Joanie: There are a few things that might help you in your journey. The first thing that caught my attention is your mold keys. They look like they are good in theory, but not in practice. The mold keys need to be made along with your rubber mold piece, not attached to it afterward. In plaster mold making, you want your keys to be round (not squared off) and tapered, but in rubber you can be less finicky. Build the keys in as you paint on the rubber. You can glue them on between layers, then paint more over them maybe.
Me: So it sounds like another approach I could use is to have the rubber snakies ready to go, but paint them in to the last coat of rubber as I'm applying it, not glue them on afterwards?
Joanie: That would work better. Depending on layer thickness, one or two layers need to be applied over the top.
Think I would consider one long ridge down the rubber where the plaster goes. Like a short separating wall. Then painted with rubber. Would make a fine key. The wormies could be made of clay. Or whatever. Or just build up as you paint.
Me: What about Cabosil? It's used to thicken rubber, it's made of ground glass.
Joanie: Oh that crap. Stay away from it. Moon dust. Tried it once. It's not safe, and not good. Try laying a strip of oil clay then painting over it with the rubber. Doesn't need much ridge, just something to 'register'.
Me: Is it OK for the clay to stay inside the rubber after the mold is made? Or do I need to get it out somehow?
Joanie: Should be okay. As long as it isn't too close to the inner surface. Might bulge inward if so. But won't hurt the mold long term. I think that I would try a long strip of something. Rope? Glue it on, paint over it well. Ponder that. :-)
Joanie: My other concern is how you have been told to mix the plaster. I find that method to be variable in the extreme. It's okay for art class, but not good enough for long term plaster strength. It's easier to do that method **if you already know how plaster is supposed to feel** LOL. In other words, it's okay if you have experience, but not if you don't.
Plaster strength depends on three elements: 1. Ratio of plaster to water 2. Temperature of the water 3. Mixing time.
The ratio between plaster and water is 70:100 by weight. 70 parts water to 100 parts plaster. You can get away with 1 part water to 1 1/2 parts plaster if it's easier. I've started using grams because the math is easier. 700 grams water, plus 1000 grams plaster, equals 1700 grams total. You need a scale that does grams or ounces. It needs to go up to maybe five pounds? They're cheap and very useful. Slip a big ziplock bag over the scale, so that it stays clean and dry. Make sure to zero out your bowl first.
Water needs to be very warm, 100 or so degrees. Warmer water kicks it faster. Cooler water kicks it slower.
Slaking and mixing time is crucial. Slake for two minutes, before you mix. The most mixing in the least time will give you the strongest plaster.
Me: Question... do you mix the plaster and water with your hands (as it was suggested to me to do) or do you use a mixer? I'm guessing if speed is of the essence, you might use a mixer.
Joanie: Anything under 10# can be hand mixed. Main thing is keeping your hand down in it, and moving as briskly as possible without making bubbles. Wrist action. Knock the container against the table to raise the bubbles. Plaster is ready when a finger drawn across the surface leaves a wake.
Joanie: After you have slathered, glopped, and oozed the plaster onto the rubber mold, run a flat thing over the seam line to expose where the two plaster pieces meet. You can tell the different colors... you shouldn't have to attack the mold with a knife and hammer.
Going back a little more, when you are pouring plaster against plaster, first make sure that your plaster is smooth. Make sure that your keys are smooth, round, and tapered.
Then use mold soap as a separator. You can use Murphy's Oil Soap too but it has detergent in it. Vaseline will hurt the long term strength of your plaster. Rots it. Mold soap will make it stronger. Wipe on a coat of soap, let set a minute, wipe off with a barely damp sponge. Repeat three times. You can see the satin sheen if it's right. Also, when soaping, use good sponges. You know the kind. Natural. One for soap, one for water. Wax on, wax off, Mr Myogi says. Never put the water sponge in the soap. Dilutes it.
Me: How do you get the plaster smooth if it is already set? Or do you take down the divider while the plaster is still soft?
Joanie: You smooth the plaster with a tool. Take off the divider, then take a tool (such as a dental tool) and scrape it. That is the first thing I learned that set a light bulb off in my head when first learning to make molds. You **work the plaster** after it starts to set. If the plaster that you are pouring against is rough, or not well sealed, you're going to fight it. It doesn't have to be smooth smooth.... it only has to offer no resistance to being separated. Tool marks are okay, little indents are okay, but big bumps, holes, or wacky keys are not okay.
Me: How do you fill any air holes in the plaster?
Joanie: If you have holes in the plaster, fill them with whatever. Clay, oil clay, whatever. You don't care if the oil clay stays in there. Makes no difference. But you DO care if the plaster breaks while you are trying to separate it. LOL
Joanie: I prefer to make my keys into my plaster after it has set. You can carve keys faster and better than you can sculpt them. Use a nickel, or a washer like you put on a bolt and nut. Turn it sideways and carve in with a twisting motion. Twist, twist, twist. You'll make a perfect, smooth key in no time. So for the plaster, don't be afraid to work it. Learn to love it in all of its loveliness.
Me: When would you remove the clay dividing wall? I was told to remove it after the plaster started to cool. Would this allow me to work the plaster as you suggest, or should I remove the clay wall earlier?
Joanie: When the chemical reaction starts, the plaster gets warm. It will still be soft enough to scrape with your fingernail, but it will be firm enough to stand on its own. Remove the clay dividing wall then. Usually, by the time I clean the bucket and my hands, take a sip of iced tea, and sit down again, I can start removing clay from the plaster. Depends on ambient temp of course, and thickness of casting. Thicker castings kick off faster and get warmer. The warmth (exotherm) keeps the cure going. But there is no rush. If it feels too soft yet, wait.
Carve your keys into it. Then let it cool for a while before doing the next one. If you are doing four plaster mold pieces total, you could do two that don't connect, first. Then the two that are between those other two.
When you do those second two, you are pouring plaster against plaster. Use the soap. Wipe on, let set, wipe off. Three times. Then test with a fingertip of water. The water should sit on the surface and not penetrate at all. More soaping is always better if in doubt.
When pouring plaster against rubber, plaster will not stick to silicone anyway, no need for a separator if what you are using is silicone or rubber. If you want to feel better about it, wipe one coat of soap over it.
Joanie: This clean hands-and-buckets business with the plaster is nonsense. Plaster is made in a way that drives off one of the two chemically bonded water molecules, and by mixing water back in, you start a chemical reaction that rebonds. Now, if you have a sloppy, uncleaned bowl you will have bits of already catalyzed plaster floating around, but there isn't any sympathy between the plaster that has already started to kick, and the plaster that has not yet kicked.
Joanie If the plaster doesn't kick, its old. Throw it out. If it feels gritty, throw it out.
Plaster absorbs atmospheric moisture then is bad. It must be kept dry, sealed if possible. Just bought some plastic kibble bins with rubber seals around lid. Keeps it longer.
Problem is that if plaster's old, you can't seal it and pour against it. Never could figure out why, but old plaster sticks to the next pour no matter how much soap you put in it. Ruin a few molds and you learn. Nothing like chiseling it all apart and losing everything.
Me: Ugh! So if it's bad, it can still set, but it won't seal?
Joanie: Yes. And you don't know for sure until your mold is made.
Me: That's good to know.... I guess... Is it better to use fresh plaster on a project just in case?
Joanie: Always. It's cheap. Time is more expensive.
Shortly after I posted my blog regarding my first adventure in moldmaking http://www.sanssoucistudios.com/blog/adventures-in-moldmaking, to Facebook, the moldmaker extraordinaire Joanie Berkwitz responded that she had a few suggestions to help me out in my next attempts, which I have posted below. One of her plaster molds is also pictured below.... it has no fewer than **twenty one** pieces!! I would bet money that Joanie is one of the most gifted ceramicists in the US right now, and I am very fortunate to be able to own some of her equine figurines.