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I'm relatively new to slip casting, but I just cannot figure out how to use 2 piece bowl molds.  Each time I try, it either comes apart in two pieces or dries completely and shatters in the mold.  I pour the mold, trim the spare when the shine is gone, and flip the mold over.  What am I doing wrong?

Edited by Kammymckenna
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If nothing comes out when you flip the mold over, then you might not be filling the mold up completely. If the walls are hollow that would explain why you are getting two halves instead of a solid bowl.

You need to screen or thoroughly mix the slip to remove the lumps. You may also need more water in your slip if it is not able to run all the way into the mold before it dries at the base, blocking you from adding enough slip to fill the mold.

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19 hours ago, Kammymckenna said:

Am I supposed to dump the slip?  I never have extra to dump.  Is the casting supposed to come out with negative space between the walls of the bowl?

No, a space between the walls is not normal. Indeed if you use the normal "drain casting" methods there is no way for this to happen.

What you seem to have is a mould that relies on "solid casting", in which both the inner and outer layer of your bowl are cast against plaster, as in this plate mould.

If you just fill a mould like the bottom one with slip you are in trouble. The original slip shrinks as the mould draws water out of it, leaving air holes.
So you must keep topping up the mould with slip until the solid-cast portions are completely full of drying clay. (Sometimes the mould is designed
to hold all the extra slip required, in which case you don't need to top-up.)

Solid casting a a bit trickier than drain casting: it's more demanding of the quality of slip, and usually has concave sections that you need to de-mould
as quickly as possible (and no sooner!) to avoid them sticking in the mould. This web page describes the process:

Regards, Peter

PS Here is a video of somebody filling a solid cast plate mould. Note the fluidity of the slip, and the ease with which it passes through the caster's sieve.

Of course you cannot just dilute your slip to that consistency with water, it has to have both the right clay:water ration and state of deflocculation.

Sorry, cannot get rid of this dratted image/attachment, please ignore.


Edited by PeterH
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