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Clay Sparrow

First time making a slip mold

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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?

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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.

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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.

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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.

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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?

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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.

 

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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.

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I think my question was answered here but I want to be positive (newbie!)

I have a piece of bisque that I want to create a slip mold from. If I glaze fire it will I then be able to create a mold from it? I am assuming the bisque is still too porous in its current state to create a plaster mold from. 

Thanks for any information! 

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You can always cover your bisqued piece with something like wax before making a mold of it.  The wax would fire out. so you could glaze it later.

 

If you glazed it, then made a cast of it, the mold would include the detail of the glazing

 

Of course the pieces you made from this mold will fire smaller than your original - because the mold is the size the slip will take, which will then shrink as it dries and fires.

 

When making an original for a mold, sculptors typically make it from something easier to use like a plastic or wax.  Why use clay with all of it's idiosyncratic shrinkage and warping problems.

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You can make a mould from bisque, but like all the comments above say, you MUST use a mould release.  You can buy soft soap from a ceramics supplier, or use oil-soap.  If you glaze your master before taking a mould from it, you could lose detail - you might then want/need to carve that back into the plaster mould.

 

My first choice for a master would be damp/wet clay, which doesn't need a mould release, nor does plastic, oil-clay or fruit and veg!  I can attest to the damp clay and plastic, the others come from "the clay lover's guide to making molds".

 

If you use damp/wet clay, you make your master the size as you would any other clay pot, and the finished pot will of course be smaller.  If you make your master from fired clay, the resultant slip-cast pots will fire smaller than your master.

 

Good luck, I'm half way through carving a block of plaster to make a press mould.  Didn't have any new plaster to make a mould, having to make the best of the "left-over" plaster from a previous mould.

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To address a few of the questions raised on this thread:

I find it best to make molds from pieces that are inert, such as a bisque clay piece or plaster. A mold can be made from a wet/leatherhard clay piece but the moisture from the setting plaster will often deform it.  This becomes especially challenging when pouring a multiple part mold. 

Yes, a bisque piece has shrunk but only 1-2% if you fire at 06 or cooler. 

When casting products for potters I prefer they bring me a bisque piece. I then seal that piece with clear urethane. After a day, or two, I go ahead and cast a mold.

The nice thing about casting from bisque is that it gives you the freedom to make mistakes. If your parting line is not as tight as you like you can repour the offending part. 

take care

Jeff Longtin

Minneapolis

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