Haven't tried Caladiums but did you try pressing them or silica gel? Guess the method depends on what you are going to do with them when they are dried -
We have GIANT hibiscus at work (about 7-8 inches across) and I pressed some in a phone book just to see what would happen, the color deepened and they dried beautifully, I have one slipped into a sheet protector and tapped to the window on the door of my office.
good luck to you and have fun