Now that the basic structure is shaped, I siliconed all the 2" pieces together. I built a platform out of luan that mimics the aquarium's glass walls and the 45 degree overflow glass wall so I can work on it out of the tank and apply pressure without the styro coming apart (and later when I go over it with the black spray pond foam and aragonite).
Having worked with styrofoam (foamular) for museum work as well as freshwater aquariums, the key to getting it to stick together and stay under water (because it wants to float) is to not treat the silicone so much like a glue, but rather an anchor.
You drill holes in the styrofoam and fill the holes with silicone and then also put silicone between the pieces. After it cures the foam cannot separate (or lift off the glass later when glued into the tank) because first of all, silicone adheres VERY well to styrofoam, but also the siliconed filled holes act like plugs that the styrofoam cannot pull out. The key here is also to take a propane torch and melt the styro around the holes so that it becomes extremely hard like a hard plastic.
This is also done when gluing it to the glass walls. The 2" thick pieces here are also "pinned" in place by siliconing fiberglass rods into the pieces. You drill through both pieces, fill the hole with silicone and then drive the rods into the holes (deep enough so you don't expose them by further sculpting and also it gets pinned at the thickest parts of the sculpture.
Anyway, the styrofoam pieces are sitting waiting for the silicone to skin over so they won't move when I go at it later with my drill. At this point it is not necessary to let the silicone cure completely...just enough so the pieces don't move. You'll notice I have my "spade" bits on the work surface. These are ideal sculpting tools as they not only drill various size holes in the styro, but also cut it down to further shape it, and best of all (given I'm making fake dry rock) they do a great job of "tearing" chunks out of the styro. When that part is done, I go over the entire thing with my propane torch and melt the styro even further until it is exactly how I want it.
It still looks thick and chunky, but it will not when I'm done. It will be quite thinned out and much more sculpted further. It's only there to be a "base" for the black pond foam. I will only need to use one can of the pond foam because while the pond foam expands, I will not allow the black pond foam to expand much because as I go over this with it, I am going to dump a lot of aragonite onto it and press it down while it's still sticky and trying to expand. I'm not using the pond foam as much for shape as I am for putting a thick skin of it over my styrofoam. Stay tuned.
