added a slip trailed circle of clay as a foot rim. It was just enough to keep the full surface of the plate’s bottom from being in direct contact with the kiln shelf so you don’t get the warping, but still keeps the same aesthetic qualities of this style.

Results may vary with different clay bodies, but it’s something that’s worth a shot.

[Rebekah Krieger]

Another tip would be to leave a “button” in the center bottom of the plate. You can find some images of Steven hills plates/platters to get the idea.