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Hi all,

My slip cast porcelain mugs are warping. The clay body is purchased dry from Laguna clay and I mix to 1.8 SG with their recommended deflocculates. I've been trying to troubleshoot this issue for 4 months. I've even left the body of the mug in the mold until bone dry and then placed directly in the kiln without attaching a handle or cleaning the rim - it still warped.

Here are some other things I've already tried:

  • Drying slowly and evenly over a week
  • Carefully removing from mold such that they are still circular before bisque
  • Casting thicker walls
  • Compressing the clay with a sponge and rib on a wheel while still in the mold
  • Tracking placement in the kiln

Anyone else have other recommendations? If I used a stoneware slip would that solve the problem? Warping always happens in the glaze kiln, not in bisque. It is not always warped in the same direction (ie. not always slumped from the weight of the handle). I know porcelain is notorious for warping and the shape of my mug isn't helping, but I figure there must still be a way. 1 out of 10 come out circular. 9 out of 10 look like the photo.

Thanks for the help!

Warp 1.jpg

Warp 2.jpg

Warp 3.jpg

Edited by Bryan D
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Add 5% EPK (dry mix) to a test before wet mixing and since you are testing do a 10% EPK. This will stiffen the body just a bit. Maybe enough -cast a mug with handle on each test and see if the 5% or 10% works.

I assume this is cone 6 as you did not say-just a guess

 

This will stiffen the clay body just a bit and is an easy cheap fix.  I have done this with cone 10 porcelain laguna slip dry body myself years ago with great results.

Edited by Mark C.
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3 minutes ago, Mark C. said:

Add 5% EPK (dry mix) to a test before wet mixing and since you are testing do a 10% EPK. This will stiffen the body just a bit. Maybe enough -cast a mig with handle on each test and see if the 5% or 10% works.

I assume this is cone 6 as you did not say-just a guess

Use grolleg or nz if you are trying to preserve some sort of translucency, otherwise EPK is great and cheap

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3 hours ago, Mark C. said:

Add 5% EPK (dry mix) to a test before wet mixing and since you are testing do a 10% EPK. This will stiffen the body just a bit. Maybe enough -cast a mig with handle on each test and see if the 5% or 10% works.

I assume this is cone 6 as you did not say-just a guess

 

This will stiffen the clay body just a bit and is an easy cheap fix.  I have done this with cone 10 porcelain laguna slip dry body myself years ago with great results.

Mark and Liam - thank you! I'll try both 5% and 10% this week and post results. I'm using a cone 5 body.

 

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EPK is a cheap way to get alumina into your body  and that will strenghten it and the clay will not affect . I used 15% in my cast body long ago which the form  had  a huge hole in side and it slumped. Thast why the 15%. It fixed the issue  and was a easy fix. Your mug is not that warped so 5-10% may be enough .

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Bryan sometimes warping especially using porcelain is caused by kiln shelves with marred or uneven surfaces. Your mug might “catch” on a chunk on your shelf which inhibits the piece’s movement during the glaze firing. I had this problem with cone 10 reduction firings using porcelain and it was solved when I bought advancer kiln shelves. Anyways look carefully at your shelves first before you start monkeying around with the slip recipe. And good luck!

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  • 2 weeks later...

Being a slip caster myself I sometimes run into this.  From my experience, 30+ years, its mostly a process problem.  (A picture of your mold would help.) Placing a conical cup form into the casting, right after demolding, often reduces this problem. You can eyeball round but having an actual round cone form makes it much easier. Similar to a comment I made yesterday you can also minimize warping if you remove the piece from the mold and THEN cut the rim. My cup forms come out of the mold with the sprew still in place. I place the clay form on a banding wheel and cut the sprew with an exacto knife. I then place the cone form back into the cup and allow it to set up for awhile.  

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(Similar to a comment I made yesterday you can also minimize warping if you remove the piece from the mold and THEN cut the rim. My cup forms come out of the mold with the sprew still in place. I place the clay form on a banding wheel and cut the sprew with an exacto knife. I then place the cone form back into the cup and allow it to set up for awhile. )

This would be pretty slow for production casting .

When we had hundreds of molds going we designed the spew to be longer (you must do this always anyways) and cut the top inside spew with a plastic slip knife.(which does dot injuge the plast as much. Then demolded the piece and finished the cut with a sharp metal blade many at a time. This mold was a three-piece mold. We where making thousands of aroma therapy lamps for a decade

The  fill hole in the front was also the spew. The top is cupped to hold the oils the bottom was flat.These where all cone 10 porcealian.Still have the masters if anyone is interested.

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Thank you all for your help. I’ve tried 5, 10, and 17% EPK. 10% seems to have corrected most of the warping issue. 5% was not enough. I’m currently bisquing a larger run of 40 mugs and should know definitively by the end of the week if 10% EPK is the solution. Although now I think I have a bit of a glaze fitment issue and it seems like EPK is causing more pinholes even in heavily deflocculated slip (more testing soon).

 

Jeff – thanks for the suggestion. You found trimming the lip outside of the mold resulted in less deformed? The shear force from the razor didn’t cause deformation? Did you trim with the cone in place and what was the cone made of (plaster?)? 


Does deformation only happen when the clay is wet? Could a wet sponge on the lip of a bone-dry piece cause warping in the kiln? Would there be a difference between (wet) sponging the lip and using a fettling knife (dry)?


It took me a few months, but I’ve learned to reach into the mold and lift the piece up by applying outward pressure near the bottom rather than flipping the mold upside down. I can get the body of the mug into the bisque almost completely circular. I’ve seen others leave a conical piece from the casting on their mugs and then cut it off when it’s bone dry. I’d prefer not to add this extra step, but will consider once I’ve exhausted other options.

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Bryan clay has memory so it you push it out with a sponge or knife it can remember this and will later return to that form.

When we added 15% EPK it never dd a thing to our 4 glazes we used in terms of pinholing in reduction fires that where slow long fires (10-12hr) and slow cooling .

sponging bone dry will not affect any deformation unless its rewetted and moved shape .

Remember not to cut on plaster with anything other than a plastic tool and even that can cut into the plaster..

Does your mold have a longer top spew at the top? or are you cutting the top right at mold level?

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Bryan, 

For my braille cups I use a little 8 oz. plastic container. (The kind sour cream comes in.) I leave the lid on the container so the thin plastic retains its roundness.  I work in a pottery shop where we make 700 pots a day. The folks who attach handles there like to use cork stoppers. (The 3" diameter variety.) Those work just as well.

Sometimes I trim, with the plastic form in place, sometimes not. It just depends upon how things are drying. Because I attach the handle, after I remove the clay form from the mold, I try to remove it as soon as possible.

I don't like to cut clay, against plaster, because it scratches the plaster and distorts the rim ever so much.  When you cut the clay, outside of the mold, you can cut it "with the grain" and minimize the distortion. When you cut it against the plaster you're pushing it in one direction. (The one sided distortion reappears after firing.)

I also leave the conical sprew on the piece until it comes out of the mold. I cut it, however, as soon as it starts to stiffen. (Soft leather hard, you might say.) After cutting the sprew I clean the rim with a sponge and then attach the handle. Once it becomes routine its not that slow and it improves the roundness considerably.

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