Warping Of Platters

[AndreaB]

Hi everyone,

 

Can you help please?

 

Since my spine has decided to work against me! I've had to forgo the wheel and have spent about three weeks slabbing platters for 

Christmas presents 

 

 

These were done in porcelain with inlay.  The problem was the warpage. They were dried fairly slowly under plastic considering time constraints. I live in a hot country and do not have a storage room, I work outside, so they were still affected by the heat.  I've had this problem before, at home and at the studio I visit on a weekly basis.  They also do not have a wet room as such, pieces are wrapped and since the owner does not fire until there are enough pieces to fill the kiln the drying time is much longer, yet all my work, and I must say most of the other student's have the same problem.

 

By the way this happens with the stoneware pieces also.  How can I avoid this in future? I want to make a set of dinner plates as a house warming gift for my daughter

 

Many thanks

 

Andrea

[Roberta12]

Those are really lovely Andrea.  You have a very fine hand for that work!    Did they warp while they were drying, or did they come out of the kiln warped?    I have increased my odds of no warping by drying the plates and platters with bags of rice on them.  That keeps the middle from rising up.  I also make sure that the clay is not too thin....that helps.  Old Lady and others here on the forum make a lot of trays and platters.  I know they have some techniques that help!

 

Roberta

[glazenerd]

Wedge in 8-10% molochite: kill the plasticity. 

 

Nerd

[S. Dean]

Hi Andrea,

 

Lovely work.  Some more questions for you in addition to Roberta's:

 

What are you using for your mold (plaster, bisque, wood, plastic, other?)

How stiff are the platters when you are removing them from the mold? 

How are you forming/rolling your slabs?

How do you transfer the slabs into the mold?

How are you removing the clay from the mold?

 

 It could be a handling issue as clay has a memory. Minimizing handling and flexing the clay might help.  However, some porcelains are very finicky for hand building and warpage is the price to be paid for working with that clay body despite careful handling.

 

Please tell us more about your process and maybe we can try to offer some long distance advice.

 

-SD

[Marcia Selsor]

I think Dean covered it. They can warp from any uneven handling during the process. When I used hump molds i had the mold and slab on a board and could flip it over to place the sub upright onto a ware board.

[AndreaB]

Dean, here's the story

 

  after wedging roll out clay with rolling pin, center outwards between two sheets of cotton

  flip over once whilst rolling

  Using a window washing blade I rib both sides of the clay

  Using the sheet I drape the clay onto a bisque mold 

  Rib clay to follow form

  Once cheese hard remove from mold by flipping upside down always supporting the inside of the form,tidy edges using a sureform and smooth edge with a moist sponge

  carve design then apply under glaze  into inscribed lines

  Scrap to remove under glaze 

  pattern the edge whilst supporting from underneath

  Dry under plastic. as mentioned the hot climatespeeds up the process so that in about 2 days the platter is bone dry

  Candle in kiln form at least 4 hours at 40 deg C

  Bisque to 960 deg C with ramp of 150 deg C per hour

  Sand with 400 grit sand paper to remove any unwanted marks

  Glaze at 1260 deg C, ramp same as bisque

 

 

 

   The warping occurs during the glaze firing

   Your thoughts?

 

Andrea

[potterbeth]

Are the bottoms warped or the rims or both? Your process seems sound, and your finishing looks clean.

 

It doesn't sound as if you're warping them in handling, but it is easy to warp a piece like this if it is lifted before it is past leatherhard. If you are not already doing so, place a board on top of the piece in the mold, flip it, and pick the mold up off of the piece. If the center of the piece will slump when you do this, it may not be ready to unmold. If you need to accelerate the process, you can use a blow dryer to get the exposed surface of the piece to leatherhard while it still sits in the mold. If it is just a little too wet, you can use a small piece of foam to support the center of the form during the flip. Once it is out of the mold, sandwich between two boards to flip upright. Use this "pancake and flip" method until the form is too dry to warp.

 

What are they sitting on when drying under plastic? It needs to be an absorbent surface (I use drywall boards for forms like this). But if that surface becomes too damp, it will actually slow the drying of the bottom which can lead to warping on a flat, wide form. To avoid this, slide the piece to a dry surface that will allow the bottom to continue to dry along with the rest of the form. A single sheet of newspaper under the form will help with the moves (too much paper can wrinkle and mar the piece).

 

Two days to bone dry! Pretty fast. Rims can warp if they dry faster in one area such as near the edge of the plastic. You might need to wrap in more layers of plastic. I have students dry each piece like this on its own board with two layers of plastic wrapped completely over the piece and under the edges of the board. It does slow down the process, but they experience very little warping.

[bciskepottery]

Happens to both porcelain and stoneware, Andrea's and others . . . could be a firing issue -- maybe firing up too fast or too fast cooling.

 

With a large surface for the bottom, you might be getting uneven heating/cooling . . . try some coils or slats under the platter to allow for more even heating/cooling. Also, uneven kiln shelf surface could be distorting as the platter expands and shrinks.

 

Not uncommon to see this problem at glaze stage vs bisque due to differences in temperature and firing schedule.