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  #1  
Old 04-02-2008, 03:13 PM
Wannabeartist Wannabeartist is offline
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Making a Plaster Mold

I have recently got into sculpting and I did 2 pieces using wax-based clay. I am thinking of making a plaster mold of one of them and casting wax into it - making a candle.

Here are the pictures of the sculpture

My fears and questions are:
1- Jitters - I am totally new to this.
2- How do I make a mold from plaster - no idea. I chose plaster because its cheap material. any good links to instructional material?
3- I was thinking of making it a 2 piece mold (right and left). Is that OK?
4- once I get the molds to dry do I need to treat it with anything - I read about soap?
5- Will I need a release agent so that I don't break the wax when demolding?
6- I was thinking of pouring the wax from the bottom but then how do I get the wick inside to come out of the top? Do i drill a hole and then fill it up with wax?

Thanks for your time.
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  #2  
Old 04-05-2008, 09:03 AM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

After no responses and my growing impatience I decided to go at it alone with a different sculpture. In my view, the other sculpture needs only a 1 piece mold. So I read up on internet sources and did it myself. I placed the procedure in my blog here.
I don't know how the results will be yet but I'll find out in a day or two.

By the way how many days should I wait until demolding???
I'll keep you posted with the progress. I am sure my experience will help all the newbies like myself out with this.

Last edited by Wannabeartist : 04-05-2008 at 12:17 PM.
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  #3  
Old 04-05-2008, 10:24 AM
dilida dilida is offline
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Re: Making a Plaster Mold

Hey Wannabe! Looks like you are off to a good start, both sculpting and attempting molds. I don't have a lot of experience with plaster molds without a rubber jacket under them, but I do know with plaster you don't have to use a box. You can just wait till the plaster starts to thicken, and then just scoop it on with your hands. (brushing a first coat on is good though). Make sure you get ample thickness over protrusions and smooth it off level if you want it not to rock when inverted. I don't worry about that, though, I just prop the mold up if I need it level. I pull the mold off as soon as it cools. Not very long at all. This topic has been discussed quite a bit here, try using the search feature at the top, I know you will find a lot of good info, and consider learning how to make a rubber mold first, followed by a plaster mold, then undercuts aren't such a problem.

Good start!
lisa
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  #4  
Old 04-05-2008, 11:48 AM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

Thanks Lisa,

That "without a box" idea is quite true now that I think about it. I figured that the simple plaster would take much longer to harden so I needed a "mold" for it. Thanks for the tip.
I'll also do a more extensive search on this site. For my other sculpture of the woman blowing a kiss I will probably need to do a rubber mold since it has many undercuts. Any suggestions for good brands of rubber molds that may be cost effective (or rather not too expensive)?
Thanks again.
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  #5  
Old 04-05-2008, 12:32 PM
dilida dilida is offline
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Re: Making a Plaster Mold

I use a one-part silicone, GE adhesive construction grade sealant, many others use a two-part brush on rubber. Silicone is the best, and so it is expensive. Once again a search of this forum will provide much info. The companies Polytek and Smooth-On have very informative websites, and another artist on this forum likes a rubber called Quantum (I think). Different rubbers work differently with what you are casting your artwork in. Some materials are not compatible with each other, so do your research. My experience is limited mainly to silicone rubber casting in wax.

lisa
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  #6  
Old 04-05-2008, 03:42 PM
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ahirschman ahirschman is offline
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Re: Making a Plaster Mold

Wannabeartist, I had no time, and still don't, but I want to give you a bit of advice before you go on.

The mold you have made may work OK, but the next mold will probably not work. Any mold with undercuts (when the mold material can get trapped under or in a part of the original) needs to be made either extremely carefully with a rigid material (very difficult) or must be made with a flexible material so that the original will release.

I have used many plaster waste molds, where you make a plaster mold on top of a soft sculpture (clay?). Once the mold hardens you can remove (Destroy) the original sculpture and pour plaster into the plaster mold. You then carefully break apart the mold to release the new sculpture.

A very good release for plaster on plaster is "Murpheys soap." You apply several lightly diluted coats, and wait for them to dry, then pour your plaster in.

Another good bit of advice is to add some type of coloring to the first thin layer (That you normally splash onto the original sculpture). Having a thin colored layer allows you to slow down with your chisel and hammer when getting near the new sculpture and avoids damage.

If you are going to attempt a rubber mold (highly recommended) then go and read this nice tutorial at Polytek (Brand I use and like) and go with something like 74-20 or 74-30. You can cast most resins into those rubbers and you can also cast plaster and almost any thing else (With the proper release, such as 2300).

Good luck, and take your time. You can always make a small test mold out of any thing around before going at your sculpture.

Ari.
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  #7  
Old 04-06-2008, 10:39 AM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

Ari,

You seemed to have a premonition of what was coming. My sculpture is locked in the mold. I used no release agent. I understand from what you are saying that I need to destroy the sculpture (made of wax-based clay) and then cast plaster into the mold after I treat it with murphy soap oil. The plaster cast will replace the sculpture.

Is there another way???

If not, I think the best way is to slowly heat the clay so that it melts out of the mold. Is this correct?

I should have done it as Linda mentioned - without the box.

I documented the process with pictures on my blog here.
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  #8  
Old 04-06-2008, 09:46 PM
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ahirschman ahirschman is offline
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Re: Making a Plaster Mold

I am sorry, but I do not have much experience with heating of oil based clay. You do need to remember that heating plaster above a certain point will weaken it, or destroy it. Plaster, when it sets, binds with water molecules. Heating the plaster enough causes the bond to break, and the plaster goes back to its pre-hardened state (I think). That is one reason that plaster is such a good fire blocker (the trapped water bit).

Any way, with patience, and more patience, you can probably save the piece. clean, out the old sculpture leaving a void in its place. That could take a long time. I destroy my originals a lot of the time, and as long as the mold comes out OK,I do not care.

Once you have removed the original then treat the plaster with a few diluted coats of Murphys oil soap, letting it dry between coats. Now, this release is used if you are going to cast another plaster sculpture into the plaster mold.

What type of plaster are you going to use for your final piece? Molding plaster is quite soft, and breaks easily, so I do not recommend it. I have used it and it is OK, but just OK. Do a search and you will find a lot of info here about plaster, or go exploring at US Gypsum. They have a lot of info. Landseer, another member of the forum has lots of info on plaster/plaster content on many of his posts.

Once you cast your final sculpture you will have a hard time separating the sculpture from the mold. You will have to carefully chisel the mold apart being very careful not to damage the sculpture. You may consider adding a dye to the inside of the mold before applying the Murphy's soap. This may or may not work. As I mentioned before, I used to add coloring to the first layer of plaster (Very thin) which would alert me when chiseling away towards my sculpture.

Keep in mind, that no matter what happens you will have learned a lot more than you could have by just reading. When you now read about mold you will be doing so from a vastly improved perspective.

Your second piece should probably use a rubber layer before the plaster or you will have to work very very hard to get the original out.

Good luck again.

Ari.
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  #9  
Old 04-07-2008, 01:14 AM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

Thank you Ari,

As usual your advise is very informative and helpful.

I am thinking of maybe casting concrete into the plaster mold instead. Since this a one-piece mold which is open on one side, I figure I can effectively pack in the concrete. In addition, it would be easier to break away the plaster mold since concrete is harder.
Moreover, my next piece (which will be a mermaid), I will want to make a two piece mold plaster and rubber mold and use a special concrete mix (which I again have no experience with). Therefore I figure that the experience I obtain with this piece will assist me in the next sculpture I plan on making.

Any advise on concrete???
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  #10  
Old 04-07-2008, 12:38 PM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

I was able to demold by heating up the plaster with the wax clay at low temperature (~110C). The whole process with pics is here.

I will treat it with Murphy soap oil as you said if I decide to mold it with plaster. I still think I should go with Concrete though. Would I be able to capture the small details of small sculptures in concrete? My guess is that it depends on the make-up of the concrete mixture. Any advise on this? I tried to look it up on the forum but couldn't (as of yet) find info.
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  #11  
Old 04-21-2008, 11:54 AM
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sfp sfp is offline
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Re: Making a Plaster Mold

I have a book I wrote on silicone moldmaking if your interested. It's Book 1 in a 4 book series I'm working on. It isn't on my website yet but I can print them myself in a pre-release plastic binder state.

-Mike
www.sureformprototyping.com
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  #12  
Old 04-21-2008, 11:00 PM
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amport amport is offline
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Re: Making a Plaster Mold

http://www.plastermaster.com/usg/
Has some great free information on plaster and plaster/cement mixes.
I use Hydrostone. When mixed and dried properly it is very hard.
However the surface is so hard it is difficult to sand.

This is what they have to say about it.
One of the hardest and strongest of all gypsum cements, HYDRO-STONE® is recommended for producing high-quality novelty and statuary castings requiring extremely hard surfaces. This product is self-leveling when poured and not suitable for hollow cast applications. HYDRO-STONE® must be mechanically mixed for best results.
You can find it here:
http://www.plaster.com/HYDROSTONE.html

I do not think you want to use concrete for castings this small. The aggregate would serve no purpose…….I think.
Good luck.
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  #13  
Old 04-22-2008, 02:02 PM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

Thanks for the websites and the replies. Mike, any info would help.

I am currently in a state were I cast the same plaster into the plaster mold after I treated the mold with 2 layers of dishwashing soap and then a thin layer of Vaseline. I couldn't locate any Murphy's Soap Oil in Israel.

Now I have a block of plaster that seems to be locked in place. I tried to get it out and now I started breaking away the mold but it is an arduous task that does not seem like its going to work since the cast and the mold seemed to have fused.

Am I missing something? Is this supposed to happen? I captured the casting process here. Any assistance would help. - S.O.S
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  #14  
Old 04-22-2008, 03:07 PM
Stevem Stevem is offline
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Re: Making a Plaster Mold

When ever I make a plaster mold I use dishwashing liquid and brush it in the mold to a good lather and then use a clean brush to brush out the foam. I then repeat this process. After that I usually spray a little bit of pam cooking spray into the mold and brush that around as well to coat all surfaces. This makes a nice release for me. When trying to de-mold the cast I will usually tap the mold all around the cast to break the static seal of the cast and mold. Once I develop a line around the actual cast I use an air blower and my compressor to blow air between the mold and cast. The only time it hangs up is if there are any undercuts. If there are any undercuts it will hang up tight and then you will have to chisel away the mold. This is best done by creating a waste mold as mentioned in earlier posts. I have made plaster piece molds that you can work around the undercuts but that involves making plaster shims and then making the mold to cover them. That is a much more involved process and that is why we now have rubber molds. All I can suggest is to keep diligently chipping away at the mold with the utmost care and not to get discouraged. Even if the piece breaks it's plaster. It can all be fixed. Even when I do a waste mold it's not uncommon to chip or break a piece. But with a little dry-wall spackling or more plaster you can fix all the imperfections. Look at it this way. A plaster cast is usually an intermediate material to work out all the imperfections of the final product anyways, so there is usually expected to be some touch up work.

Best of luck to you,
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"Success is a little like wrestling a gorilla. You do not quit when you're tired -You quit when the gorilla is tired. -Robert Strauss
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  #15  
Old 04-25-2008, 04:03 PM
Wannabeartist Wannabeartist is offline
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Re: Making a Plaster Mold

Steve,
Thanks! Took you great advise - "All I can suggest is to keep diligently chipping away at the mold with the utmost care and not to get discouraged".

Did just that and I was always to pull out the sculpture with some damage which I will fix up.

I captured the process here.
Again - Thanks a bundle.
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