About 6 years ago, I popped in to an awesome cocktail bar in Long Island City, NY called “Dutch Kills”. The bartender was cleaning up after two customers who had just left and, in one of the glasses, sat the remnants of the clearest chunk of ice I’d ever seen, about 2-inches square. It was completely unblemished, uncloudy, and – frankly – beautiful.

The bartender noted my gaze and, in an effort to clean up, she pulled a “fresh” cube out of the freezer and sat it on a plate in front of me. I couldn’t help but stare at it. I asked her how in the world they made it and she admitted that they don’t. They buy the ice from a nearby company that makes ice for sculptures and she wasn’t exactly sure how they were made.

When I got home, I started researching how this was done. None of the ice I had ever made was crystal-clear. Not even close! (This frustrated me more than it probably should.) When I fill a tray with water, stick it in the freezer, and take it out a few hours later, all I have are a bunch of milky, uninspiring cubes riddled with haze and bubbles. The only (barely) clear part is around the edges. There aren’t all that many variables. Ice isn’t “complex” and it’s certainly not made of obscure ingredients. What was that ice company doing differently?

Google wasn’t much help. There were plenty of wrong answers about how to do it. Using distilled or bottled water was the first thing I tested and dismissed. When I compared tap water against bottled, the results were subtle (to put it mildly). The second trick (which did actually help slightly) is to boil the water first. The reason this works is akin to why an opened bottle of soda will go flat more quickly when left out in a warm room than it will in a refrigerator. Hot water has a harder time holding dissolved gasses. Boiling simply reduces the amount of air dissolved in it. Still, neither of these techniques got anywhere near that glass-clear look I was after.

For the first step, I just sat down with a few of my cloudy ice cubes and really pondered what I was looking at. A key factor was noting where the bubbles are (hint – they’re in the center of the cube) and thinking how they got there. The fact that lake ice and icicles are clear was another big clue. They have something in common: they freeze in only one direction. A lake freezes from the top-down while icicles freeze in layers, both vertically and horizontally. It’s also worth noting that the purest water freezes first and the impurities are washed away. (This is why ice that forms on a river is clear.)

However, water in your freezer does not freeze in only one direction; it freezes inward from all directions. Hence, all the bubbles (and impurities) get pushed to the center of the cube. The first goal is to freeze the cubes in such a way that the bubbles never get to the center.

The bubbles, as it turns out, are only half the problem. As water freezes, it also expands. (If you’ve ever forgotten a glass-bottled beverage in a freezer or if the pipes in your house weren’t protected from the elements, you’ve learned this lesson the hard way.) That expansion is insanely strong and will break just about anything that surrounds it – including other ice. As water freezes from the outside in, the inside ice expands and fractures the outer ice, then continues freezing, fractures again, and so on. Apart from the bubbles, the haze in the center of the cube is also caused by that fracturing.

The ultimate solution (and the way that the ice sculpting places do it) is to freeze water in only one direction. This serves three purposes:

It prevents the bubbles from getting trapped in the ice. It allows the ice to expand as it freezes which avoids fracturing. Since the purest water freezes first, impurities are (mostly) pushed out of the way.

Great! All I needed to do was figure out how to freeze water in only one direction.

Simply control when different parts of a container freeze. To do this, take a good sized plastic container and wrap it in some kind of insulator. Just about anything, an old sweater or scarf, styrofoam, bubble wrap, or even a kitchen towel will work. Wrap the edges and bottom of the container with the insulator and leave the top exposed. Place the container, wrapped in the insulation and filled with water, in the freezer for a day or two. (The insulation slows the process of freezing down considerably so plan ahead. It can take quite a while.)

You won’t wind up with a perfectly clear block (or cylinder) all the way through, but if you did the insulating right, the majority of the ice will be clear. Simply cut or melt the cloudy portion of the cube away and you’ll be left with ice suitable for keeping a cocktail cold for quite a while; just try not to stare too hard at it.

Note: Despite the size of the cube, the relatively small amount of surface area means that a single, large ice cube is better for keeping a cocktail cold but not necessarily getting it cold. It does, however, limit the dilution that would otherwise happen with lots of smaller ice cubes.

This article was adapted from a larger blog post I wrote. You can view the original here.

Daniel.