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Textree

Choosing a clay for slip casting big pot

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I've been kicking around of making a bonsai pot for a year or two. I don't have experience but Iv e done some research , found a place to fire it, and a ceramic store in town for supplies. 

I'm not skilled in pottery but I am pretty good with tools so I decided to make a wooden version the pot, use it to make a plaster of Paris mold, and pour a casting slip I prepare into it. 

My question is about the clay. The store has armadillo clays 2 or 3 are non absorbent enough at cone 6.  And a few other types and brands. They have a cone 5 porcelain (which can be fired to cone 6) , something called buffalo wallow with and without grog. They also have another brand of porcelain VPP (very plastic porcelain). If I am just turning them into slip is one harder to use than the other?  Is one more or less prone to breaking in the kiln? If it matters the pot would be big , like 18 inches by 15 by 5. The amount of plaster to make the mold, the amount of clay ill need, cost to fire the pot and glaze will make for an expensive project. Id prefer to pick something that I'm likely to succeed with. I like the idea of smooth vitreous porcelain but if its not going to work I can live with the other

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I decided to do this with half of the stoneware type , half of 213 porcelain and add some kyanite. I read up on mixing and deflocculating the slip. It seems like the difficulty is letting it dry very slowly. I have a garage freezer not in use so ill put it in there. 

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There are casting slips available that you can buy ready to go. I believe Standard Ceramic is one source. Otherwise, you can take any commercial body and make it into a casting slip. If you can't buy the clay body as a dry mix, then you'll need to buy moist clay and let it dry, then break it into the smallest pieces you can before making slip with it.

If you're just planning on making one or two of these pots, I would slab or coil build it. A mold that large is a big undertaking, and IMO not worth the effort for one or two pieces, especially if you're new to mold making and slip casting.

Here are my calculations for the volume of plaster you'll need, for #1 pottery plaster:

Your piece is 18x15x5 = 1350 cubic inches

If you make the mold 2 inches thick all around, that's 22x19x9 = 3762 cubic inches.

Subtract the volume of the piece from the volume of the outer mold, and that leaves 2412 cubic inches of plaster to make the mold.

You need 16.312 grams of plaster per cubic inch so 16.316 x 2412 = 39,354 grams of plaster. Divid that by 454 grams/lb and you get about 87 pounds of plaster. That's a big, heavy mold. You'll need almost 6 gallons of slip to fill it, so that's another 70 pounds or so added to the weight of the mold when it's full.

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That's almost exactly how I did that math so I'm on the right track. Thank you . like you, I figured out it will take 3 or 4 bags of plaster and was thinking 75 lbs of clay. The reason for the mold would just be I think I could make a nicer finished product that way. Whereas if I built it, it would look more amateurish . thanks again I might scale it down 

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