[Clay Sparrow]

So, I haven't done this just yet, but I'm preparing to within the next few days. I have a few pieces waiting to be fired at a paint-your-own pottery shop, but the person I've talked to told me that creating a slip mold from fired goods would not be possible. Is this true, or does it depend on the nature of it? For example, one is vase-like, and I had planned on laying the bottom half in clay, casting the top, and repeating the process for the bottom half, so it would be a two. Does it have to be greenware for this (she also said after casting, the piece within would be destroyed), or can it be ran through the kiln and cast afterwards? If so, does it need a release compound?

[bciskepottery]

Yes, you can make a plaster mold from a fired object; did that this past Friday -- plaster mold from a glazed bowl. Yes, you will need a release compound -- I used Murphy's Soap.

If you make a plaster cast from a piece of greenware, you should not fire it. You likely will get plaster into the greenware and it could cause lime popouts during firing. More than likely the greenware piece will be damaged/destroyed while removing it from the plaster mold.

[Clay Sparrow]

Thank you for your help! I wouldn't have been so worried about it, had I not put such a great deal of work into these pieces. Will much detail be lost when coating the vessel to be cast with a release compound? Some of the detail is pretty intricate.

[bciskepottery]

The Murphy's Soap should not cover up detail. Put on an even coat and let it dry, then do the casting. I've found I get better results when giving the release time to dry than when it is still wet.

[Lucille Oka]

A few things to take into consideration when making a multiple piece slip casting mold, you will need to make ‘keys’, and make a ‘spare’. Also depending upon the design, you may need 'sprues'. Make note there will be considerable shrinkage in size of your cast from the original model size.
Finally get a good book on mold making.

[Started Late]

Lucille Oka, on 09 April 2012 - 07:51 PM, said:

A few things to take into consideration when making a multiple piece slip casting mold, you will need to make ‘keys’, and make a ‘spare’. Also depending upon the design, you may need 'sprues'. Make note there will be considerable shrinkage in size of your cast from the original model size.
Finally get a good book on mold making.

What are 'sprues' please?

[Lucille Oka]

Started Late, on 09 April 2012 - 08:03 PM, said:

What are 'sprues' please?

Sprues are ‘channels’ that you attach to your model using small thin coils of clay. It is used in areas of the mold that may have air pockets. Sprues allow an extra bit of room for the slip to travel in the mold. If not used, the result may be an incomplete casting. Sprues must be cut off and fettled during clean up of the cast.

[INYA]

But making mold form a glazed piece is easier (at least for me) because there is no chance of accidentally destroy the piece. It is firmer and you can really grab it or make nice clean line between the parts without destroying the positive.
I make my molds from plaster (or plastic when the designs are too complicated) turned on woodturning mashine.

If you use glazed piece you advantage is also- that you can make many molds with it.

[Diana Ferreira]

I bought a couple of moldmaking books. the best one I found online, was by Sasha Wardell. Slipcasting

• ISBN-10: 0812219988
• ISBN-13: 978-0812219982