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Slip casting globes- form?


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#21 Marcia Selsor

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Posted 02 July 2013 - 11:44 AM

Super markets sometimes sell balls in kids area. I live near the beach...maybe that is why.
I have used glass forms for half spheres. Mold release really necessary.
I like kids balls about 10" diameter. There are usually bins of several sizes.

Good luck finding what you are looking for. I lived in Montana and it was difficult sometimes.

Marcia

#22 perkolator

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Posted 02 July 2013 - 01:50 PM

buy kids balls for playing (like bouncy balls or even pool/beach balls) and fill with expanding spray foam to make rigid. might even work with a heavy-duty mylar balloon - call a balloon shop they may have round balloons in various sizes and you can make them rigid.

#23 Kohaku

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Posted 02 July 2013 - 07:46 PM

Thanks for the ideas everyone.

This will be my first time slip casting. I have three balls to work with, and plan to make molds tomorrow.

It occurs to me that all of these things float. I was going to use a cottle and pour plaster up to the meridian... but I'm suddenly worried that the balls are going to float skyward on me as I do this. Any suggestions?
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#24 OffCenter

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Posted 02 July 2013 - 10:06 PM

Thanks for the ideas everyone.

This will be my first time slip casting. I have three balls to work with, and plan to make molds tomorrow.

It occurs to me that all of these things float. I was going to use a cottle and pour plaster up to the meridian... but I'm suddenly worried that the balls are going to float skyward on me as I do this. Any suggestions?


Oh yeah, we forgot to mention the hard part and the reason you should have bought a bowling ball. The balls will have to be held down in the plaster. Place the ball where you want it inside the cottle boards leaving at least an inch between the bottom of the ball and the table top, then mark where the middle of of the ball is on the ball. Have a way to tie the ball down after you sink it into the plaster or weights ready to put on top of the ball (hard to balance but doable), or be prepared to hold the ball steady for 5 minutes for the plaster to set (not easy). Pour plaster then sink ball to where you marked the halfway point then tie it down or weight it down or hold it down until the plaster sets.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#25 Marcia Selsor

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Posted 03 July 2013 - 07:43 AM

Hmmm
I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.
Spray with WD-40 and pour the plaster around the upper half.Turn this part upside down and cast the other half.

I never had much success trying to hold something down in plaster.

Marcia

#26 OffCenter

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Posted 03 July 2013 - 07:57 AM

Hmmm
I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.
Spray with WD-40 and pour the plaster around the upper half.
I never had much success trying to hold something down in plaster.

Marcia


Damn! I wish I had thought of that! I can almost hear my dad saying, "There's the right way and there's Jim's way."

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#27 Kohaku

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Posted 03 July 2013 - 08:19 AM

Hmmm
I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.
Spray with WD-40 and pour the plaster around the upper half.
I never had much success trying to hold something down in plaster.

Marcia


Glad I saw this before forging ahead.

How about the second pour? Once you flipped and removed the sand/cardboard, how would you make sure the ball stayed put during pour #2? I was things of forming a pouring spout from clay and using this to hold the ball in place as I poured...
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#28 Marcia Selsor

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Posted 03 July 2013 - 10:28 AM


Hmmm
I usually cut a circle in cardboard and put the ball in sand up to half way. seal any gap with clay at a minimum.
Spray with WD-40 and pour the plaster around the upper half.
I never had much success trying to hold something down in plaster.

Marcia


Glad I saw this before forging ahead.

How about the second pour? Once you flipped and removed the sand/cardboard, how would you make sure the ball stayed put during pour #2? I was things of forming a pouring spout from clay and using this to hold the ball in place as I poured...

that sounds feasible. make sure you carve the keys in to the first half before you pour the second half.Do you mean the slip reservoir or is this a pitcher.?
Marcia

#29 Kohaku

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Posted 03 July 2013 - 01:01 PM

that sounds feasible. make sure you carve the keys in to the first half before you pour the second half.Do you mean the slip reservoir or is this a pitcher.?
Marcia


Sorry about the fuzzy terminology- slip reservoir is what I meant. My goal is to get as close to an immaculate, make-a-mathematician-weep sphere as I can get.
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#30 DanMehlman

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Posted 23 July 2013 - 07:23 AM

HI- I'm a professional model and mold maker in the ceramic and glass industry and I would like comment on your question and the responses so far. I've made such molds before.

 

1- You can't beat a bowling ball for that size sphere. Borrow one or get one at a thrift shop. They are so round that you can rotate the ball and cast both sections from the same side of the ball, thereby avoiding the finger holes. Billiard balls are excellent for their size. Bocce balls?

2- If you are trying to make a nice clean mold, don't try to push a rubber ball down into wet plaster, you will trap air and get voids on the casting surface. Always pour plaster on top of a model so air bubbles rise away from the casting surface.

3-  Any kind of inflated ball will give you problems because it will float and possibly distort. Even if you can secure it down with clay, tape, etc. for the first section, it will float when you pour the second section. The one possibility for section #2 is to create a structure to hold it down from overhead and bury that in the pour hole model. If the ball is soft, this will likely create distortion and may still float enough to come out of section #1 a bit, ruining the mold. If the ball is rigid, you may get away with this system.

4-  If I had to make a mold from an inflated ball I would do this: set the ball up in a clay pad up to the seam line. Splash on the first half of a plaster waste mold. Turn it over, carve keys, apply release to the seam line, splash on a second mold section. There will be no floating issues with a layed-up mold. Open the waste mold, remove ball, seal and soap the mold, reassemble, and fill with plaster. Break the waste mold away. You now have a plaster reproduction of the ball, which will not float when you make the working mold.

5- Personally, I would (and I have) make a plaster ball from scratch, either by turning two halves upright on the plaster wheel, or by turning it in one piece on the lathe... but this takes experience and equipment that you may not have.

6- Contrary to advice on this thread, do not use WD-40 or any oil as a mold release, it will seal the mold and impede the slip casting. Plastic or rubber balls need no release at all. Use mold soap as a release on the seam line when you make section #2.

 

Good luck with the project

Dan

www.MehlmanDesign.com

 

 

 



#31 Kohaku

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Posted 23 July 2013 - 01:26 PM

HI- I'm a professional model and mold maker in the ceramic and glass industry and I would like comment on your question and the responses so far. I've made such molds before.

 

1- You can't beat a bowling ball for that size sphere. Borrow one or get one at a thrift shop. They are so round that you can rotate the ball and cast both sections from the same side of the ball, thereby avoiding the finger holes. Billiard balls are excellent for their size. Bocce balls?

2- If you are trying to make a nice clean mold, don't try to push a rubber ball down into wet plaster, you will trap air and get voids on the casting surface. Always pour plaster on top of a model so air bubbles rise away from the casting surface.

3-  Any kind of inflated ball will give you problems because it will float and possibly distort. Even if you can secure it down with clay, tape, etc. for the first section, it will float when you pour the second section. The one possibility for section #2 is to create a structure to hold it down from overhead and bury that in the pour hole model. If the ball is soft, this will likely create distortion and may still float enough to come out of section #1 a bit, ruining the mold. If the ball is rigid, you may get away with this system.

4-  If I had to make a mold from an inflated ball I would do this: set the ball up in a clay pad up to the seam line. Splash on the first half of a plaster waste mold. Turn it over, carve keys, apply release to the seam line, splash on a second mold section. There will be no floating issues with a layed-up mold. Open the waste mold, remove ball, seal and soap the mold, reassemble, and fill with plaster. Break the waste mold away. You now have a plaster reproduction of the ball, which will not float when you make the working mold.

5- Personally, I would (and I have) make a plaster ball from scratch, either by turning two halves upright on the plaster wheel, or by turning it in one piece on the lathe... but this takes experience and equipment that you may not have.

6- Contrary to advice on this thread, do not use WD-40 or any oil as a mold release, it will seal the mold and impede the slip casting. Plastic or rubber balls need no release at all. Use mold soap as a release on the seam line when you make section #2.

 

Good luck with the project

Dan

www.MehlmanDesign.com

 

Dan- that's really helpful- thanks.

 

I went on raku binge over the past couple weeks, and put this mold project on the back burner. I was gearing up to try with rubber balls though... and so appreciate the timely warning.

 

We do have a bocci set, actually. Unfortunately, the size I'm looking for is a bit bigger... so maybe a bowling ball is- after all- the way to go.


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#32 Marcia Selsor

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Posted 23 July 2013 - 01:42 PM

I have used balls for the beach. They come in various sizes and are solid enough to hold their form.
Marcia

#33 soilandpolish

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Posted 26 July 2013 - 06:57 PM

In fact on my last visit to our local pottery supply, the proprietor and I were discussing slip casting (I was wondering about using paper forms dipped in slip -- I've since seen some work done like that seaching online, for example http://www.australia...E/origami.html/).

 

She told me that she once dipped a hollow plastic ball in slip, poked a small hole in it, and fired it and it came out well, with just a bit of ash or residue near the hole. Thought I'd pass that on.

 

[edit: oops, I see Marcia already said that this works in the post right before mine, tried to skim them all before posting but missed that, pfft]






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