How can I make a mould out of organic material? Birdnest=cement

Silicone. It is thick enough to not creep through the cracks and flexible enough to be used as a mold. Make sure to cover the entire surface and let dry 24-48 hours or longer depending on your humidity levels.

Hello Shirleyandboston :) No where online could I find SPECIFICALLY how to make a cement bird's nest from cement, but I HAVE FOUND the link below using organic materials to make a silicone mold in order to make wax candles. I am thinking that since cement isn't hot like melted wax the silicone molds should work fine for cement too (you just don't have to put a hole in the bottom of the mold for a candle wick). It shows how to remove from the mold any organic material you cannot remove by hand, and I'm sure you will need to clean out the silicone mold that way from the bird nest you take a mold from. AND because a nest is practically 100% porous it will need to be sealed (heavily, I would suppose) with several layers of a Sealer and Releaser as shown in the video below (like POL-EASE 2350 Sealer and Releaser Agent) and then wait for it to dry completely before beginning the molding process. I believe you will have to carefully glue (and maybe then nail) the bottom of the nest to the bottom of the container that the silicone will be poured into, and care taken when pouring the silicone INTO and AROUND the nest in order to maintain the integrity of all the bumps and edges so it will look as realistic as possible once you get all the organic material cleaned out and the cement poured in and then set solid and released. It will all make more sense when you watch the video. Good Luck Shirley, and thanks for coming to HOMETALK for answers.

https://www.youtube.com/watch?v=0LjmrDWyncg