Crumbling during bisque firing

[Carol B]

I’m a newbie at doing my pottery solo., without instructor. I have had a bottom of a mug and feet on another piece crumbling during firing. It doesn’t happen every time which leads me to thinking that it’s in the way I’m attacking. I slip and score and attach. The point is attachment is fine one at the seams, it’s more away from the slip and score. I Let dry completely. Bisque fire to 04 with a hold of 5 minutes in an electric auto Skutt. Any help is certainly appreciated.

[Bill Kielb]

5 minutes ago, Carol B said:

I’m a newbie at doing my pottery solo., without instructor. I have had a bottom of a mug and feet on another piece crumbling during firing. It doesn’t happen every time which leads me to thinking that it’s in the way I’m attacking. I slip and score and attach. The point is attachment is fine one at the seams, it’s more away from the slip and score. I Let dry completely. Bisque fire to 04 with a hold of 5 minutes in an electric auto Skutt. Any help is certainly appreciated.

If you can post a couple pictures that would help. From your description it’s hard to figure interface, parent material or attached item. Since it’s on the bottom, gravity usually keeps these things in contact during the firing even when not attached well unless these legs are configured and extend outside the plane of the mug enough to produce a twisting moment on the joint.

[Carol B]

Unfortunately I have already tried taking the feet off to salvage piece. But I’ve scraped up crumbles (pic) and show underside after scraping off remaining fit (pic). I cannot compress my files to attach. Will have to read up to do so.  Will be back when I can figure out.

[Carol B]

Oh it didn’t break at point of attachment but crumbled in the middle of foot that was not attached to bottom.

[Callie Beller Diesel]

Did you by any chance put the pieces in while they were still a bit damp on the bottom?

[Bill Kielb]

40 minutes ago, Carol B said:

Oh it didn’t break at point of attachment but crumbled in the middle of foot that was not attached to bottom.

Hmm, defective legs or the leg maybe was too thin to support during firing. One interesting rare thing comes to mind. When wares are fired they need to slide along the shelf as they basically contract. Likewise as the shelf is heated it expands then contracts when cooled, usually at a different rate than the pot. As long as the pot slides easily during this process, usually no issue.

Adding legs puts  the entire weight of the pot  on them. These turn out to concentrate the weight and friction in the shelf in the area of the leg or point load in engineer speak. Sort of opposite of snowshoes .............. or a pot without legs but lots of surface area to slide with.

so assuming the clay in the leg was not defective, they could have pulled themselves apart as the pot and shelf moved in competing directions. A solution might  be as simple as making heavier (larger cross section) legs or even placing some alumina on the shelf under each leg so it can slide easily.

Maybe an outlier thought, but something that strikes me as possible and hard to figure out if true.

[Carol B]

I think you are right about the feet/legs. The piece was a pretty good size and weight with several attachments on top and then feet on bottom. This weight probably added to the stress on the feet. The moving back and forth makes sense too if I’m understanding what you’re saying. If the piece is heavy then heavy duty feet are needed. And if not heavy enough they will not allow for movement during the firing?  Maybe more solid feet required. I think you’ve given me an idea to attach the entire surface of foot to pot. Wish I could compress my photos to post. Still working on it.

[Bill Kielb]

10 minutes ago, Carol B said:

I think you are right about the feet/legs. The piece was a pretty good size and weight with several attachments on top and then feet on bottom. This weight probably added to the stress on the feet. The moving back and forth makes sense too if I’m understanding what you’re saying. If the piece is heavy then heavy duty feet are needed. And if not heavy enough they will not allow for movement during the firing?  Maybe more solid feet required. I think you’ve given me an idea to attach the entire surface of foot to pot. Wish I could compress my photos to post. Still working on it.

No worries, hope it turns out to solve your issue. Don’t forget the alumina on the shelf trick. This is an old  proven cure for large platters that have a propensity to crack when they can’t move easily and smoothly. Platters usually  have a large foot area but are so heavy with respect to the foot surface area  that they are prone to this type of stress during firing and cooling.

[Carol B]

Thanks so much for the help. I will apply the alumina neat time. I may have fixed photo problem. 

[mnnaj]

If your piece is large, try making a waster slab of the same kind of clay.  Roll it out about the same thickness as the pot, let it get to the same moisture content and when you fire put the pot on the slab for both bisque and glaze firing.  Doing this has saved many pots.  I use wasters on flat bottom things all the time.

Nancy

[Carol B]

17 hours ago, Carol B said:

I’m a newbie at doing my pottery solo., without instructor. I have had a bottom of a mug and feet on another piece crumbling during firing. It doesn’t happen every time which leads me to thinking that it’s in the way I’m attacking. I slip and score and attach. The point is attachment is fine one at the seams, it’s more away from the slip and score. I Let dry completely. Bisque fire to 04 with a hold of 5 minutes in an electric auto Skutt. Any help is certainly appreciated.

Thank you for the suggestion. I will try this in the future. 

[oldlady]

carol, welcome to the world of clay!   my question is about the thickness of the total piece because you mention the word 'heavy".  is it thinner than your little finger? 

a lot of people work with very thick clay.   i do not understand the notion that hand built things SHOULD be a quarter of an inch thick or more.  that is not necessary and simply makes your work heavy and slow to dry thoroughly.  

if you can use Paint on your computer, you can shrink and  post a photo more easily.  if you take the photo with your phone, make sure it will come out topside up.

[Carol B]

Oldlady I roll my clay to 3/8 as a rule. I recently rolled out a slab to 1/4 and of course had it break before the bisque firing.

I can’t post my pics because my files are too large even when compressing them. I will need to get on my laptop and use a program to further compress. I mainly use my tablet and that’s what causes the problem, not like a computer with different programs. I’ll work on it and will post.

 

Thanks for the help.

[Bill Kielb]

20 hours ago, Carol B said:

Oldlady I roll my clay to 3/8 as a rule. I recently rolled out a slab to 1/4 and of course had it break before the bisque firing.

I can’t post my pics because my files are too large even when compressing them. I will need to get on my laptop and use a program to further compress. I mainly use my tablet and that’s what causes the problem, not like a computer with different programs. I’ll work on it and will post.

 

Thanks for the help.

Quick tip, if you are using an IPad  tablet: select your picture then look for the “choose image size prompt” that appears below. Select  and  then select small for an image size.. That usually reduces them enough and is quick and easy but hard to notice if you are not looking for it.
See below: