To use a large ice sculpture mold, clean it thoroughly, place it securely in a freezer and fill it about 90 percent full with water. Depending on the size of the mold and the temperature of the freezer, leave the mold in the freezer for about 48 hours. Allow the mold to sit at room temperature for a few minutes before disassembling it.
While it may seem counterintuitive to only fill the mold to 90 percent of its capacity, this is necessary due to how water molecules behave as water freezes. While most substances become denser as they freeze, water actually becomes less dense. During freezing, water molecules repel each other and the empty space between them increases. This is the same property that makes ice float in liquid water, even though both ice and liquid water are made of the same substance. Because water expands as it freezes, the ice eventually fills the entire mold.
Tap water contains a large amount of dissolved oxygen, nitrogen and carbon dioxide gas. As water freezes, these gas molecules become trapped, resulting in a bubbly, cloudy appearance. Eliminate these bubbles and produce crystal clear ice by boiling the water for your sculpture for 30 minutes and letting it sit at room temperature overnight prior to freezing. This allows all of the gas molecules to escape.