Clay as a positive mold for a hydrocal sculpture?

[Daniel Smith]

Hi all, 

New here as a ceramic arts daily member, but i have been browsing this forum for great info for years! 

I am currently trying to make an outdoor sculpture for a project I am working on in architecture school.  I am thinking about using Hydrocal® White Gypsum Cement to create my sculpture because of its durable qualities and hopefully  easy to finish nature (please feel free to advise me if I am right or wrong on these, I am new to working with the material).  

The question I have involves casting the Hydrocal to create the sculpture.  I am thinking about creating a hollow clay coil mold for my sculpture, into which i would pour the hydrocal mix to allow for it to conform to the coil mold's interior shape and form.  I would pour-cast this sculpture as the clay mold (laguna bmix) would be in the leather-hard shape.  To limit the weight of the final sculpture, I would want to let the outer portion set up and be able to pour a significant amount out, so the final hydrocal sculpture itself is hollow too.  

Does this sound like it would work?  I am worried that the hydrocal would stick to the clay mold, or that it would be too thick a consistency to pour out to make it hollow.  

I hope that this makes sense to you all.  Please respond with whatever feedback you think that i need, with whatever information you could offer me.  I appreciate all the help! 

-Daniel 

[Dick White]

45 minutes ago, Daniel Smith said:

I am worried that the hydrocal would stick to the clay mold, or that it would be too thick a consistency to pour out to make it hollow. 

Daniel, welcome to the zoo. I will address your questions in the order asked, though the answers are more relevant in the reverse order. 1) Hydrocal (and other plasters) will not stick to the clay mold. The clay can be peeled right out and any remaining residue can be washed off. 2) Hydrocal is a plaster-like material. It does not dry in the mold like casting slip, allowing you to pour out an excess from the interior of the mold. Rather, it sets by chemical action and that action is simultaneous throughout the wet plaster. The whole thing will harden all at once.

[neilestrick]

If the sculpture is of any size, I would not rely entirely on the strength of the clay to hold the wet plaster. Some sort of external support may be necessary. I mostly agree with Dick on the setting up issue, however there is a chance you may be able to get it at just the right point where the plaster is starting to thicken but can still be poured out, leaving a sufficiently thick layer in the mold. Alternatively, depending on the shape of the piece you may be able to stick something into the middle to create some negative space. Being plaster, even thickness and all that precision that is required with clay is not necessary, so you could just use a bunch of rolled up paper.

[JohnnyK]

Hey, Daniel,

Since the H-cal sets up chemically, you might consider making a clay plug for the area that you want to hollow out. You could build the plug around an armature of the appropriate size and suspend it from the lip of your mold (maybe a tapered cone shape plug), then pull the plug before the h-cal sets up hard...or wait until it sets up and then scrape it out of your piece.

JohnnyK

[Rae Reich]

But definitely build your clay form thickly and with inflexible exterior support for it. The plaster in such large quantities (even with center-void form in place) will be very heavy and will try to escape through any weak spot. Don't know how intricate the sculpture will be, but simple forms are best to start with. 

How tall/wide will your piece be?

[Daniel Smith]

Wow!  Thanks so much everyone for the advice, it all sounds great.  I am excited about the project.  

For supporting the clay mold during the pouring process, i have been advised by a former ceramics professor to either build a box and put the mold in it with sand around, or wrap the mold in a stretchy cellophane to counteract that outward pressure.  

 

Thanks again! 

Daniel 

[Daniel Smith]

4 hours ago, Rae Reich said:

But definitely build your clay form thickly and with inflexible exterior support for it. The plaster in such large quantities (even with center-void form in place) will be very heavy and will try to escape through any weak spot. Don't know how intricate the sculpture will be, but simple forms are best to start with. 

How tall/wide will your piece be?

Rae, 

The piece will be approximately 13" high, 15" long, and 9" thick.  

[Mark C.]

I see a few issues with the clay form not supported comes apart when trying to turn off to drain.

Heres an idea-build the clay form and put it in a wood box which is screwed together t o come apart. Put some clay walls to the form and they go to the box wall. Pour plaster (any kind will do as this is just for support) between clay form and box walls-The clay walls will  make separation easy when unscrewed.Pour your mold and then you can flip the box dumping out what you need without touching you form(encased in plaster and then clay.

When set take box apart break away plaster-peel clay and you have your Simi solid hydrocal form

I hope you can visualize this process??

[Rae Reich]

6 hours ago, Daniel Smith said:

Rae, 

The piece will be approximately 13" high, 15" long, and 9" thick.  

Then your surrounding form should be twice that, except for the pour end or side. 

I like @Mark C.'s idea (which might be original - I've never heard of it!) but it will then be very heavy so "flip the box" will need helpers and thoughtful positioning. Maybe "tip the box" on its side will do.