Warped Piece

[sarahz]

Hello, I'm a self taught potter and recently been using a very large hump mold for a platter. I had luck once, but ever since everything seems to warp. My edges curl up and it doesn't stay flat. I usually let it sit for a good 5 hours so that it doesn't crack. It looks totally normal when I pull it off, but the next day after it's dried it warps. Any advice? I've attached a file of my glazed piece that turned out okay and a photo of the leather hard warped piece.

[neilestrick]

Clay shrinks as it dries. With platters and wide, low bowls, the lip will dry faster than the rest of the piece, so it shrinks and moves. You need to slow down the drying by covering the lip with plastic. In addition, you may need to slow down the overall drying time by loosely wrapping it in plastic.

[preeta]

how long do you keep it on the hump mold? what is the hump mold made of? plaster, bisque or wood?

[sarahz]

It's very large so I keep it on for about 5 hours so it can take the shape. It's made of plaster. I just pulled it off and covered it with plastic. Crossing my fingers!

[Babs]

How are you placing it after you take it of the mold.

You say you pulled it off, hard to get off??

Even drying, drying raised onto bats so air circulates underneath and all of what Neil said.

[MatthewV]

Warping happens because of the way you handle the clay. So you nicely rolled it out and put it on the hump mold. After you are done working on it, slide it around just a little to make sure the clay isn't sticking. Then wait awhile. I personally put it away in a slow drying cupboard for a day but your five hours should be fine too.

Now here is the step where people mess it up. You need to flip it out instead of lifting from two edges. Then do the rest of your cleanup work hopefully on a banding wheel or other means to avoid having the handle it. You can move it about using two full and open hands.

As long as the piece stayed round for its entire journey, the piece has no reason to become warped. Well, unless you dry or heat or look at one side too much.

[Mark C.]

If you warped it at all during forming it will want to go back as clay has memory . Keep it flat then press in mold and let dry enough so it stays in that shape. I like slump molds way better than hump molds for the reason that the clay shrinks away from mold as it dries. just keep in mind clay has memory. Also the right clay for this is important as is the drying as Neil said above.

[Marcia Selsor]

as said above, how you handle it during the making will be remembered. In forming , you may need to beef up the edge. Let it dry on the mold if it can move when shrinking.If the form was styrofoam with plaster shell, it would be light enough to flip off the mold. Keep thinking of ways to overcome the removal without distorting the form.

 

Marcia

[Dick White]

Another option is to make lemonade from the lemons. Instead of insisting to yourself that the rim has to be flat and straight as if it were a commercial platter, form the rim into a gently undulating edge with your fingers at soft leatherhard. Any warping will become part of the flow of the rim.

[sarahz]

Thanks all. I appreciate your feedback! I have always flipped it to get the mold out, but it's pretty heavy to lift out without bothering the clay. I'll glaze my warped piece and use it for my Thanksgiving turkey and think of you, Dick White!

[preeta]

a slump mold can be anything right? even a 5 gallon bucket as long as you are careful about pressing down the rim too much?

[GEP]

I make a lot of plates, trays, platters on hump molds. One thing I always do is take a flexible rib (a red Sherrill rib) and throughly burnish the slab against the mold, using fairly firm pressure. This erases any memory the slab may have had, and realigns the clay into the shape of the mold. I also leave the piece on the mold longer than five hours, usually overnight.

 

This is another process where practice and repetition also make a big difference. When I first started using hump molds, I made lots of warped plates. As time went by, the warping gradually disappeared. Not through any conscious technique changes, but I think I slowly learned how to handle slabs soundly and consistently.

 

I also think your footring is too small for that platter. If you make the footring wider, the rim will be better supported.

[Min]

Thanks all. I appreciate your feedback! I have always flipped it to get the mold out, but it's pretty heavy to lift out without bothering the clay. I'll glaze my warped piece and use it for my Thanksgiving turkey and think of you, Dick White!

 

If you make a lot of pots and find your mould too heavy to comfortably flip over it might be worth making a lighter weight one.

Piece of styrofoam insulation board, surform it to slightly smaller than you want the mould to be. Use profile ribs, just cut out one out from cardboard. Clean of all the dusty bits, raise it up a couple inches off the table (put it on a throwaway container) then slowly pour plaster over it. (plaster needs to be just starting to set when pouring). Jiggle it a couple times to smooth it then when nearly set run a rib over it. Can leave plaster or the edges or surform them down when plaster is fully setup. This one is 50cm X 20cm and weighs just under 4 lbs.

[sarahz]

Man, everyone is so helpful here. It's nice to get a chance to talk to other people who know what the heck I'm talking about. Thanks all!