Going Clear (in cocktails)

“Surprisingly, though, taste is not the only consideration. Presentation is the most important thing - garnish, colour, the glass. If the drink pleases the eye, the customer's mouth will start to water. Aroma is the next thing, then taste. We apply these criteria when judging a new cocktail in a competition.”

~ Peter Dolelli, The Savoy Cocktail Book

In every high-end bar (and even some of the lower ones), there is one ingredient common to every cocktail they make, an ingredient so subtle, it's downright transparent (or, at least, it should be). Depending on the device that made it, ice can be cloudy with an increasingly dense galaxy of bubbles towards the center, or it can be clear as glass. Achieving that distinction is no simple feat. (More on that in a minute.) First, let's talk about how ice got in cocktails in the first place.

The origins of ice in drinks are fairly simple and understandable. Someone discovered that in warmer weather, people preferred (but needed to become accustomed to) cold alcoholic beverages. Cold softened the harshness of liquor and, as the ice sacrificed itself in the chilling process, it also served to dilute the spirit slightly. For a bar owner, anything that would sell more liquor was worth paying attention to and paying for. With that, the ice trade was born.

This was, of course, well before refrigeration was invented so it was up to Mother Nature to do the freezing. Large blocks of ice frozen in lakes and rivers were harvested and stored in insulated warehouses for the next year – literally, an ice house. That's right; the ice you enjoyed in August was put in storage in February.

One man built an empire shipping ice from New England to as far as Calcutta, India. His curious story can be found in a book entitled “The Frozen Water Trade,” a biography of Frederick Tudor.

These days, ice in cocktails is largely taken for granted. Still, ice should be treated with the same care as any other ingredient. Any flavor ice might impart is certainly a consideration, but it’s easy to overlook its aesthetic contribution to a cocktail. Simply put, clear ice is more attractive than cloudy ice. However, making clear ice is somewhat trickier than one might expect.

Those oddly-shaped “cubes” generated about every 30 minutes or so by your refrigerator – utilitarian, unattractive, efficient, largely opaque, but abundant – are maybe suitable for a glass of iced tea or a Rum and Coke. They’re also handy for chilling sparkling, white, or rosé wines. But when ice is given a starring role, it should spruce itself up a bit and “look the part”.

Fed up with ugly ice, I decided to just make my own. (It’s one of the easier things to make.) I took tap (and even filtered) water, poured it into an ice cube tray, and got results very similar to the ice maker. I compared my results to a cube I got in a drink at Dutch Kills in New York which was perfectly clear. How could the results be so different? My water isn’t drastically different than theirs (both consisting almost exclusively of two parts hydrogen to one part oxygen), and it’s not like there are different “kinds” of cold. What were they doing that I wasn’t?

I did the obvious and searched the internet, finding opinions (or, at least, theories) everywhere from Yahoo Answers, to the marketing materials of ice machine manufacturers, to the engineers responsible for The Ice Hotels in Canada and Sweden who need to make massive amounts of clear ice. I discovered as many ways to do this as there are cures for hiccups, and just about as many actually work.

We’ll start with two obvious points:

1. Filter the water. Even if what you filter out isn’t contributing to the appearance of the ice, you’ll be drinking some portion of the end product and filtering will make it taste better.
   
   
2. It is primarily bubbles and fracturing, rather than impurities, that contribute most to the cloudiness of ice. 

Many online “authorities” advise boiling the water which does have some solid science behind it. At boiling point, water can no longer “hold” dissolved air and will release it (counter-intuitive given the number of bubbles created during boiling, but those are different.) This is very similar to why carbonated beverages in an open container will go flat more quickly in a warm environment than a cold one. A good boil reduced the density of bubbles considerably, but essentially you're just sending fewer bubbles to the center of the cube rather than eliminating them entirely.

I then began to wonder if the speed of freezing affected the results. Rather than sitting in a tray and buffeted by the blower in the freezer, I actually put a container of water at the bottom of the freezer inside an insulated bag. This, too, made it slightly clearer but it still wasn't quite right. There were still bubbles in the center of the cube – this was a huge clue.

Clearly, the last bit of water to freeze is the one which contains the air; bubbles were herded to the center of the cube and frozen in place. What if the center of the cube wasn’t the last to freeze? What if it was the top or the bottom? What if the water froze, say, from the bottom to the top and pushed all the bubbles out before freezing? Assuming this would work, how might one control the direction in which the water freezes?

There is another factor I wasn't quite capable of grasping on my own. Ice cubes generally freeze from the outside in (which is why all the bubbles get pushed to the center). Now, think about what water does when it freezes – it expands by about 8-9% to the tune of about 30,000 pounds per square inch. In the same way ice easily cracks pipes in cold weather, it does the same thing to itself when it freezes. The outside freezes first, then the inside begins to freeze, which expands and cracks the outside. Then the inside freezes deeper, cracking the outside again, and so on.

We’re back to the direction factor again. Not only does freezing in only one direction get rid of the bubbles, it prevents fracturing because water being frozen is never encased in already-frozen ice.

One method used in restaurant-grade machines is to continuously run water over a cold plate building up ice in layers; a somewhat impractical method in a home refrigerator (though I did consider drilling holes in mine and installing an aquarium pump to cycle the water). The purest water freezes first. By simply running water over the same surface, you’ll get clear cubes. (This is in part why icicles are so clear.)

Rather than the proactive approach shown above, there is a sneakier “passive” method. Simply insulate part, but not all, of the container. The part most exposed will freeze first and dictate the direction in which the water eventually freezes. While this still isn't a perfect method, it gets mighty close.

For comparison, here are 4 examples.

The first 3 all used the same container. 

Example 1 was placed in the freezer as-is with no insulating layer. Note the amount and location of bubbles as well as fracturing.

Example 2 is much closer to ideal, but the insulation wasn’t packed tightly enough against the container causing the lower section to freeze too early. While the top section is fine, it doesn’t yield a very big cube.

Example 3 was in the same insulation as #2, but the insulation was held tighter to the container.

Example 4 is just a larger version of #3 created in a 1-liter water bottle.

Here's how I did it:

1. Take a 1 liter plastic container (it needs to be flexible, of course, or ice will tear it apart as it expands) and cut the top off just above the label. You'll want one with smooth, straight sides to allow the ice to slide out of the container.
2. Wrap the very bottom in a thicker layer of insulation. (An old blanket, sweater, or even a folded kitchen towel works well for this.)
3. Above that, wrap a thinner layer, and leave the top of the container exposed. Note that a good insulator should block the airflow generated by the fan in your freezer.
4. Fill the container with water and place it in the freezer, preferably with the open top positioned near where the fan in your freezer blows (usually near the ice maker if you have one). Note that, by insulating the container, you’re controlling the direction of freezing, but you're also slowing down considerably the rate at which it freezes.
5. Leave it in the freezer overnight. Depending on the thickness of your insulation, it may take longer. Check the progress carefully or you'll likely introduce a bubble into the otherwise pristine ice block. (I know this because I did it; twice.)
6. Within 24-48 hours, depending on your insulation, you should wind up with an extremely clear majority of the “cube” with all the bubbles concentrated at the bottom. You can chisel or melt away the bubbled section. Alternatively, if you can get the timing right, you can stop the freezing process before all of the water has frozen and avoid the bubbly end piece.
7. A cheap bread knife makes a surprisingly effective tool for scoring the block and then you can crack it into more useful pieces. Cut a groove in the ice, maybe 1/8th inch deep. Then, holding the knife against the block, take a mallet and smack the back of the knife. 

There you have it. Perfectly (or, at least, relatively) clear ice is probably the smallest detail of a finely-made homemade cocktail, but it is clearly an important one.