Firing large flat wall hangings

[kswan]

I'm making some large wall hangings (between 18" and 25" on the longest side) which is larger than I've done before. I realized with the first few I fired that I made them so thin they are sagging. All of them have four nubbins on the back. 

I am using a sculptural paper clay instead of my usual clay, which cracks when it gets too big. I bisque fired a few on edge thinking that it might prevent the cracking, but I noticed they warped a little even in bisque. They were way too thin, about 3/8 inch thick, I realize now they should have been about 1/2 inch or so. 

I place some of my 1/2" posts under the edges to stop the downward movement, but the center sagged in between the nubbins. It doesn't look so obvious unless the light hits off the glaze, then you can really tell. At any rate, I'd like to try to keep them a bit flatter for the next sets and thought maybe someone might have some advice for me. My idea is to put some coils of the same height as the nubbins along the bottoms for temporary support. I was thinking the coils in conjunction with the posts might keep everything from sagging too much, but I might also have a weird ridge from the coil. Would this work?

In the photos, I'm showing how I had the posts in the kiln, the nubbin positions, and the possible rolled coils to help give temporary support.

I cannot wait to be done with this project!

[Min]

I’ld roll out a cookie the same size as your wall hanging and just a hair over the depth of the nubbins, cut out the clay where the nubbins are and fire it on that. Cookie doesn’t need to be bisqued. Since it’s paperclay it’s probably okay firing on the bottom shelf and not get a cooling  dunt   but it would be safer not to fire on that or the very top shelf. 

[Mark C.]

44 minutes ago, Min said:

I’ld roll out a cookie the same size as your wall hanging and just a hair over the depth of the nubbins, cut out the clay where the nubbins are and fire it on that. Cookie doesn’t need to be bisqued. Since it’s paperclay it’s probably okay firing on the bottom shelf and not get a cooling  dunt   but it would be safer not to fire on that or the very top shelf. 

Yes that-a waster slab is needed forget the coils as it will just warp over them.

[kswan]

Okay, thanks, I'll try the slab method, that's a good idea. I'm going to try it for just inside the nubs and use the shelf posts for the outer edges as that seemed to work okay there.

I've got 9 more of these to fire so I should get it right by the time I'm done! 

[Babs]

Do what Min suggested.

Murphy will make it slump else where for sure

[Babs]

The biscuits can be used a number of times, just saying.

[Mark C.]

support the whole piece not just the inside the nubs. Clay is cheap and warpage will happen if not 100% supported-as Babs said just saying

[kswan]

Yup, I'll get it all underneath! You're right, I don't need to test fate on the rest of these. 

[karenkstudio]

I've fired a lot of slabs and never add any nubs on the back. Anything needed for hanging can be made separately and glued on after your firing processes are finished.  I sprinkle kiln wash on the shelf which helps with shrinkage in the bisque firing and the shelf is my support for keeping the slab flat.   If all pieces are already bisque fired, refer to the advice given above.

 

 

[kswan]

@karenkstudio I'll definitely consider that next time (if) I make things this large again. I was worried that attachments made after firing wouldn't be as strong as those made into the clay. These are pretty heavy, so I don't want them crashing to the floor. They're all already made with the nubs on the back and I'm going to just support them underneath as the others have suggest at this point. Thanks!