A little simple information that can save time, money, worry

These are a few small things I didn't know when I started out molding and casting. Perhaps sharing them will spare someone else a little anxiety and money.

Silicone mold-making tips:

1. Clean silicone adheres to itself, and it's possible to add more silicone to a mix you've started. So, if you come up short in your estimation of how much silicone to mix for a mold, just mix up some more and pour it in. Everything will set up fine as long as it's the same kind of silicone. You don't have to wait for the first batch to cure before adding more, but it will work either way. Just make sure no mold releases, dust or other materials get on the mold in progress.
2. Clean, well-mixed silicone can also be recycled into itself, and this can go a long way toward stretching the expensive material. If you mix too much silicone, just let it set up in the bowl and then cut it up into small blocks with a knife. You might even put it through a strong meat grinder  (not something you plan to use on food) to make smaller chips, or what we call "croutons" at work. Then, next time you make a block mold, simply sink some of the recycled material into the new mold in deeper areas after pouring rubber. My boss likes to mix the croutons into the mix while we're making it.
3. It's also possible to recycle old molds. When a mold is worn out, give it a good cleaning with rubbing alcohol or mold cleaner, then cut it up and use it as described above.
4. Temperature makes no difference in silicone setup. Silicone will cure in heat and cold. However, tin-based silicones (the most common) do require humidity to cure, and cold air is drier, so it may take silicone longer to set up on a cold day. It's possible to speed up the cure time by adding humidity to a room with a humidifier or vaporizer.

Resin casting tips:

1. Casting resins heat when they set up. The more resin in the batch, the hotter it will get and the faster it will set up. Small, thin batches take longer to harden than bigger batches. Bigger batches, because of the heat, seem to shrink more. I've seen large castings pull away from the mold a little toward the end of the cure time.
2. You have more working time if the resin is colder when you start using it. However, don't allow it to get down to freezing temperatures.
3. Resins do adhere to themselves, so it's possible to add more resin to cured resin. You get the best adherence if you do it soon after the first batch cures. Basically, this means that if you're casting a piece and come up short in the amount of resin you mixed, just let the piece set up completely and then add more within a few minutes to top it off. However, because of the shrinkage mentioned in the first tip, this won't work well if at all with very large solid castings. The new resin will leak around the area where shrinkage has occurred and you'll have a resin "skin" on your casting that you'll have to try to peel off.
4. Do not add fresh liquid resin to a partially cured batch of resin. The fluids will mix together, throwing off the chemical reaction. The casting will look like it has set up, but resin "bleeding" is likely to result, in which fluid oozes out of the casting for months or longer.
5. It may be possible to "shock" a bleeding casting into curing. Set it out in the sun for a few days or submerge it in boiling water (don't use a kitchen pot!) to see if that works.
6. Recycle bits of cured resin by submerging them in new castings. Just take broken bits and sink them into the liquid resin to stretch the material. I use bad castings and, particularly, pour spouts this way.