When you freeze rose petals, the water forms ice crystals which can then be sublimated into water vapor. The petals start in a large chamber chilled to -20 F. A deep vacuum is pulled on the tube with another freeze chamber in between the large chamber and the vacuum pump. The middle freeze change is kept at -60 F. The ice crystals become airborne water vapor in the first chamber then are sucked through the second chamber. The second chamber is so cold that the airborne water vapors get stuck to the side. (Kind of like when you stuck your lips on a cold flag pole as kid.) The technical term is Lyophilization. No the process of freeze drying, not the lips on the flagpole thing. That is called Youthful Exuberance.

If you are really into the science part of this take a look at the Virtis Companies Lyophilization 101

Back to the petals!