A while back I came across an easy idea for an ice sculpture centerpiece. From memory - Fill a bundt pan with water and stir in i.e. cranberries. When it is unmolded it will look like whatever you put into the water is suspended in air. I will try to find the exact directions and post. If anyone has tried an ice mold, would love to hear how it turned out and how you made your sculpture. This site has some great tips and ideas. Using a cut-off milk carton, is one I'd like to try - and, perhaps adding some food coloring to the water.
http://www.sculpturesinice.com/tips.html
http://www.sculpturesinice.com/tips.html
