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I'm firing a cone 4 vitrified slip cast body which looks lovely but seems to slump particularly with the larger pieces. I'm going to try firing at a lower temperature but thought I would ask on the forum to see if anyone had any other suggestions. I'm using a fairly large electric kiln which I recently purchased. I also will try taking out the lower plug on the kiln as I heard that helps keep the temperature more even. 


Any advice would be greatly appreciated!



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@Harriet  Welcome to the forums.

Plenty of folks to help solve the issue; but info and pics help a lot. You already stated "vitrified at cone 4". So are we to assume this slip is rated for cone 4?  What type of slip? Porcelain, earthenware, etc? What do you mean by large?  Premixed or your own recipe?  Room temp at the time of casting?  Does it state a range of cone firings? 04 to 4.. For example.  We need info to help assess this issue.


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Thanks Tom!

The slip is from a clay/glaze book that I have that says to fire to cone 4. it's a medium temperature body sometimes called a soft stoneware. The largest size is 11" diameter by about 10" tall, room temp was about 55 degrees.  it doesn't state a range of cone firings. I am mixing it myself and am super precise about the measurements. 


Thanks again!

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Harriet: I did not have the time yesterday to analyze your recipe; this morning I did. ( all values in molar %)

cone 4  22% OM4,  20% Tenn.1, 58% Nep Sy.   (molar). KnaO 8.97.  Alum. 18.67.   Si 70.49.  COE (est.) 7.57

cone 4 mod. 22% OM4, 20% Tenn1, 40% Nep Sy, 18% EPK. KnaO 6.87,  Alum. 21.19, Si 70.05  COE (est.) 6.85

cone 6 standard porcelain recipe.  KnaO 3.94,  Alum. 20.41 Si. 74.08. COE (est.) 6.00


Direct Anaylysis  cone 4 KnaO 8.97. Cone 4 mod. KnaO 6.87. Cone 6 KnaO 3.94 ( molar) 

Alum.  Cone 4  18.67.  Cone 4 mod. 21.19. Cone 6 20.41. 

Edit note: the original recipe was all ball clay, which would put it in the stoneware or earthenware catergory. Molar levels for cone. 6 stoneware runs from 2.89 to 3.29% molar. In which case this recipe has nearly three times the KnaO levels of cone 6 stoneware.

I had mentioned in another recent thread that the lack of alum levels play a larger role in slumping rather than flux levels. Your original recipe had larger pieces slumping, and smaller ones in tact.  It is a good case study for alum vs. flux levels. The reason I posted in molar was to give a direct comparison to KnaO: your recipe was 8.97, and typical cone 6 was 3.94. Your recipe has over double the KnaO level of cone 6, although you are firing to cone 4. The alum levels were 18.67, and yet it still almost with stood over double the flux levels. 


Edited by glazenerd
Info added @ edit note.
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  • 4 weeks later...

Hi Tom and Mark,


Thanks again for for your help. EPK definitely helped with old recipe but still slumping. Just tried Tom’s recipe and it’s way better. I have another question- I’m making colored clay and the colors were much more vibrant with the old formula, they greyer and lighter with the new reciie. If I tried a white kaolin would that help? Was thinking of trying Groelleg or Tile#6 but wanted to get some expert advice. 


Thanks in advance!


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