Rock success!

My first attempt to cast a rock in a latex mold with Great Stuff spray foam ended in a big mess. I then decided to try making a casting with CelluClay. As you can see in the above photo, this worked.

The only problem was that it took about a week to completely dry out and harden, and a couple of cracks developed due to me mishandling the unhardened casting.

I think the long drying time was due to the casting being about 1/2" thick in places and me mixing the CelluClay with a little too much water. For future castings I'll try a little drier mix, and may try drying the casting in a toaster oven set to keep warm.

But, once dry, the casting is very hard and quite light.

It was then hot glued to the location I had in mind for it. The above photo clearly shows the casting's crack. Once filled and painted it shouldn't be noticeable. 

This side view photo shows the big gap behind the casting and the mountain. The hot glue holds the casting firmly in place. 

I got out the Great Stuff spray foam and carefully filled the gap with some. Once the warty mess is dry I'll carve it and blend it in with the rest of the mountain.