Because I messed up my plaster mold, I had to dig my sculpt out and redo the details. Cleaning the sculpt got me thinking about my puppet’s design, and I wanted some advice.
I plan to have my monster character with a separate head and body so that I can attach them later. However, I decided to do the “cut off” where the base of the head meets the neck, which I’m now thinking is too risky, especially if I’m casting this guy in silicone. What do you think? Should I redesign the head to include the neck and have the “cut off” where the base of the neck meets the body, or should I go with what I got? Please let me know what you think.
Hey Alzoman7! What happened to your mold? The clay build-up looked good, was it just a plaster layering issue? I think you should elongate the neck just a little so that the head has something to squish onto and create a seamless look. The head to neck area is a natural looking place to have a "seam." I think that would look better than having the head and neck in one piece attached at the chest (unless you were doing clothing that hides the neck bib). Great body sculpt, I love the angular choices and fat belly!
That's a bummer! I use UltraCal, which is a stone based material. I don't know if it's the same process, but I let each layer dry a little before applying the next one. That may be helpful or redundant! Can't wait to see how it all comes together.
Can I offer some advice on plaster?
First, use Ultracal, or Crystacal R or anything that describes itself as Dental Stone. Do not use plaster of Paris or anything that is weaker than dental stone.
Second, you need to ideally make each half of your mould with a single mix of plaster (there are ways round this...) The mistake most people who are unfamiliar with dental stone make is to have too much water to start with. You should use a clean container and always add plaster to water, not the other way round. The approximate ratio of plaster to water is 3:1 volume. So if you half fill your container with water, it will be overflowing before you have got anything like enough plaster in there. Put only enough water to fill about 1/4 of the container. If necessary get a bigger container!
Eventually you get good at estimating how much water you need for a given quantity of plaster. This is the way you mix it. Pour plaster into the water until it makes an island in the middle sticking up out of the water. Then add a little more plaster. Just let it soak up the water. You will almost certainly think you have added too much, but will be surprised when you start mixing. It is easier to add a (tiny) bit more water than it is to add more plaster, so err slightly on the too thick side.
Now mix your plaster. It should combine easily with the water if you have let it soak for a couple of minutes. The consistency should be that of yoghurt or somewhere between single and double cream. If the surface breaks with little cracks it is too thick. If it is very runny, too thin.
First task is to brush on a layer of plaster to the mould surface, making sure you get it into all the nooks and crannies. Have a bowl of water handy and wash your brush out, as set plaster will ruin it! Once the surface is covered - and you can blob plaster onto plain areas - then you can start to build up the mould. Get a layer about 1/2" (12mm) thick over everything. I like to bang the mould on the table to release any trapped air bubbles if it is that sort of mould, but this is often not possible with bigger moulds, or the mould may not move without the walls being disturbed. The next layer should have some reinforcement in it - either glassfibres or burlap (hessian). Make sure you do not press these down to the mould surface, as you want that to be the best it can be. Then just add more plaster and reinforcement until you have got a plaster mould thick enough - 1" (25mm) is good for a large mould. By this time the plaster should be starting to thicken up, and it becomes possible to smooth the final layer into place, creating a good back surface.
OK, so you are halfway through and are running out of plaster....Eek! No problem. Add a quantity of water (remember the 1:3 ratio) to the bowl with the plaster mix already in it. Then add the plaster, sprinkling it in. You will find that it doesn't do the absorption stage very well, and usually at this stage you just want to get the mould done, so go ahead and mix straight away. It will be a lot harder to get a smooth plaster, likely it will be quite lumpy, but this should not matter as it is the back of the mould. By mixing it in with the original plaster this helps to get it to set off at the same or similar time as the original. You should never build up in layers as one layer will be going off and expansion and contraction will take place at different times, causing problems within the mould structure.
OK, so once the plaster is done (and you have rescued that brush!) it starts to set up. This is a chemical reaction. The mould will get hot and then it will cool down again. Once it is cooling, it should gain its strength. Test a bit of plaster in the pot. It sets but crumbles at first, then after the heating it goes much more solid.
Drying a mould is a separate process. Let us assume you have done both sides of your mould and have prised them apart and very gently released your clay from inside. There may be a bit still stuck, in which case you can put the mould into the oven and warm it up a bit to soften the clay. Not too much or the clay seeps into the pores of the plaster. Once the mould is clean, put it in the oven at about 120 C until the mould feels dry. This may be an hour or more. Then let it cool inside the turned off oven, so there is minimum stress on the mould.
Now you should have a mould ready for your silicone. Use soft soap as a release agent for the silicone.
Hope this helps.
I do it a little differently than Simon, but it works for me. I do work in layers, but the trick is to get the next layer of plaster on as the previous layer is thickening up, but not fully set. That lets the layers bond together. I dab on with a brush, too, which mixes the layers a bit. My first 2 layers are Hydrostone or Ultracal with no reinforcing(so I don't get the sisal fibre or hessian/burlap or fibreglass matting going through to the surface), then an Ultracl layer with some reinforcing in it. Then I do a mix of Ultracal and ordinary casting plaster, with more reinforcing, then a couple of layers of just casting plaster. That's mainly a money saving thing, I use the Ultracal, which has to be imported from the US and costs more, on the first layers so I get a good hard surface. If in doubt, stick to the Ultracal only. I don't pour into a box, I build up, so my moulds aren't as blocky.
Ideally I open the mould as it is cooling down, the warmth will have softened the plasticine. Leaving it longer and re-heating it is risky, too warm and the plasticine melts in and is hard to remove. Sometimes it does anyway, just from the heat of the plaster setting. After years of using no release agent on the plasticine sculpture, I have had a few where the plasticine won't come away cleanly - possibly changing to Chavant NSP so as not to inhibit the silicone has made this worse? It took a lot of scraping the clay out, then washing with Shellite, to clean the mould. So I am now tending towards putting some release on the sculpt. I brush on some pva release, a liquid that dries clear and can be washed off with warm water afterwards. Then I put a little mould release wax on. Not too much release or the details get softened.Wrinkles and the lines between scales can get filled. By all means use Krylon instead, Kathi knows what she is doing.
I notice the scales on the chest look like cut lines in the clay, with burs sticking up on the edges. If you are having to re-sculpt after the mould went wrong, you can improve on that. If scales and wrinkles are deep or large scale, I round the edges with a tool, and brush over with Shellite (Naptha) to smooth it. For finer textures, try laying some cling wrap over the clay, and press your scales in through that. Provided the clay is not too hard, It makes the edges round off and look more natural. You can warm the sculpt to make it softer and easier to press details into it - the same goes with texture stamps if you use those.
For silicone bodies, I make the neck part of the body, with the join to the head going under the chin. The head is cast in a seperate mould. That makes it easier to pour the silicone in to head and body, without getting air pockets. For foam latex monsters I usually sculpt and mould it all in one piece, since i put the foam in with a brush.
Hey stop mo nick. Vaseline as a release agent for the sculpt would work just as fine right? I’m on a bit of a budget.
Probably, but I have never used it for release.
I have mixed in some vaseline to soften some old plasticine, though, so I am thinking if the plasticine could melt into the plaster, so could the vaseline, and it might still stick if the mould gets too hot. It could blend together. So I am more comfortable with the pva release which does not melt with heat. (It is not pva glue, which is tougher.) Or a layer of shellac, then wax, like I do sometimes, though usually for a sculpt that is smooth and does not have a lot of fine texture to clog up. Those are things I have had in stock for years, so it's no expense for me, different for you.
So sorry, someone else who has used vaseline for release needs to answer this one.
I do actually use a spray release agent, although it is not essential. So, yes, adding that step is fine. The PVA approach is also good. Vaseline does work, but has a tendency to leave brush marks. You could thin it a little with white spirit to reduce this.
Nick is right about ideally opening the mould as it is cooling down, once the plaster is no longer crumbly but snaps when you break a scrap piece. It's just that I sometimes leave it too long and have to reheat. But do it very gently, taking the mould out of the oven frequently to remove a bit more softened clay. You certainly don't want to get it too soft and sticky, just enough to be able to ease it out from the crevices.