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Plates flopping after glaze firing


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I've been struggling with this for a while. My dinner plates look fine after trimming and bisque firing. Then after the glaze firing a lot of them flop over and I get a bump on the inside around the outside of where the footing is underneath. Sorry if that doesn't make sense, I can't really get a photo of it.

Basically after drying the inside is a smooth curve, but after glaze firing the inside has a bump in the curve. 

I throw them to 12" from 2kg of clay. After trimming they are around 0.8 - 1kg, after glaze firing they are 700 - 800g. The foot ring is trimmed to 8" at the widest point. I try and trim all areas to the same thickness.

There doesn't seem to be a relation between weight and warping, even some of the thinnest ones are ok. 

Any ideas? Maybe more clay, wider foot ring? 

Photo is good one on right, flopped on left

 

 

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Yes wider foot ring

If the plates are slumping because the foot ring is not supporting the curve (I cannot see in you photo what is going on) Then its a wider foot ring. If the foot ring is fine and the plates are trimmed to thin then its more clay. 

You can decide or show us a bottom and top photo of the slumped plate looking down at it and I can see whats happening -I assume cone 10 as well?

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Other thing I would look at is how thin the clay is in the area I pointed to with the arrow below. If this is too thin the rim will slump. I'ld try break a couple slumped ones open and look at the profile. 

594929629_ScreenShot2021-08-13at11_55_08AM.png.58631502c777a20c4845ae5bad0c5342.png

 

 

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Thanks @Mark C.  Would you think wider like thicker footring, or wider like further to the outside? These are cone 8.

@Callie Beller Diesel thanks, I could try that but wouldn't that be more if the centre was slumping? If anything the centre is lifting a bit, maybe it's slumping in the centre first in the firing I guess.

@Minok I'll look into that

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1628933461528.jpg

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I think and this is a guess at this point in the foot ring needs to come out just a little and make them a bit thicker as well.

I throw porcelain plates and fire at cone 10 without movement . I do have a small nubbin in center for slumping and they are thicker so they do not move-they are in my photo album if you want to see them.-

At last weeks Art show I had 3  eight piece plate sets(glazes differently)  with salad plates-sold one set.They bought matching bowls in two sizes as well. Biggest sale at show for me. I tend to stock lots of dinnerware as most potters do not.

Edited by Mark C.
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Looking at your pics, I think the answer is in the shadows. The last pic shows a shadowy ring that does not look like a trim ring. I would say as others have said that you need a thicker plate, and possibly a wider foot ring, maybe even two if you are going to trim thinly.

 

 

best,

Pres

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On 8/14/2021 at 5:39 AM, tomhumf said:

These are cone 8

What is the recommended cone range for you clay body ?  If you're firing at the upper end, maybe firing just a tad cooler would stop the slumping, but still be 'mature enough'.  

Could be that there's some interaction between clay & glaze that is lowering the melt point of the clay - especially if, as others have suggested, it's a little on the thin side.  I was doing some testing of local clay several years ago, and fired identical pieces side-by-side, with one un-glazed, and others with various glazes, and a couple of the glazed ones slumped badly, when the un-glazed piece didn't move at all.

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tom, it looks as though you are using a black clay.   haven't i been reading the sad stories of how difficult it is to work with black clays?  didn't the consensus agree that firing to cone 5 was better than cone 6?   

are you having the same problems as those folks?    yes, they are in the US and you are in the UK but what do you really know about your clay?

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Posted (edited)

Hmm, it's not black just fired in reduction and the lighting was bad in those photos. I guess the glaze could be having an affect...

It's the clay on the image, my probe doesnt go much above 1200 so I think it should be ok in theory.

https://www.valentineclays.co.uk/product/stoneware-special-fleck

I'm about to fire some more with a slightly wider and further out foot ring, will see what happens with those. Thanks 

 

Screenshot_20210817-143005.png

Edited by tomhumf
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19 hours ago, tomhumf said:

my probe doesnt go much above 1200 so I think it should be ok in theory.

Just a thought,  if you are not using cones then for this clay and this shape and whatever cone this was fired to, it might tend to slump. I would confirm with cones what this is being fired to and also at some point probably throw some low flat lids just to get an idea of it’s limits. Lots of porcelain bodies I won’t fire more than an eight inch span without significant crown in them to keep them held up. Might just be structural and good ole gravity, but confirming with cones is always a good thing IMO. Cone 6 reduction, also interesting!

Edited by Bill Kielb
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  • 1 month later...
3 hours ago, tomhumf said:

I've just seen this, I do use cones and get to cone ^8  but not much above 1200C usually, maybe my thermocouple is in a cold spot - it's above the burner port so probably.

Cones measure heat AND the effect of heat over time so the temperature on a thermocouple is not nearly as important in clay as: what cone did you make it to?  One can fire to cone 5 with a forty  minute hold and make it to cone 7 overfiring their cone 5 clay and glaze yet never exceeding 2167 degrees (1186C) . In this case the thermocouple was right but all my stuff slumped and was ruined.

Edited by Bill Kielb
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