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Potterylover

Bisque hump mold problem

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I don't use bisque vessels as molds, but see if you have any undercuts which are preventing the removal of the cast. Try putting water in the bisque hump mold and wait a bit (about ten minutes), then twist the cast off. Prehaps you can start researching plaster casting.

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are you sure the clay is dry enough to release? If there are no undercuts then leave it longer before trying to get it off. You could try laying strips of damp newspaper on the mould before putting the clay on it, the paper certainly won't stick to the bisque and you'll be able to release the clay easily.

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Try dusting your bisque mold with corn starch before pressing the clay on it/in it. Or dusting your clay slab with corn starch before draping it on the mold. If using a bisque bowl as a hump mold, cut your clay 1/8th to 1/4th of an inch above the rim . . . that gives you a bit of room for lifting the form off the mold and prevents undercuts where the edge of the clay wraps over the edge of the mold.

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Thanks to all that responded. Very helpful! This morning, I see two cracks on the sides of the clay bowl and could lift out the bisque mold. I'll try your suggestions!

 

I'm going to try to repair the cracks in this one though.

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Those cracks are there forever so you are better off just starting over.

 

When you use a bowl as a hump mold there is always the problem of the clay shrinking as it dries and the resulting stress makes the bowl crack because the mold won't. One way to get around this is to remove the pot when it has dried enough to keep its shape. Another option if the bowl shape is a bit more open is to loosen the clay pot when it is cheese hard by gently pressing on the edges. Once it pops up a bit you can leave it to dry a bit more but you always have to be aware of the fact that the clay is shrinking and getting tighter. This also happens with cup shapes that you wrap around a cylinder. These need to be taken off almost immediately or they never come loose.

 

If you don't want to move the piece before it is totally dry then you have to give it a surface it can shrink on. I sometimes use the cotton batting quilters use as a buffer between the clay and the mold ... that way there is some give and the bowl can shrink without cracking.

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IIf I use bisque bowls for a hump or slump mold I do one of 2 things. Wet newspaper and apply it to the mold, or roll my clay on plastic wrap and then do my forming with the plastic wrap side down.

 

If using as a hump mold the plastic method is the better alternative because it has more tensile strength and you can get it removed at a stage before it shrinks enough to crack, then use the female,slump portion of the piece to give your work support for the rest of the drying process. This prevents wall slumping on bowls and other such items because they have the support of the original form.

 

Kathy

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IIf I use bisque bowls for a hump or slump mold I do one of 2 things. Wet newspaper and apply it to the mold, or roll my clay on plastic wrap and then do my forming with the plastic wrap side down.

 

If using as a hump mold the plastic method is the better alternative because it has more tensile strength and you can get it removed at a stage before it shrinks enough to crack, then use the female,slump portion of the piece to give your work support for the rest of the drying process. This prevents wall slumping on bowls and other such items because they have the support of the original form.

 

Kathy

 

 

I would often use bisque hump molds with students made from bowls that I had thrown on the wheel. I would let them place their clay over top of the mold, get everything tight, and trimmed at the base, and then either put it on a heat duct from the univents in the Winter, or in Spring and Fall use a heat gun to warm the bowl from underneath. In about 15 minutes, the clay would be able to separate when on the heater, heat gun maybe 5-8. Once out of the form they would store them until the next day with the piece setting on its rim. This often allowed two pieces to be made for one hollow form to be assembled when leather hard. I like it also to allow wet clay distortion once the piece is assembled.

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I have some large forms I use only as slumps because I don't have a good idea how to keep the humped ,soft form holding together when I take it off the hump before it starts to shrink. Anyone have instructions for how to manage that part of the hump process when using larger forms?

Willowtree, how do you get the form from ouside ,on the hump, round to inside the same form, the female side, if it's soft?

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With large forms you have time to watch and wait before they dry too much. When the form starts to set up spread both hands over it and gently move it just enough to release from the mold. Then watch and wait again until it sets up enough to hold its shape when you lift it. I don't quite understand why you would then put it in a slump mold rather than just making it in the slump mold to start with ... Is there a pattern or something?

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I have some large forms I use only as slumps because I don't have a good idea how to keep the humped ,soft form holding together when I take it off the hump before it starts to shrink. Anyone have instructions for how to manage that part of the hump process when using larger forms?

Willowtree, how do you get the form from ouside ,on the hump, round to inside the same form, the female side, if it's soft?

 

 

I agree with the last two posts. In order to use molds effectively as hump molds you need to be able to watch and `wait`until the form is just stiff enough to flip over onto a board without rim distortion and if it is large or oval -oblong you need another board to sit on top to prevent warping.

 

A heat gun is another option but it has to be used carefully or there can be cracking due to uneven heat.

 

Kathy

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With large forms you have time to watch and wait before they dry too much. When the form starts to set up spread both hands over it and gently move it just enough to release from the mold. Then watch and wait again until it sets up enough to hold its shape when you lift it. I don't quite understand why you would then put it in a slump mold rather than just making it in the slump mold to start with ... Is there a pattern or something?

 

 

 

Chris, I make sets of plaster molds for my larger (16-20 in) pieces from the `Slump-Hump`commercial molds. I also carve the hump part of the mold so it needs inverting. I tend to make my pieces on the hump mold first and then invert them into the slump mold for covering and drying. Saves distortion and cracking on those large slab platters and ovoid forms because they dry to almost bone dry in the slump plaster. If it is a bisque bowl I normally just use the inside curvature and let it set up in the bisque bowl.

 

Not sure whether you were asking me the question or not. I have just lost two many of those long platters in the past due to my own careless drying that I just bit the bullet and pour an innie and an outie when I pour plaster.

 

eg: http://www.flickr.com/photos/stonewear/5845038165/in/photostream/

 

fishplatter

 

 

 

kathy

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I have some large forms I use only as slumps because I don't have a good idea how to keep the humped ,soft form holding together when I take it off the hump before it starts to shrink. Anyone have instructions for how to manage that part of the hump process when using larger forms?

Willowtree, how do you get the form from ouside ,on the hump, round to inside the same form, the female side, if it's soft?

 

 

I agree with the last two posts. In order to use molds effectively as hump molds you need to be able to watch and `wait`until the form is just stiff enough to flip over onto a board without rim distortion and if it is large or oval -oblong you need another board to sit on top to prevent warping.

 

A heat gun is another option but it has to be used carefully or there can be cracking due to uneven heat.

 

Kathy

 

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Wobbly slumped, or humped bowls that are hard enough around the rim, but whose bottoms are not totally trustworthy, may be placed upside down on a non-absorbent board, with a home made pillow support under them. The pillow can be made of styrofoam peanuts contained in a kitchen trash bag. Then, plastc wrap the entire project, rather loosely, until the bottom becomes more reliable.

 

A slumped piece may be placed back into the forming bowl, with a plastic liner between the bowl and the mould; wrap the plastic over the edge, so that the bottom may firm up. Somtimes a humped bowl, with texture, will fit into the concave side of a mould if the curve is shallow.

 

 

Another alternative, say you have optomistically applied a footring that threatens to indent a not-too firm bowl bottom, is to put the bowl, bottom down, into the pillow support, cushioning the whole piece, and then wrap an Ace bandage around the rim to keep that secure until such time as the project has firmed up properly. Wrap loosely as before.

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Slump mold are so much easier to deal with as you can walk away and let the clay dry as it shrinks away from form.Hump molds require constant attention as the clay will crack on the form unless you get it off just right. Consider making a slump mold from your hump mold and that way next time it will go smooth.

Side note-

slump forms usually take more plaster than hump forms.

My 2 cents.

Mark

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