Plaster mastery

In the first article in this series, I discussed why glass casters should build a parts library of mastermolds. In this article (part II), I’ll talk about the easiest of mastermold-making techniques: The plaster mold.

Silicones, urethanes, and resins–the stuff I make most of my glass casting mastermolds from–can be spendy and hard to find. The moldmaking techniques they require can be time-consuming, with a steepish learning curve.

So don’t use them when you start out; use plaster of paris, or inexpensive bisqueware, instead. There are a lot of advantages to making mastermolds with plaster of paris:

• It’s readily available in most craft and hardware stores
• It’s cheap (less than a tenth the price of good silicone)
• It’s very fast
• It requires minimal equipment and supplies
• If you’ve ever done kid crafts, you probably already know how to use it

Plaster molds let you make a LOT of waxes very quickly, and because they’re so inexpensive, they are also a good choice for one-off parts that you’ve no interest in making again. (Another choice for one-offs: Make the mold out of reusable materials such as Gelflex, Elastak, or Composimold. More about that in another post.)

So what do I make plaster of paris mastermolds of?

• Leaves and plant parts
• Basic shapes (such as the paisleys in the picture at top)
• Small plates and bowls
• Twigs, sticks and branches
• Lettering and handwriting

Plaster of paris is NOT an appropriate mold material for every component in your casting library. First, because plaster doesn’t flex, which means you can’t wiggle your wax out of undercuts in the mold without distortion (or breaking the mold).

Models that work for hard, inflexible mold materials must have negative draft (see the diagram below) and no “locking” details that might prevent removal of the casting. Basically, if a plaster mold is correctly made, and will easily release the wax, you’ll be able to look straight down from the top and see every single detail, nook and cranny in the mold without obstruction.

You CAN make plaster molds of more complex shapes using multipart mold techniques, but it’s generally faster and easier to move to flexible mold materials.

Second, because plaster of paris molds have a limited lifespan. This is mostly because they’re far more fragile than flexible masters, but they’re also more vulnerable to heat and moisture. They simply won’t last as long as the identical mold in silicone or urethane, so if you want to reuse the master over a long period of time, or you’re trying to preserve an original sculpture, opt for long-lived silicone.

Q: Why can’t I cast glass in a plaster of paris mold?

Plaster of paris molds are NOT refractory casting molds. (Well, they can be fired, but they tend to shrink and crack like a dried-up lakebed.) If you want to make molds that can be filled with glass and used in a kiln, use an appropriate refractory plaster mix.

Plaster of paris MOLDS aren’t great in the kiln, but dry, sifted plaster of paris is a great addition to your firing toolchest. It’s a great (and cheaper) alternative to using kilnwash or refractory mold mix to smooth over coarse fiber paper, level out gaps between kilnshelves, fill mold cavities in pate de verre vessels, etc.

What you’ll need to make a plaster mastermold

• Plaster of paris
• Water
• Plastilene (oil-based clay), polymer clay or earthen clay
• Deli container(s)
• Measuring cup
• Sifter
• Sharpie or crayon marker
• Plastic spoon for scooping and mixing plaster
• A plastic or paper bucket for mixing plaster
• Vaseline or PAM (about the ONLY time I’ll recommend either as a mold release)
• Models

Step 1: Prep your model(s)

Let’s start with something simple: A leaf. Bit late in the year, but I picked a few fall leaves for this demonstration (they’re a little brittle, but they still work, and they’ve dried a bit, which increases texture–that’s good!). I gently cleaned them off, then picked the ones with the deepest, nicest texture.

Tip: Choose the parts that show the most texture possible and remember that the back of the leaf is frequently better than the more delicate top.

You’ll be making a negative mold of the leaf, then a positive wax from that mold, and then a refractory mold of the wax, and then filling that mold with glass; that’s four impressions away from the original. No matter how careful you are, some detail will be lost, so go for maximum texture.

If you simply laid the leaves down on your worktable and poured plaster over them, the resulting mold would be too shallow to hold enough wax for a good model. You want to create a reservoir for the wax, so roll out some clay to at least 1/4 inch (here, I’m using soft white plastilina). Gently lay your leaf on top.

Leaves are not perfectly flat; they’ll rise and fold on the clay. You’ll tuck bits of extra plastilina wherever there’s a rise until the leaf is smoothly resting on the clay in a natural position.

Trim away the clay around the edge, but don’t get too precious about being exact; it’ll be easier to carve excess wax away than it is to exactly fit plastilina underneath a sawtoothed edge like the leaf above.

You’re going to be putting your leaf assembly into a mold box for pouring plaster, so you’ll need a container. I usually use a clean deli container an inch or two bigger and deeper than the model. Ideally, it’ll be flexible, with slightly sloping sides (wider at the top), so it’s easier to remove the plaster once it sets.

Your mold box will determine the actual volume of your mold, so do that first:

• Measure the height of the model, and add a half inch to find the thickness of your mold
• Mark how high that is on your mold box (measure from the inside bottom of the box), and pour water into that box until it reaches that level
• Pour out that water into a measuring cup
• Pour out a little less than half of the water–this is the amount of water needed to make the correct volume of plaster

Smooth a thin film of vaseline into the container, and set your leaves on the bottom inside, making sure they’ve stuck fairly well. You don’t want them to float up when the plaster is poured, and the less plaster that seeps under the model, the easier it will be to clean the mold later.

Smooth a thin film of vaseline on the top and sides of the leaf models, too. Be careful not to use too much, because that would obscure detail. (BTW, vaseline and PAM are commonly suggested as mold releases, but they are absolutely lousy in just about any moldmaking exercise EXCEPT this technique. For everything else, buy a real mold release.)

Step 2: Make the mold

Technically, you need a 2:1 plaster:water ratio, by weight, to mix plaster of paris. Practically, though, the volume method I described works just fine for this purpose.

Sift the plaster of paris into the container that already contains the water you’ve measured out. The plaster will sink to the bottom; keep sifting until it forms an island in the middle of the cup that rises above the water level.

Wear a mask or respirator, and gloves. Plaster is very hard on your hands, and lousy to breathe.

Let the plaster slake (rest) for 2-5 minutes. If the plaster island sinks below the waterline during this time, sift in more. Then mix the plaster thoroughly until there are no visible lumps.

You ARE wearing a P100 particle mask, right?

Stir the plaster until it’s well-mixed; don’t worry about a few bubbles and lumps; they won’t matter in this application. Now pour the mold, as in the video:

Step 3: Use your new mold

Making a plaster mastermold is fast. Using them is even faster.

Unlike silicone mastermolds, where you pour the wax in and wait a couple of hours to let it cool, wax in a plaster mold will harden and cool almost instantly, letting you make several copies in an hour. You can also use them to make clay impressions, but the process is a lot messier, so I usually reserve plaster molds for the following process.

What you’ll need:

• Clean pan about twice as deep and big enough to comfortably hold your mold
• Cold, clean water at least an inch deeper than your mold
• Wax pot (I use a crock pot from Goodwill)
• Sculpture wax (see my post on selecting waxes for more information)
• A ladle or metal cup with a handle (for pouring waxes)

Make sure the crock-pot is on a heat-proof surface, plug it in and melt your wax until it’s very liquid (depending on your setup, this could take an hour or so). Then follow the process in the video:

Fresh plaster degrades pretty quickly in water. I left a sample in water overnight, with the top just peeking out, and in the morning the piece had lost about 1mm of plaster right up to the waterline. So it’s best if you can let the plaster cure for at least 24-48 hours before you use it, and don’t leave it in the water when you’re done.

Making waxes with a bisque mold

So…I got to thinking: If I can do this with a water-soaked plaster mold, why wouldn’t it work with ready-made bisqueware? Bisqueware is slip-cast clay forms, usually dishes and serveware, which you can pick up at just about any paint-your-plate place, or a good ceramics supply house.

It’s basically earthen clay that’s been fired once, somewhere between 1800F and 2100F, until the organics are burned out and the clay is hardened into a solid form. It can be sanded and drilled, but is also porous enough to accept glazes and stains well. A ceramic artist glazes and fires bisqueware in a second, much hotter, firing until it fully sinters into ceramic material.

I figured I could take advantage of bique’s porosity, i.e., the fact that it can absorb and hold cold water, to make waxes just as I did with the plaster molds. So I picked out a bisqued bowl with a nice outside shape, and and tried a little experiment:

These aren’t the most perfect waxes around, but they’re a first attempt (and they were also made holding the video camera in my left hand while making wax bowls with the right–I suspect that two hands would do a much better job). The cool bisque chills and stops the wax almost instantly, so unless you get complete coverage on the first try, you’ll see marks in the wax. I can probably resolve that with enough practice.

Shoving damp bowls into a wax pot also releases water into the pot, which causes some bubbling; the more you dip, the more chance you’ll add bubbly bumps to your wax bowls. And if you keep going, the wax could start spitting–a bad thing to be around. After the second bowl, there were enough bubbles that I covered the wax pot and let the water steam out.

Even so, I got some very serviceable, thin wax bowl shapes I can use to build up wax onto, carve into, and just generally turn into starting points for a series of sculptured bowls. This is definitely a technique worth exploring.

How to take care of your plaster mastermold

Plaster absorbs just about everything (that’s why the water technique works), so if you leave it out in your studio it’s liable to stain. It will probably also chip and crack; that’s one reason to make the mold at least a half-inch thick. It’ll last longer.

I loosely cover mine in bubble wrap and label what’s inside. Then I secure it with a piece of tape and put it on the mold shelves. You can stack these, but not more than one or two molds deep.

In the next post, I’ll talk about making the same leaf mastermolds…but this time in silicone and urethane.