You can learn a lot about ice just by looking at it. A pristinely clear cube tells you it’s made from pure water in a perfect crystal lattice. Cloudy ice signals impurities, absorbed gases, and irregular crystals. Those impurities (such as chlorine or fluoride) impart unwanted flavors, while absorbed gases (like oxygen and nitrogen) and irregular crystals weaken cubes, making them more prone to shattering while shaking. This creates many unwanted ice shards that will overdilute your cocktail. Bartenders go to obsessive, time-consuming lengths to achieve perfectly clear ice for craft cocktails, but we just wanted an easy at-home technique to get us as close to clear as is practical.
Adapted from A New Way to Cook by Sally Schneider, (Artisan 2001).