A friend in my NARGS chapter recently asked me to make a couple new planters for him. We discussed many designs when he stopped by but settled on two that I don't make often. The first is an oval bowl. For this I made an inner mold of laminated layers of 2" rigid foam glued together and shaped with a utility knife. The hypertufa bowl was cast upside down, packed onto the dome-shaped mold, with gravity and diligent packing with my hands helping to make it tight and uniform. Keeping the hypertufa mix on the dry side is very important as if it's too wet it will just slide off the mold.
Before casting I wrapped the mold in shrink-wrap as a sort of release agent. It also gives me something to pull on once the bowl's cured and I need to remove the core.
The finished trough is pretty good-sized at 24 x 20 x 10 1/2" high. Those are my boots for scale.
My friend admired an organic looking planter on my deck and asked if I could make a similar one. I probably made that planter about 6 years ago, with cardboard boxes as a mold. Almost all of my molds are rigid foam or plywood now, and it's been a while since I've done one with cardboard boxes. Above is the mold after I cast my friend's new trough. The outer carton has the flaps folded down, with shrink-wrap around it for safety. The inner carton is filled with sand. The best and worst part of using cartons for a mold is that they begin to fail as you cast the trough.
The trough was thicker than normal, since the outer carton started sagging outward almost immediately.
I trimmed the inside walls with the sharp end of my brick hammer and then scrubbed the outside with a wire brush. It's 21 x 21 x 14" high. It definitely has that organic feeling that my friend wanted.
Now they need a few weeks to cure.