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Kohaku

Slip casting globes- form?

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I'm interested in slip casting some globes of varying sizes (ranging from maybe 4 inches diameter to possibly as large as 12 inches diameter).

 

Anyone have any suggestions for where to buy or find a graduated series of rigid spheres, ideally on the inexpensive side of things? I don't want to use my Japanese fishing floats!

 

I use globes for water features. I find that a perfectly spherical, closed form is pretty hard to throw, however... and deviations from the spherical (even small ones) don't work well for this application....

 

IMG_1551_zpsa48e4c4d.jpg

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How about balls? The inflated kind for children are pretty tough and very cheap. I just used a big one (about 15 inches in diameter) to cast a slump mold from which to make a hump mold. I remember someone using one to make a one-piece (or almost one piece) spherical slip mold by casting the whole thing in one piece with just a hole left in it to deflate the ball and pull it out.

 

Jim

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How about balls? The inflated kind for children are pretty tough and very cheap. I just used a big one (about 15 inches in diameter) to cast a slump mold from which to make a hump mold. I remember someone using one to make a one-piece (or almost one piece) spherical slip mold by casting the whole thing in one piece with just a hole left in it to deflate the ball and pull it out.

 

Jim

 

 

Jim- that was my first thought. I talked with a local potter who does a lot of slip casting, though... and he thought that the plaster would tend to deform anything inflatable. There are things like bocchi balls that are rigid... but it's generally hard to buy these outside of a set.

 

If anyone's used inflatable balls for this purpose, I'd love to hear about it...

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To work with an inflatable, try filling the cottle boards with your plaster and pushing the ball down into it whilst it is still liquid. Draw a line on the equator so you get it to the right place. That will allow you to see if the plaster does deform the ball, and an inflatable ball is only a few bucks. Or visit the local Goodwill store and get a selection of used sports balls. A baseball should be solid enough, and a billiard ball is solid. Maybe you could borrow a bowling ball?

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To work with an inflatable, try filling the cottle boards with your plaster and pushing the ball down into it whilst it is still liquid. Draw a line on the equator so you get it to the right place. That will allow you to see if the plaster does deform the ball, and an inflatable ball is only a few bucks. Or visit the local Goodwill store and get a selection of used sports balls. A baseball should be solid enough, and a billiard ball is solid. Maybe you could borrow a bowling ball?

 

 

I may have to try this. The options I've looked at online are all pretty pricey (for example- the duckpin bowling balls price out in the 40-100$ range). There are some online venders that sell plastic (polyethylene) balls- but you have to generally order these in lots of 500+

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Kohaku,

 

A search of www.ebay.com for plastic sphere returns;

 

- a variety of "plastic mould half sphere"s from 4" to 12"

which you might manage to use to create a registered pair of plaster half-moulds.

 

- a 1 gallon plus plastic fish-bowl (obviously not a full sphere)

 

 

Regards, Peter

 

PS you don't say if you need a full sphere, or how you are going to handle this in

the mould [e.g. removable plug to enable slip pour and partial drain; trim cast

hole and insert plug; invert mound to cast onto plug; remember to make a pin-hole

before firing].

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Do you have access to vertical turning lathe for plaster? You can make make 2 half spheres and build a mold from those semispheres.

 

If you do not have access to turning lathe, you can use your throwing wheel and quarter of a circle (cut it out from sheet metal, firm plastic, plywood? etc) as your "mold".

First you make a hump out of clay and then you keep adding thick plaster mix (not too runny) on top of it while wheel is spinning slowly. You need to make sure it's soft enough so your mold can scrape it easily to the proper shape. Adjust the plaster/water ratio accordingly and mix small amount of plaster.

 

huh.. let me draw you a picture smile.gif

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here it is

post-19541-137263175169_thumb.png

 

Clay is there to save you some plaster :)

 

PS! This vertical rod is important because it will help you to keep the "mold" centered.

Mold and rod are connected so that mold spins freely.

post-19541-137263175169_thumb.png

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How about balls? The inflated kind for children are pretty tough and very cheap. I just used a big one (about 15 inches in diameter) to cast a slump mold from which to make a hump mold. I remember someone using one to make a one-piece (or almost one piece) spherical slip mold by casting the whole thing in one piece with just a hole left in it to deflate the ball and pull it out.

 

Jim

 

 

This. I've never had an issue with balls deforming unless not properly inflated. I think the largest one I made was about 32" in diameter. Just make sure you put YOUR seam around the balls seam ... oh and fill the plug with clay. But in reality ... I just took a sonotube, cut my section I need for the total height, split it down the side so I could remove it later, and made a mold like I normally would (build up back side with clay around the ball, make key holes and a pour spout, pour plaster on top, flip and remove clay, inspect mold and patch if nessessary, pour second half after using release agent.)

post-25644-13726388002_thumb.jpg

post-25644-13726388002_thumb.jpg

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Do you have access to vertical turning lathe for plaster? You can make make 2 half spheres and build a mold from those semispheres.

 

If you do not have access to turning lathe, you can use your throwing wheel and quarter of a circle (cut it out from sheet metal, firm plastic, plywood? etc) as your "mold".

First you make a hump out of clay and then you keep adding thick plaster mix (not too runny) on top of it while wheel is spinning slowly. You need to make sure it's soft enough so your mold can scrape it easily to the proper shape. Adjust the plaster/water ratio accordingly and mix small amount of plaster.

 

huh.. let me draw you a picture smile.gif

 

 

Cool idea. Maybe a bit of a naive question... but if you form plaster shapes on a wheelhead or bat, does the plaster come off easily once it's set?

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How about balls? The inflated kind for children are pretty tough and very cheap. I just used a big one (about 15 inches in diameter) to cast a slump mold from which to make a hump mold. I remember someone using one to make a one-piece (or almost one piece) spherical slip mold by casting the whole thing in one piece with just a hole left in it to deflate the ball and pull it out.

 

Jim

 

 

This. I've never had an issue with balls deforming unless not properly inflated. I think the largest one I made was about 32" in diameter. Just make sure you put YOUR seam around the balls seam ... oh and fill the plug with clay. But in reality ... I just took a sonotube, cut my section I need for the total height, split it down the side so I could remove it later, and made a mold like I normally would (build up back side with clay around the ball, make key holes and a pour spout, pour plaster on top, flip and remove clay, inspect mold and patch if nessessary, pour second half after using release agent.)

 

 

Sure, unless the surface is not smooth. It's good idea to put some soap water on all the surfaces, especially if you are pouring the second half of the mold.

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How about balls? The inflated kind for children are pretty tough and very cheap. I just used a big one (about 15 inches in diameter) to cast a slump mold from which to make a hump mold. I remember someone using one to make a one-piece (or almost one piece) spherical slip mold by casting the whole thing in one piece with just a hole left in it to deflate the ball and pull it out.

 

Jim

 

 

This. I've never had an issue with balls deforming unless not properly inflated. I think the largest one I made was about 32" in diameter. Just make sure you put YOUR seam around the balls seam ... oh and fill the plug with clay. But in reality ... I just took a sonotube, cut my section I need for the total height, split it down the side so I could remove it later, and made a mold like I normally would (build up back side with clay around the ball, make key holes and a pour spout, pour plaster on top, flip and remove clay, inspect mold and patch if nessessary, pour second half after using release agent.)

 

 

Sure, unless the surface is not smooth. It's good idea to put some soap water on all the surfaces, especially if you are pouring the second half of the mold.

 

 

A release agent should be involved in every step of casting, whether that is thin fine mesh slip, soap, varnish, whatever. It helps. On play balls that are thick rubbery plastic, even when textured will flex and are made to not stick to a whole lot of stuff and have never had an issue with not using a release on the ball itself. Little bit of compressed air will burp it open from initial molding.

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A release agent should be involved in every step of casting, whether that is thin fine mesh slip, soap, varnish, whatever. It helps. On play balls that are thick rubbery plastic, even when textured will flex and are made to not stick to a whole lot of stuff and have never had an issue with not using a release on the ball itself. Little bit of compressed air will burp it open from initial molding.

 

 

I had an amazingly frustrating morning running around town trying to find balls. Goodwill and Salvation Army are a bust. Wallmart (where I hate to tread) had a few sports balls (basketballs and the like) and a couple hot pink barbie nerf specials. Off to Big Five and the local craft store next.

 

Really though... it's pretty sad when the biggest retailer in the nation barely carries a ball on its shelves (but 50+ varieties of play guns).

 

Gad- I sound like my grandpa.

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A release agent should be involved in every step of casting, whether that is thin fine mesh slip, soap, varnish, whatever. It helps. On play balls that are thick rubbery plastic, even when textured will flex and are made to not stick to a whole lot of stuff and have never had an issue with not using a release on the ball itself. Little bit of compressed air will burp it open from initial molding.

 

 

I had an amazingly frustrating morning running around town trying to find balls. Goodwill and Salvation Army are a bust. Wallmart (where I hate to tread) had a few sports balls (basketballs and the like) and a couple hot pink barbie nerf specials. Off to Big Five and the local craft store next.

 

Really though... it's pretty sad when the biggest retailer in the nation barely carries a ball on its shelves (but 50+ varieties of play guns).

 

Gad- I sound like my grandpa.

 

 

I thought they were pretty common. I picked up 5 big balls at a Kroger for about $3 or $4 each and used them for casting then forgot about them in an old falling-down storage shed where they were exposed to the weather and a year later dug them out and used them again. They were strong, very smooth and did not deform when used to cast plaster. The dog even played with one of them and wasn't able to burst it. Try grocery stores, drug stores, Toys R Us, etc.

 

Jim

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I thought they were pretty common. I picked up 5 big balls at a Kroger for about $3 or $4 each and used them for casting then forgot about them in an old falling-down storage shed where they were exposed to the weather and a year later dug them out and used them again. They were strong, very smooth and did not deform when used to cast plaster. The dog even played with one of them and wasn't able to burst it. Try grocery stores, drug stores, Toys R Us, etc.

 

Jim

 

 

Finally found a couple balls in the local drugstore, and a rigid, smooth foam ball at the local craft store. ($9.00 for a five inch chunk of styrofoam!). Three sizes to experiment with- good enough for now.

 

I live in bumbleputz, Idaho... so my options are limited...

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The styrofoam balls work well, I haven't done it but I watched someone make a sphere mold in college sculpture class. I believe it was acetone that he poured in the mold to melt the foam. Denice

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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

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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.

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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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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

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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

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