I consulted the experts in the lab at Silicones Inc., where I get my RTV silicon rubber. I was told that it was okay to use a 50/50 catalyst split, or even a very hot 100% catalyst mixture. I hadn't know that I could use their "Fast" catalyst separately, feeling it would produce too hot a mixture. As it is, the fast catalyst makes a slightly more brittle, or harder mold. You could use both description. And the mold library life might be a bit lessened, but the library life is already up to fifty years, and I am not too concerned with whether a mold lasts for 50 years or 35. (I'll be dead by then anyway).
I worked upon a few theories that I had developed as I made the other molds. This is how the "learning" curve can contribute to "revolutionizing" your work. My first thought was that a mold that was contiguous, that is, not divided by a shim would leave less of a shim-kerf, and thus be more accurate. It was better to "cut" the mold after it had cured than use shims to divide the mold before application of the silicone. I applied two thickened (Thixotropic catalyst) layers before beginning my shim wall, as you can see here:
The shim wall itself caused me great concern, and I played around with materials. I had some plastic sheeting which I had gotten as waste from some industrial use, and found that hot glue-- you know the glue sticks and gun used by "crafty" people-- actually sticks this material together.
Inch by inch I made my way around the figure already covered by two layers of silicone, hot gluing, and in some cases resorting to duct tape, making a "floating wall" with no support on either side. In my mind, it would either be the mold maker's version of Parkour, or it would be an epic fail. Here, you can see that the shim line is not held by anything. This is its underside:
The next problem to solve was how to keep the silicon, which is sticky stuff, expensive, and nasty to clean up, from dripping off the sides of the plastic floating shims. Hot glue to the rescue again, as well as at least a hundred popcycle or "craft" sticks. Here, I start to hot-glue the craft sticks to the plastic material to create the retaining wall.
Here I am beginning to extend the shim from the body:
Since having no shim at all creates a thinner shim line, I would remove the white plastic shim after the silicon "side" had set up. I painted on soap as a mold release, and used the craft stick wall as a foundation for the second hot glued craft stick run. Here is the second craft-stick run that has allowed me to pour the second side of the shim. You can see that it is totally floating in this shot:
I also made one layer of the old cheese cloth method to stiffen the mold, but only on the figure and not on the shims. One gallon (9 pounds) of silicon is expensive, and I was nearing the end of my gallon. After the entire mold was made, I had 200 grams of silicon left over.