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What Clay To Use For Mold Making


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I've finally gotten to the point where I like the look of some characters I've created (round little fun animals) and I'd like to try making a mold of them for slip casting.  I'm a little lost as to how to go about it.  Do I use a different type of clay that doesn't harden to sculpt and then create the mold with plaster? or use regular clay and let it dry (or even fire it??) then create the mold.  I'd prefer to avoid firing the piece so that I don't have to worry about keeping it hollow and can just concentrate on the design.  any advice would be appreciated! or links to tutorials.  I've been searching but haven't found the right one.   The attached picture is an early design but gives you an idea, I assume i'll have to modify for molding to avoid undercuts.

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post-66966-0-62816500-1443800976_thumb.jpg

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I am not a molding expert, but when you have a form this complex you will be looking at multi part molds. For that to work well you will need some expert help. Look at the top left of this page and click on Ceramic Arts Daily ... Scroll to the bottom and you will find a link there to mold making tutorials.

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I prefer and would sculpt with #2 Medium Roma Plastilina or Chavant Clay.  Roma has Sulfur Chavant can be purchased without sulfur. For a plaster mold sulfur will not matter, but if you ever make a mold with silicone it will. I find the Roma type clays are easier to use and I end up with a better sculpture.

 

I sulpt with Roma and use the water based clay durring mold making for part lines. Water based clay removes from the Roma clay sculpture easier.

 

You can use water based clay for the sculpture if you like sculpting with water based clay. Water based clay is ever changing as you work with it, from wet to dry, some people like that. I find water based clays have too many issues for sculpting items that will be molded, it can crack and if it's dry, it's hard to add additional clay. For one of a kind sculptures and simple quick sculptures water based clay may be the way to go. You have to understand the limitations of each clay, the clay that you use is up to you.

 

The only reason that you would sculpt an object and then fire the sculpture before making a mold is to make a master for making multiple molds. This works great for simple items not so great for complicated stuff. It's a cheap way to make multiple molds, but you have to account for all of the shrinkage. If you make your mold with plaster, keep in mind plaster expands, try use the expansion to your advantage. With plaster you will have waist molds...When casting plaster on plaster or something hard like bisque chances are really good that something will break because of the expansion or get permanetly stuck..

 

I think the best way to make multiple molds, is to make and mold your sculpture, test the mold, then using urethane, cast the mold. The urethan mold can be used a bazillion time to make perfect plaster molds. Urethane is expensive and generally best left to those making molds for sale or heavy production of cast ware.

 

 

I would highly reccomend Andrew Martins book "Mold Making and Slip Casting" He does not really discuss molding super complicated pieces, but the information to lead you to that point is certainly there. My favorit part in his book is when they talk about molding a complicated piece, then modifying the mold removing undercuts until the mold works.  Complicated figurines are cast in multiple pieces, assembled leather hard, cleaned up. The high end really detailed figurines are additionaly sculpted and textured after they are assembled and cleaned up. Sometimes porcelain items are assembled, bisque fired at a low temperature then cleaned up, because porcelain breaks so easily in the greenware stage.

 

I can see how you could modify the sculpture, removing some undercuts to make a mold. I might try to make a mold part line right at the ears.

Good luck I hope this helps

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  • 3 years later...
1 hour ago, startaro said:

Can I use regular low fire clay to build up areas for mold making?

You can use any clay you have lying around the studio to make molds from; toss the clay once its been in contact with plaster. Not worth the risk of introducing plaster into the rest of your work.

If Im making a slip mold Ill try to avoid getting any oils on the plaster, which can cause negative issues when casting (use proper mold release, etc). Oil based clay when it is cold does not give off much of a residue, but the plaster will get hot as it cures, which could drive some of the oils off and into your mold surface. If your object was sculpted in wax/oil based clays then I might consider making a urethane/silicone mother, pouring a plaster master, and making your production mold off the plaster casting.

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To the OP'er. Unless you cant absolutely avoid it, trying to keep the number of parts to a mold to 3 or less is a good idea. Without seeing your object in real life, Id guess you have at least 2 parts per each paw, and another 2 per ear, plus 3-5 for the rest of the body. Thats a HIGHLY complex mold with a bunch of small pieces. Even just getting all those pieces put back together to make a single casting would be a challenge.

However, if you wanted to cast your cat in another material other than clay, you could make a mold off that quite easily. Using silicones/urethanes, and plaster you could make a mold in which you could cast different gypsums, acrylics, etc. Of course youd have to find different ways to decorate the surface as you couldnt fire those materials in a kiln to glaze them, but maybe thats not a big deal for you, or preferable to use acrylic paints to color?

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