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Cracks In Rims


SarahJC

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Hello everyone,

 

I've been using a bowl I made for the past few weeks and it recently developed a vertical crack at the rim, and I am trying to figure out why. I'm a newbie, so please go easy if these are stupid questions! :)

 

It's a fine crack but goes all the way through the clay (see photos). It's a cone 6 clay, and it's glazed only on the inside with a pretty thin layer of glaze (over a white slip). Another bowl I made developed a similar crack, but after the bisque firing. This makes me think I'm maybe doing something wrong which is causing a weakness, which later leads to the cracks..

 

I don't have much info about the firing (I'm part of a pottery club and the studio manager does all the firing), except that the kiln has sometimes been misfiring and I think these bowls were a little under fired. Other things I've made haven't cracked.

 

The two reasons I can think of are - I'm throwing them too thin for this type of clay, which is not a fine clay. Or, when I trimmed them I attached them to the wheelhead with a little water, not with coils of clay. I wondered if I wasn't very careful with that, and combined with the thinness of the bowl it damaged the rims somehow. Could either of these be the reason, or something else? I don't think it's glaze fit because the other bowl cracked at the bisque stage.

 

Thanks for any advice!

 

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You've done a pretty good diagnosis . . . the thin rims/wall and stress fracture from trimming that does not show up until firing. Chances are the crack was there at bisque but just not noticeable. Another factor could be glazing only the inside; not that that is a bad thing, but the glaze may be creating an unequal tension with the slip/clay body that results in delayed crack or dunt.

 

If your look is for thin rims/walls, rather than trim on a flat wheel, center a large lump of clay that is rounded at the top and put your bowls on the hump so the edges are above the wheel rim. The soft clay will hold them in place for trimming. Or use a bisque chuck with a coil of soft clay on the top rim to cushion the bowl.

 

As to firing, put a note in your pots and ask that they not be stacked for bisque firing -- that could cause a stress crack later on.

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The crack could be from picking up the pot by the rim when it is being handled after the rim has reached leather hard or drier.  Rims on green and bisque ware are not normally designed to handle the stresses that occur when the pot is lifted by a single point on or near the rim. 

 

a good rule of thumb is to use two hands and lift from the bottom not the rim. 

 

LT

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Here's a question, because I can't tell from the picture: is your glaze crazed at all? If it isn't, this may be your problem. If this crack was not apparent at the bisque stage, and it "rang" after being glaze fired as well, you could have a glaze fit issue. That is to say, if you're your glaze actually fits the clay body properly and exerts pressure on the pot as it should, it may be pulling itself apart if the outside is left unglazed.

I had similar cracking issues when I was using a tightly fitted glaze as a liner on some pots, and a not-fitted-at-all shino on the outside. The cracks tended to drift into a "u" shape.

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  • 1 month later...

I like the red and white stripes in the first pot.  In addition to the other suggestions, I would add that rims are under more stress than any other part of a pot because they get handled, bumped, and cracked in the making of the pot, then in use after it is made.  If you make your rims thicker, they will be stronger and subject to less stress.

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