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Slip Casting 2Ft Tall Bulbous Form

Slip- casting large forms pressure

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#1 rienerjm



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Posted 29 September 2013 - 11:24 AM

I am trying to slip cast a large form that is about 2.5 ft tall and 1.5 ft wide at its widest point. The slip is being drained by sucking out the excess, but when removing the form, one side usually caves in. I figure this is just from uneven pressure in the drying process and the weight of the shoulders on the form. I heard about a technique where pressure can be put into the mold after removing the excess slip so the walls stay pressed against the plaster.Does anyone have more info on this technique, such as how much pressure (in terms of psi)/ how long pressure should be applied, and how to successfully close off the pour hole so there is no air escaping? Thanks in advance!

#2 PeterH


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Posted 29 September 2013 - 02:02 PM



>when removing the form, one side usually caves in

Are you convinced that it is caving-in when you de-mould, and not earlier? If so, does the

shape of the mould prohibit you from simply letting it dry fully in the mould.


How thick are you casting your walls? Can you cast the bulk of the body thicker, thinning

near the neck if necessary for appearance.


If the caving-in is happening earlier in the process, would the structure be more self-supporting

if you dried it upside down? I've got a cast-off mould from a friend which seems to need this --

and letting it get really dry before de-moulding also helps. In my case this is drying overnight,

rather than for an hour or two.


If the collapse is occurring before or during de-moulding, filling with something like expanded vermiculite

to give it a bit of support might help (waiting until the casting is touch-dry). Empty when dry and hope any

residue doesn't leave a mess after firing. [A similar process using something like silica sand might help if

the form droops during firing. I believe people do this sort of thing, but don't understand how shrinkage is

catered for.]


Applying pressure to the mould during drying sounds fun, and why not apply it through the pour-hole?

But remember even at say 1 psi there are going to be big forces on a mould of that size. [I've never seen

a mould explode.]


Remember that "glugging" during emptying a mould can detach the casting from the walls. Are you

certain your slip-removal process is not reducing the pressure within the mould, even temporarily.


Regards, Peter


A quick google didn't find any refs to this pressure-during-drying process, and I would welcome some.

I've heard of using pressure during casting, and as an aid to aid de-moulding, but not during drying.



#3 Denice


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Posted 29 September 2013 - 03:30 PM

Is this a mold that you made?  I have found that if my mold is thinner in a particular area the clay will be wetter and cause problems when I unmold it.  Since I toss the first one anyways I just let it dry longer.  Denice

#4 Mark C.

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Posted 29 September 2013 - 07:36 PM

Since you are sucking out the slip and the form collapses during removal-I suggest leaving it in mold longer so its sets up more and has some strength-This is a simple fix.


Mark Cortright

#5 JBaymore



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Posted 29 September 2013 - 08:22 PM

Pressure casting is not done the same way that the slip casting you are thinking about / using is done.  One of the places I work in Japan utilizes this method.  The molds are make of specail dense plaster and have metal bands on the outsides, they are usually done in "gangs" of multiple molds that "feed" the slip to each other, and they are held in metal racks that clamp the molds to keep them from popping apart when the slip is added under high pressures.


The molds are specialized... the pieces are SOLID CAST ...... by injecting slip under pressure into a negative space left in the molds.  It is very, very much like lost wax casting of metal.  You'd need a mold that had a core form that was the INSIDE profile of the bulbous form.  This would almost for sure necessitate a more than one piece cast clay form that was then assembled after casting.


Try leaving the cast layer in the mold a bit longer to stiffen. 


If you can't get the excess slip out of the mold without flipping it... and the flipping action is what is causing the sides to collapse.... pump the slip OUT. 


OR.. ... design a casting mold with a drain on the BOTTIOM so that you fill it from the top with the drain plug in place..... and then pull the drain plug to drain the mold.  Patch the hole in the bottom later.





John Baymore
Adjunct Professor of Ceramics; New Hampshire Insitute of Art

Former Guest Professor, Wuxi Institute of Arts and Science, Yixing, China

Former President and Past President; Potters Council



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