Two Tips On Casting: seam on the seal/ammonia in clay

Can't remember exactly where I read these, but they're good tips to keep in mind when casting:

1. instead of just making keys, like with hex nuts, also dig a trench around the first half of the mold. This will create a nice little seal all the way 'round the cavity, which will help keep the casting medium from seeping out of the mold.

2. when building up a layer of clay in making the first half of the mold, make sure the clay is ammonia-free, as ammonia inhibits the curing of silicone.