Ruth,
I have made latex molds, and also a few silicone molds from concrete leaf castings. I have NOT used the silicone in the tube, but a two part putty that is generally used for culinary work. For either the latex, or the silicone that I have done, I have not used a release agent of any kind. It will feel like the mold (latex or silicone) is sticking to the master (concrete leaf), but will release when pulled off of the master. For latex molds it is very important to sprinkle the back of the mold prior to peeling it off with talcum, baby powder, or cornstarch because in it's new state it will adhere to itself. Once the latex mold is removed sprinkle the veined side.
Others here have used release agents for mold making and am hopeful they will respond.
Happy casting,
Dena

Thanks Dana! I've been holding off on it because I don't want to make my leaf unpaintable by coating it with something I can't get off later. I have the silicone in the tubes, so I'll try a little bit and make sure it releases okay. I guess if there's a problem with that, I could paint it first and then use a release agent to make a mold from it.

I was up your way today, taking a friend to SeaTac to catch a flight to Denmark. It was sure a gorgeous day for a drive!

-Ruth