Creating Flat Ceramic Plank

[Ben xyz]

Attempting to create and fire a flat ceramic plank (unglazed)  say 12" x 18" x 1/2" thick, to use in a sculpture (dimensions are somewhat variable at this point). The issue I have is warping. I've heard of placing a weighted bag (rice or sand) on top of it, or perhaps a flat board on top while it dries. The warping in firing is the next issue. Lying horizontally flat on a shelf is one option for (cone 5). It will pick up kiln wash, but may have to live with that. Another option is creating a clay box with slots (to minimize  wrarping) to hold the plank on its side. It will not be glazed, so running is not an issue. Type of clay may help, too. Ideas? Thanks!

[Russ]

Drying it sandwiched between two pieces of drywall sheetrock wallboard whatever you call it in your area works fairly well. Make sure its pretty dry before unsandwiching it because if its only partially dry it may still warp. When firing lay it flat on a shelf with fine silica sand under your slab.  ...works fairly well but some of the other fine folks here might have other ideas they can send your way.

[PeterH]

+1 drying between wall-board.
In my limited tile-making efforts I also turned the slab+boards over from time.

Re the use of silica sand, I found this a powerful analogy.

PS I was taught that non-optimum rolling can make the slab more likely to warp: frequent turning & unsticking from the rolling surface, gradual reduction in thickness, rolling in multiple directions, don't let any thickness guides constrain the slab's edges, ....

Edited January 13 by PeterH

[Jeff Longtin]

To give nuance to Peter's suggestions:

The big key is to stretch the clay in lots of directions while you're rolling it out. Will you be making the slab on a slab roller or doing it free hand? If doing it free hand you will want to use various thicknesses of wood sticks to guide you in reducing the thickness. Plop down sticks that are 1" thick and roll the slab. Then pick up the slab, turn it over, spin it 45 degrees. Then roll it with the 3/4" sticks. Pick it up, turn it over, and spin it 45 degrees again. Then plop down the 1/2" sticks, roll the slab again.

Rotating and flipping is the best way to minimize warpage on big slabs. If the clay is too wet you can sometimes stretch it too much at the corners, when you pick it up, so its best to let the clay stiffen a bit between rolls.

Firing the slab is another challenge: are the kiln shelves flat? Is it worth it to go out and buy new shelves that are perfectly flat for the project? (Sometimes it is.) All the silica in the world won't help if the kiln shelf is warped. (Yes, a bed of silica can mitigate a warped shelf but a not warped shelf is the better place to start.)

Clay intended for throwing is considered "plastic". That usually involves shrinkage. Hand building clays tend not to have so much plastic clays in them. Meaning...less shrinkage. Making a large slab is usually best with a hand building clay.

The drywall trick does work well. In place of it several sheets of newspaper, under weight of a flat piece of wood, also reduce warpage in drying.

 

[davidh4976]

Minimize bending the clay while rolling it out or when moving it from the roller to the drying boards. Dry between sheets of newspaper sandwiched between drywall boards or cement board untill completely dry. Change boards when they are damp by changing the top board, flipping the sandwich, and then changing the other board which is now on top. Allow to completely dry before firing. Fire on edge. Yes, fire on edge.

[neilestrick]

Also, pound the clay out into a slab until it is close to the thickness you need, then finish off by rolling. The less rolling you can do the better. Fire it on silica sand or on a waster slab. Firing on edge may or may not work depending on the type of clay you're using, especially in the cone 5 firing. For porcelain or other bodies that fuse pretty tight it's likely to warp.

[Ben xyz]

Thanks! I always try to remember to rotate and flip the clay when rolling through the device with its thickness increments (but good to be reminded of). Pressure from as many directions and angles appears to be the key.  Pounding the clay first makes sense, which I'll try. The 'memory' of a clay's previous bending is a reality. Will avoid it when transporting the slab to work station after rolling. Will look in the silicon sand, too. Probably trying different clays might help (some more warp-able than others). Appreciate all the suggestions.

[oldlady]

you might want to contact Marcia Selsor about the way she handles the large slabs she uses in raku.  not that you want to do raku, but the way she prepares the slab when she does the horses that are a part of her work.   look at her website and here in albums to see the results.