I bought a gallon of Smooth-On's Smooth-Sil 910. This is really soft stuff (Shore A hardness rating of 18--kinda like a Gummi Bear) that takes 24 hours to cure, with a 5-6 hour post cure at 125 degrees Fahrenheit recommended (but not necessary in Texas). It's got a peculiar 20:1 mixing ratio, so you've really got to use scales to mix it up properly.

VOLUME ESTIMATES: The scientific way of figuring the quantity of silicone is by filling your molding "box" up with water, and getting the volume from that. Then you have to dry it off really good because moisture & silicone are enemies. I'm impatient. I eyeball the volume. (later: Exactly the right amount. Damn, I'm good. Damn I'm lucky.) You're supposed to be able to add silicone if you come up short, so it's not really a big deal, just a hassle.

WEIGHING: I highly recommend an analog triple beam scale (instead of a digital), because there's a much more intuitive & natural feel to the weighing. Now, let me explain how I do my weighing. First I open my spreadsheet. I weigh the empty cup. Then, I weigh the cup filled with the eyeballed amount of silicone. The spreadsheet tells me how much the total mixture should weigh once I add part B. I just add part B until the scale balances at that setting. In this case it's really easy since Smooth-Sil 910's B catalyst is a clear liquid which I can add with a pipette. The spreadsheet formula is also pretty easy: Part A's weight is calculated by subtracting the weight of the container from the weight of the container filled with Part A. Part B's weight is calculated by multiplying Part A by the mixing ratio (in this case, x 0.05). I made an online Javascript version which you can try if you've got a Java-capable browser.

MIXING: When you mix, be sure to mix the stuff thoroughly. This means getting the stuff on the walls and in the cracks at the bottom of your mixing cup. If there's any stuff left unmixed, it won't cure properly and you'll end up with streaks of gooey stuff in your mold.

VACUUMING: Smooth-Sil 910 does need to be vacuumed to get air bubbles out. Even molding a relatively simple piece produced a cluster of air bubbles along a vertical side. Soooooo... I constructed a cheap, cheap low-tech vacuum chamber.

Here it is-- It's a manual vacuum pump from a toy that connects through the fruitcake tin housing to an air extraction nozzle on the floor of the chamber. I cut a piece of craftstore rubber foam to form the gasket & contact cemented it down. The bell jar is actually a plastic cookie canister that I stole from my wife. It's really sturdy. I have no idea what the pressure is, but I know that it's sufficient to make a bunch of bubbles form at the top of the mixed silicone. It also causes the volume of the silicone to double, at least. And that's a good thing. It's resting on top of a jewelry cleaning gadget, which by virtue of its vibratory powers, is attempting to hasten the movement of the bubbles to the top and away. I use this thing after I've poured the molding compound over the pattern, too.

This picture shows the base for "Queen of the Set" about to be covered with the degassed silicone. It's just a little smaller than the tub that I'm using so this should be fairly volume efficient. I've parked the base on top of a small clay circle. This is going to be a one-piece mold, poured from the bottom. I don't care how the bottom looks, but I'll try to take advantage of the molding compound's softness by putting the copyright info along the edge on the bottom. The mold should pick up a border along the underside with the writing on it, and demolding won't be a bitch because the mold will be really flexible & stretchy. (We'll see about that. --08/10/97)

When pouring silicone, you're supposed to pour slowly in a narrow stream from about six inches above, and into a single lowest spot. Let the rubber flow around and engulf the pattern.

08/11/97-- Success! Even though I cheated and demolded at about half the recommended cure time, the mold came out good. (impatience can get you in trouble, but not always...) Not a single pinhole or bubble. I still waited before I poured the first casting though. I wasted a casting trying to figure out the exact amount. (er... sometimes I don't guess right!) Even the scribbling on the bottom edge of the base was reproduced. I just wish I had put more effort into cleaning up the bottom before molding.

Last modified: Saturday, January 6, 2001 6:20 PM