Molding a full size bust

Hi Folks,

I'm working on my first larger (full scale) scale sculpture and have so far only ever made molds for something much smaller, and generally just a box mold. I've had a look around at tutorials, so I have a broad understanding of the techniques, but I'm hoping for some more specific guidance here if possible. I'm assuming I should be brushing on the silicone and making a shell.

Any advice would be greatly appreciated. Cheers!


  • For a sculpt like this you would most likely want a 2 part silicone mold with a rigid shell (mother mold) made out of something like fiberglass, plastipaste, plaster bandages, etc.

    The basic method would be to determine your case line, which would most likely be across the tops of the shoulders and over the crown of the head, and then clay up a wall along the back half of the sculpture (or use shims) to mask off half the sculpt.  Then you would add keys, mix up a thickened batch of silicone to fill in deep areas and undercuts to simplify the form, then brush on a few layers of silicone over that.  Then you would lay up your mother mold on top of that, remove your mold wall or shims, and do the same process on the second half making sure to add release so the silicone and mother mold of the first half do not bond to the second.  

    That is just a brief overview of the process skipping a lot of details and steps.  I suggest watching some of our lessons that feature mold making to learn about the finer points like keys, pry points, bleeders/vents, setting up a solid rim, etc.

  • Awesome, thanks Chris!

    What are the benefits of making the silicone in two parts rather than doing it as one piece, with the parting line raised up to a wall then cut with a mold knife, like in the image below.

    The clay wall part sounds the most intimidating, especially since the sculpture is in WED.

  • By making it in 2 pieces you can have keys integrated into each half so they line up properly.   With the method you showed it is possible to cut in a zig-zag fashion to give the halves a way to line up, it can produce mixed results and may not be as durable as the cuts can introduce tears later in the molds life.  If you are not looking to produce lots of castings and are ok possibly having to clean up seams on your casting in the event the alignment is not perfect you can totally go with that method.  For example, if you are just making 1 casting the time needed to clean up the seams is feasible, but if you need to cast 4, 5, or 20 you could be adding a lot of time down the line.

    The more advanced mold-making techniques often take into account the cost/time further down the line for multiple castings.  Sometimes budget, time, or other resources can dictate a simpler method like the example you provided.

  • Thank you!

    Although I have no grand plans for how many to create, as a learning experience I think I will go for what you've suggested and try a two part mold.

    I've got a premiere subscription so I'll look around. It seems like I'll need to gather the methods from a few videos.

    One I can see that matches is "How to make a latex rubber mask" by Timothy Martin. He's making a stone mold for latex, but it's a similar size and it looks like the method for how he sets up a wall is relevant to what I'm doing. 

    A question I have in regards to that is the keys he makes are those big angular wedges. Are those the types more suitable for when it's going to be a stone mold? The ones I've seen for silicone seem to be either circular impressions, or long tracks that go all the way around. The larger ice cube shaped ones on the outside of the silicone mold to allow the plaster mother mold to lock in. Is that right?
  • There are 2 main types of keys for this style of mold.  The keys that help the silicone halves join together, and the keys that help the silicone seat into the mother mold.   Both can have a wide variety of shapes.  I usually do the angular/ice cube shapes for pretty much everything.  Sometimes if there is a small area in the mold where I want to add a key I'll do a round impression to create a hemispherical key.  There's no specific number of keys needed, usually, you want as many as possible to help keep everything perfectly aligned and prevent pieces from slipping, sagging, or otherwise falling out of alignment.


  • Cheers Chris, you've been so helpful.

    One last question: What's the best way to seal the WED clay for making a silicone mold? I'm going to go with the same "white clay" Timothy used in his videos for the wall, and I'm assuming I want the WED sealed in some way to avoid that joining.
  • A common method for sealing WED clay before molding is to spray it with a couple of coats of Krylon Crystal Clear.

  • OK, so bit of a disaster. I won't go into what went wrong, but here's what I have. It's fine, I know the detail is captured.

    My question now is related to the surface of the mold, and whether I need to work to smooth things out for the plasti paste shell? I'm thinking I need to build up more of a ridge for the seam where I'll be making the cut (got a mold knife that will create the key as it cuts).

    I painted on those raised sections running perpendicular to the main seam to work as keys for the mold jacket. I'm thinking I need to make the seam larger.

    Any tips of where tyo go from here would be greatly appreciated. 

  • Slight update  - added more thickened silicone to raise the seam and "keys".

    It's really messy, but I'm not sure how much that matters. The plan is to build a clay wall tomorrow to make the shell. One piece for the back then two pieces for the front.

  • The outside needs to be as perfectly smooth as possible so the mother mold can fit flush to the silicone.  Any bumps might not align properly and distort the silicone, or cause the silicone to lock onto the mother mold preventing them from being separated and re-seated properly.  This would result in distorted castings.

    What may help is mixing small batches of thickened silicone and troweling it on to fill the deeper areas and even things out.  A lot of the roughness can happen as the silicone starts to set up.  After a certain point, you have to stop working the silicone or it will not smooth back out again.

    I would suggest shaving off any of the rough edges or bumps with a razor or sharp knife first, then trowling on the small batches to even things out.  Same for the keys, you may be able to shave those into smooth shapes so they can work properly.

    It's all a process, and the learning never stops!  Keep at it, and I look forward to seeing how everything goes!

  • Cheers Chris, could to know it's salvageable. I lost some sleep thinking about that ice cream covered abomination in the other room.

    The mold knife is great from trimming down the excess, so I can shape out the seams and keys into cleaner forms, then trowel on thickened silicone.

    I wonder if I could use foil, seran wrap or soemthing to help create an average of the surface for the plaster to form to
  • Any material. (like foil, etc) added to the outside could be subject to move and would create voids between the silicone and the mother mold during casting.  Those voids would collapse when the mold is full of casting material, resulting in deformations in the casting.  The mother mold's purpose is to hold the inner silicone mold firmly along its entire surface so the surface can not slip, fall away into the cavity, or bulge outward under the pressure of the casting medium, so it needs to be a perfect fit to avoid deformed castings.

    You can totally save this, but it will take patience and a good amount of effort.  First I would shave and trim everything down as much as possible, then I would go in small sections with small batches of silicone.  For example, starting on one half (or smaller) of the front side of the face, then the other half, then do the same for the back of the head.  The goal is a smooth, almost machined look with no pockets or air bubbles between layers, no undercuts that would cause your mother mold to lock onto it, and clean keys that align the same way every time and can socket into the mother mold firmly.

    You got this!

  • Oh wow, hadn’t realised I had to be so precise. 

    Here’s where I’m at, which suddenly feels way off. 

  • Wow, awesome. progress!   You're not far off at all (like 98% there), there are a few small lumps here and there that you can smooth out (like some of the rough spots on the key in the bottom right of the back) but you are very very close to where you need to be.  It does not have to be glass-smooth or perfect, just as long as there are no lumps that lock the keys into the mother mold or (or prevent them from going back into the mother mold after being taken apart) and no irregularities that prevent the mother mold from seating firmly with the silicone.

    You're in the home stretch!

  • It worked!!!! Kinda!!
    Turns out the first batch of resin I used may have been a bit old, so there’s that two tone color thing going on. I’ll be priming and painting so that’s fine. 
    So first thing I tried to cast in was the smooth on duo matrix plaster/resin thing. I went for a hollow cast and it didn’t survive the demold, too brittle and the mold needs some tough love to get the thing out. Next one, which is what you see, is resin on the outside then filled solid with hydrocal.  
    The problem with the mold is that the bottom is completely open, so trying to do any rotational cast is tricky and leaves the bottom section by the opening very weak. It happened here since the resin and hydrocal don’t bind, and the layer of resin is thin near the opening. That can be fixed though. 
    Next time I’d go for the neomatrix on the outside then fill with hydrocal. 
    Either way, I’m really happy with it. It’s a really clean cast. It needs really minimal dremel work in a couple of areas. The seam is super clean, no raised areas, just a small fringe of resin that peels away easily. 
    Perhaps there’s a way I can temporarily seal the bottom when casting in order to be able to get a more consistent hollow cast. 

  • Nice!   Glad you are getting castings and things worked out!

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