A large number of people have become interested in clear ice. The best way to answer the clear ice question is to think about icicles.
If you live in an area where icicles form in the winter, you know that icicles are normally clear and beautiful. There are two things that make icicles so perfect:
- Icicles are made from pure water in the form of melted snow.
- Icicles are created in layers. Water drips down the icicle and freezes in progressive layers rather than freezing all at once. This approach avoids entrapped bubbles.
If you ever look inside a restaurant ice maker, you will find that it makes ice in layers. Cold water runs continuously over a plate or a grid where the ice is forming, and the ice cubes (or ice disks in some machines) grow in layers.
If you would like to try creating clear ice at home, start with distilled water (to eliminate the minerals) and boil it (to eliminate air dissolved in the water). Make the cubes small or thin to get closer to the way that icicles are formed.
Robert Fulghum adds the following:
A technique used in most ice manufacturing plants that make large blocks of ice is to put a tube in the center of the container of water that is to be frozen. Through the tube they bubble a very low pressure stream of air. Before the tube becomes ice-bound, they remove it, and they pour or suction the water that is left in the center of the ice block away. All of the impurities -- dirt, dissolved air and minerals -- are forced into this water by the crystallizing ice. They fill the void with fresh water (or not) and continue freezing. The core of the block is clouded but the rest of the block is clear. If the core is not refilled and frozen, the entire block is clear.