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March 26, 2004

Mad With Plaster

I almost forgot to post about today's little excursion into Plastermania.

Wednesday I set up half the mold form for the first pour: six plastic eggs buried up to their midriffs in clay. Today, after some time spent fussing around and bitching to myself about how other people leave the clay area a mess, I set up the cottle boards and poured the mold. I was able to get both halves poured and cleaned before the guild work time was over.

If you're wondering what the steps are for making a plaster mold for slip casting, they're quite simple.

Find the line. The first set is to find the line on your object where two halves will not make an undercut (undercuts get stuck in the mold). On most cheap plastic toys, this line is really obvious because it will not have been fettled off. You bury the object up to that line in clay, so that one half is sitting above the clay. All angles should be at or very close to 90 degrees; you don't want sharp edges on the plaster. You want to make sure you put on a pouring spout at the same time, or you won't have any way to pour the slip into your mold. I like to make my pouring spout almost perfectly straight. Other people like more of a taper. It should be wider on the outside, but not too narrow anywhere, or a plug will form during casting. For really large objects, you will want to put on a pouring spout and also a draining spout on the other side of the mold, because slip weighs a lot more than you think it does.

Set up the boards around the object. The boards used for mold-making are just 1-by boards that have been sprayed down with WD-40 so they will resist binding by the plaster. The ones we have at school have L-brackets on the ends so they slide together. I've also seen ones with a piece of angle iron along one short end, as a clamping surface. At school we clamp the boards in place (roughly) with clamps, then seal them in place with wads of clay.

Seal the cracks. I put a coil of clay around the bottom of the cottle boards on the inside, then up each of the four joints, and smooth them down completely. I also seal the outer cracks in the corners and along the bottom with clay, although not as completely, because I rely on the quality of my seal on the inside. Because everything you see on the surface of the clay will appear on your mold, I make a real effort to do nice, clean work sealing the cracks. I still have to do some cleanup, but not as much as more sloppy mold-makers will have to do.

Brush on release agent. Some people do this before putting up the board setup, but my hands are small, so it makes little difference to me. We use Purelube's pure soap product at school; at home I usually use silicone as a release agent. Apparently something called "green soap" is the end-all, be-all of release agents; I'll have to try to find some of that sometime. One of the critical things with the release agent is making sure not to allow bubbles to form. Bubbles will appear in the surface of the object, so unless you really want bubbles all over the place, you've got to be super tidy.

Mix the plaster. We mix by volume at school: 10 plaster to 7 water. Makes a decently hard mix and is easier to do than weighing, although weighing is a more reliable method. I put the water in the bucket, then the plaster. The plaster gets mixed in by hand, smooshing the lumps and mixing fairly slowly. You can speed up the cure of the plaster by mixing faster, but I prefer a slower-setting plaster.

Splash on a coat of plaster to make sure the object is covered. I like to do this even though it's not strictly necessary for every object. It's just a few handfuls of plaster splashed on the object to be sure you get all those odd nooks and crannies that might form a bubble and not get molded.

Pour the plaster. Once I finish doing the splash coat, I pour the rest of the plaster into the mold. I knock the sides to release any bubbles that might be in there, and skim off any froth on the top (froth will cure hard and then flake off all over everything, so it's best to get rid of it either at this point or during cleaning). Once the mold is sitting and curing, I rinse the plaster bucket with cold clean water and dump it -- not into the sink. We dump plaster water in a barrel at school. When I do plaster at home I dump it in the back yard and the dog eats it. Chacun à son gout.

Twiddle thumbs. It takes about twenty minutes to half an hour for the mold to be ready to be released. Once it gets really hot, it's set up enough that if you unmold it it will not fall apart and ruin all your work. At that point I scratch the name of the object ("Kitty") and my name on the back using a needle tool. Helpful when you want to find a mold later without having to open them all up (and risk damaging it), and also to identify your molds in a shared studio space.

Unmold the first half. I release the boards, pull off the clay blobs and bits, and turn the mold over. I remove all the clay from the back of the object, keeping it buried face down in the plaster (so the edges will line up properly). If the object comes free, I try to put it exactly where it was, which may or may not be easy.

Clean carefully. With the clay peeled off the surface of the mold, I wash everything down, including the object (which is usually dirtier than the plaster). If it's just unbearably dirty and you don't want to deal with it, you can set it out overnight and all the clay will dry up and be a bit easier to get off, but you're just going to have to wash the sucker at some point.

Cut keys. You'll want to make your mold so it only goes together in one way, so you don't accidentally form a massively misshapen object and waste the time spent casting. So you use a tool rather like a melon baller and scoop out three or so rounded keys on the surface around the object (taking care not to cut into the object, naturally).

Is that pouring spout still attached? This is a good time to make sure you're making both halves of your pouring spout, if you have it coming out at the seam (the usual method).

Replace the boards. At this point I put the boards back and seal them up again, as before. Coils of clay around the edges inside, and in the cracks outside. All smoothed down nicely for clean seams.

Brush on release agent. This time, the release agent has to go on the plaster, as well as on the object. Plaster won't stick to clay very well (which is why we're using it to make the mold in the first place), but it will stick to plaster like nothing else. So you want to make sure that release agent is thoroughly soaked into the plaster. If it dries out while you're working, add more. Also lube up the object.

Mix plaster as above, splash on splash coat, pour the plaster, and wait for it to set. Once the plaster is hot, I once again carve the name of the object and my name on the bottom of the mold. Then as above I unmold the mold and remove all the clay bits.

Pull the mold apart. It should come apart stiffly. If it seems stuck, you might need to tap in the seam with a plastic wedge (lightly!) to crack apart a place where the plaster has bonded to itself. Sometimes a key will get stuck because the release didn't stick to it very well. You can now remove the object and the pouring spout, as well as the clay coils used to seal the mold to the boards.

Wash the mold off. Using a damp sponge, wash the clay residue from both sides of the mold.

Scrape off sharp edges. Using a Surform or Surform-like tool, scrape any sharp edges off the bottom, sides, and top of the mold halves. Be careful not to mess with the edges that will form the seam of the mold. You'll clean those up later. Right now you're just making it so the mold doesn't get plaster everywhere when you set it down. Using a fettling knife, scrape the pouring spout interior so that it is smooth. This will help you later when you are trimming the spouts after a pour. Once you've finished, use the fettling knife to gently knock down any sharp edges on the sides of the mold, where chunks of plaster might come off in the clay. Be careful doing this, as every bit of plaster you take off will mean more seam cleaning later on.

Wash the inside of the mold. If you used an oily release agent, wash the inside of the mold with white vinegar and water to break down the oil. Otherwise, the mold will not be able to absorb slip. I've started doing this step every time I make a mold, even though I don't use an oily release agent. It helps a lot.

Allow the mold to cure and dry completely. This process takes five or so days, or one ride under a kiln for a single firing. You have to be very careful about exposing the plaster to heat. Prolonged exposure to temperatures higher than 200F will make the plaster break down into powder. Not good. If you dry the mold under a kiln, clean it again once you remove it, as all kinds of nasty things will have gotten inside.

Make a cleaning pour. Fill the mold with slip and let it sit for 15-25 minutes, topping up as necessary. After the time is up (less time for smaller molds, more time for bigger ones; the time will determine the thickness of the clay), pour out the excess slip and let the mold sit for four or five hours before unmolding.

Unmold your cleaning piece. The first piece from a mold is bad -- it will definitely have plaster chunks in it somewhere, and it will have some red clay on it, marring the surface. So after making sure the mold is good and doesn't need repair or recasting, break the piece up and throw it away (break it up to prevent somebody from trying to "save" it from the trash and possibly making a bomb in the kiln).

The mold is now ready to be used!

Posted by ayse on 03/26/04 at 5:35 PM

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March 26, 2004

Mad With Plaster