Time for a quick tutorial on plaster mold-making. Lately I have been working directly with plastaline (and plasticine) clay as a positive mold. I apply layers of papier-mache over-top of the clay (with a release layer so the glue does not stick to the clay) and skip the plastering process. The downside to this method is the loss of detail. Then again, the masks that I have been working on lately have been lacking fine detail and so this method works just fine. The other drawback is that I have less clay to work with on other masks while I am completing the layers of papier-mache. This may, however, be a good thing as it keeps me focused and gets me to finish a mask straight through.
There was a time when I made most of my masks in negative plaster molds. One reason for this was to retain detail. The other advantage is the ability to create duplicate masks from the same plaster mold (this is sometimes possible with a positive clay mold with a few touch-ups in-between castings). The other main reason I was using plaster molds was because I was casting more neoprene masks.
I get my neoprene rubber (a sort of synthetic latex) from Chicago Latex a division of Spartan Adhesives down in Chicago. It is convenient for me because shipping is included in the price and I receive my order the following day. I will go into more detail regarding my use of neoprene in a different post but for now, the thing to understand about neoprene is that it uses a slipcasting technique.
Slipcasting is the process of pouring the liquid neoprene (or other casting compound such as latex or liquid pottery clay) into a plaster mold and filling to the top. The neoprene is left in the mold for 1 to 3 hours depending on the thickness of my mask and the dryness of the plaster mold. A dry plaster mold helps to speed the curing process as it wicks the moisture away from the neoprene. This creates a layer of semi-cured neoprene and then the inner left-over liquid is poured off (and re-used). Another few days and the mask can be released from the mold in order to dry for a few more days still.
Neoprene is more expensive and requires a plaster mold but in the end is much less time-consuming that paper-mache. Of course, the more experience a person gets with paper mache, the gap is closed somewhat. Now finishing with the neoprene tangent, back to the plaster mold casting.
[First, I will begin by apologizing for the low-light photographs of this tutorial. The basement where I am currently using plaster does not offer great conditions for my camera. Still, I wanted to share my process and so the photos are what they are.]
I start with a mask modeled in plastaline. The mask is deeper than the finished product will become because I must have an even surface in order to fill the neoprene.
I use a splattering technique for the initial application of the plaster, so I like to use a wooden box (discarded dresser drawer) as a splash protector.
For mixing the plaster, I first place an estimated amount of water into a flexible plastic bowl. I say flexible because I want to be able to break any remaining plaster out of the bowl once it has hardened and the easiest way I have found to do this is by twisting the bowl. You can also spray the bowl with a quick mist of cooking spray to help with the release but I have only tried that once (it worked well but I do not have much issue without it either).
The estimation of how much water to put in the bowl comes with experience. The other note that comes from experience is that it is better to have too much plaster than not enough. I am definitely not one that wants to waste resources but when I am working on a project, I do not want to risk ruining the entire thing by not having enough plaster. It is possible to mix up more plaster quickly but most likely the initial plaster will begin to cure and the layers may not adhere to each other. I am sure you can measure and calculate in order to come to a proper dose but I am not much for that and it was not the way I was taught.
I also do not measure the plaster that I mix with the water. Speaking of plaster, there are many different types of plaster. For this project I am using plaster of paris that I have left over from a trip to the hardware store. I do not recommend getting plaster at the craft store because it is expensive. Even at the hardware store it is relatively expensive but not nearly as bad and it comes in a larger 12lb bag. It is quicker to take the trip to the local hardware store than go out to the gypsum supply company. There are also stronger plasters that one can use but my use has never warranted the extra expense (yet).
I grab handfuls of plaster from the bag and put it into the water. I do not worry too much about sifting the plaster into the water to avoid air bubbles. The next step rids of that. I continue adding plaster until I have created an island of plaster that does not sink down. Now I let it sit for a few minutes. Above you can see a before and after shot of the island. In the picture on the right, it is possible to see how the plaster has changed and risen slightly. Leaving it at this stage, it will not cure quickly at all. It must be mixed. If you need to work with a lot of plaster on a large mold, it is possible to use two bowls/buckets. Let the plaster sit like this in one bowl and then gently remove some of the plaster/water and mix it in the second bowl. This way the first bowl will still be mixable after applying some of the plaster.
I wear gloves in the plastering process mainly for easy clean-up. That and when I have worked with plaster more regularly, my hands get very dry and cracked (they already get dry enough from riding my bicycle in the winter). I mix the plaster by hand. Having let the plaster rest for the first few minutes, it greatly helps reduce air bubbles. After a minute or two of mixing the plaster becomes a consistency of thick syrup at which point I can begin the application process. If it remains thin and does not flow smoothly from my hand, it is still possible to add more plaster. Note, that adding more plaster at this point does create more air bubbles and speeds up the setting time of the plaster (meaning less workable time). Even with the warning, I have had to add more plaster many times when I was not paying close enough attention and the molds came out just fine (and sometimes a faster set time was desired).
ATTENTION: Did I remember to use a mold release on the wood board?!This is one step that is easily overlooked but can be important. I tend not to put a mold release on the plastaline clay but I will put some on the wood board that holds the plastaline. Without the mold release, the plaster may adhere quite strongly to the board and become difficult to remove. Of course, I have forgotten this step in the past and still managed to release the plaster mold but it is not worth the risk. Having been awhile since last making a mold I nearly forgot this step on this project. I would usually use Murphy's Oil, dish detergent, or hand-soap as a release agent in this application but instead I grabbed petroleum jelly. I do not like using petroleum jelly as it is hard to remove from the plaster. I will never use petroleum jelly on my clay because the jelly will form a layer in the plaster mold which will inhibit the process of neoprene slipcasting.
Now comes the messy part. I begin to splatter/splash the clay with the syrupy plaster. I flick my fingers with force in order to break out any air bubbles that form. Most of the time this method works very well for me and air bubbles are non-existant.
Another technique is to brush the liquid plaster onto the clay to fill in the detail. I find that flicking works just as well for most applications. If I am really worried about detail, I may brush instead.
As the plaster continues to thicken I begin pour handfuls onto the already splattered plaster. If it is too thin, the plaster will run freely over the sides. When it is thicker, it will begin to hold on the form. I continue to scoop and apply in order to cover the entire form. This is when the process usually speeds up. Work fast and efficient and there will be no troubles. If a person takes too much time in the process, the plaster will harden and be no longer workable before the form is sufficiently covered.
I smooth the plaster mold as it hardens further. Since I will be pouring neoprene rubber into my molds, I like to leave a flat surface on the top so that when I flip it over it sits flat. But sometimes I am slightly short on plaster and will leave this step out because I do not want to risk making the plaster too thin at the tip of the nose.
As the plaster cures, it will heat up. Once it cools down (20 minutes to an hour) it is then possible to release the clay model from the plaster mold. I take a hammer and chisel, screwdriver, or putty knife to separate the wood from the plaster. Then I pull out the clay from the plaster mold. Sometimes this is an easy process where the clay pops out, but other times it is a slower one where I have to dig out some of the clay in order to get a good grip.
The finished plaster mold. The whole process is rather quick. Sometimes I have been impatient and needed to de-mold sooner than waiting for the plaster to fully cool down. Luckily I have not had breakage in the past, but now I generally will wait before removing the clay.
Now this was only the basic overview of plaster mold-making technique. I did not cover in this post the importance of understanding under-cuts or different ways of strengthening plaster by way of burlap or cloth. More to come in later posts but in the meantime this is plenty of information to get out there and start experimenting!
