In the last 20 years, fancy ice spheres, cubes, and columns have become a mark of quality in high-end bars, indicating to customers that the care taken by the bartender didn’t end with the construction of the drink. If anything, there’s been a bit of a nightlife arms race where fancy ice is concerned, with bartenders striving to one-up each other, carving special ice balls by hand or developing techniques to produce ice that’s perfectly clear.
But what is clear ice? Isn’t all ice clear? As it turns out, no. As researched by cocktail writer Camper English and others, conventional ice appears slightly cloudy because substances in tap water—air, dirt, and minerals—are pushed to the center as the water freezes from the outside in. In clear ice, all these substances are pushed to the bottom of the cubes as the water freezes from the top down, a phenomenon that English has dubbed “directional freezing.” Think of a lake in winter: Its water freezes from the top down, pushing the impurities and dirt toward the bottom of the lake, where it’s warmer and less likely to freeze. As a result, the ice that sits on top is perfectly clear.
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