Update 2/27/13: Like this post? Check out the 250-page, 65-recipe book that came from this research.
I was inspired to post it here by a reader named Andrew, who sent us an e-mail recently asking how to make fancy crystal-clear ice cubes at home. He’d tried everything from using distilled water to sonication, but never got the perfect cubes he envisioned.
Andrew’s not the only one – a whole bunch of folks have tried to make clear ice cubes at home that can match up to the “premium” ones available through companies like Kold-Draft, which are often used in high-end bars and restaurants. I’ve tried all the methods over the course of the last year and figured out what really works. No esoteric equipment required.
What are the benefits of clear ice?
Using clear ice for mixed drinks is not just about aesthetics.
Reason #1: Purity
If ice is clear, then by definition, it contains minimal impurities. Impurities, like chemicals used to treat tap water, minerals, and particles of food picked up from the freezer, become more soluble in water as it gets colder. But anyone who has ever marveled at a snowflake knows that water forms a crystal structure when frozen. Water’s crystal lattice structure forms most efficiently without impurities, so impurities are slowly forced out as water freezes.
Experiment: Try dissolving a small pinch of salt into a cup of water. Taste the mixture, remember how salty it tastes, and then freeze it for a few hours. Open the freezer a few hours later, and you should find that a strange ice cube has formed. Taste it. The outer wall of the cube will taste salty, but the remaining ice will be less salty and have a loose, snowcone-like texture. This occurs because the salt was forced to the outsides of the cube and into its center as the water froze. In fact, crack the cube open and a small puddle of salty water should leak out. The loose texture of the ice is caused by all the holes the salt leaves as it migrates out of the water.
Normal tap water doesn’t have nearly as much mineral content as salted water, but the same principles apply. The more slowly you freeze ice, the more impurities get forced out to surface of the cube. Takeaway? Rinse ice off before you use it to remove most of the impurities.
Reason #2: (sort of) texture and dilution
I can say anecdotally that clear ice has a harder “crunch” than normal ice and that it melts more consistently.
Kold-draft and others claim that clear ice melts more slowly than normal ice. However, I have yet to see a test that compares the melting rates of equal weights of clear versus cloudy ice and I haven’t gotten around to doing it myself.
What causes cloudiness in “normal” ice?
The transparency of liquid water is caused by the lack of interfaces where light can scatter. With liquid water, dissolved salts and minerals don’t affect transparency.
Ice becomes cloudy for a number of reasons:
Reason #1: Dissolved gases.
As gases are forced out of solution by crystal formation, they are trapped as tiny bubbles (see image above)
Reason #2: Crystal formation.
“At its freezing point, water molecules are moving so slowly that they begin to gather in loose, undefined clusters, called seed crystals. As the temperature continues to drop, molecules line up around the seed crystals in increasingly rigid formation until all of the water crystallizes. The longer it takes for crystals to form, the larger they will be.” [Joachim and Schloss, The Science of Good Food, Pg. 253]
You know how you’re supposed to freeze food quickly so smaller ice crystals form? You want the exact opposite effect when making clear ice cubes.
Reason #3: Density changes
Water is most dense at 4C or 40F, but it’s only slightly less dense at freezing temp, 0C/32F. Ice at freezing temperature is 8.3% less dense than liquid water at the same temperature – a substantial difference.
If water cools quickly, a density change can actually occur after the ice has already frozen solid. That’s right - molecules within ice continue to shift even in its solid form. Don’t believe me? Take a look at this time-lapse video of ice freezing.
The video makes it really clear (har): notice how as early as 1:00 minute in, air bubbles are trapped in solid ice. But the ice continues to grow significantly well after the entire block turns solid.
How do professionals make perfectly clear ice?
Ice sculptors have long made large blocks of clear ice by slowly freezing water from the outside in. They agitate the water in the center of the ice that remains liquid to allow gases to escape. When the ice is almost frozen solid, the very center of the block is drained. High-end ice machines spray layers of water onto a heat sink that freezes the ice layer by layer. Dissolved gases are quickly forced out of solution.
How do you make perfectly clear ice at home?
Two ways work: Directional freezing, and high-temperature freezing.
Option #1: Directional freezing
Cocktail journalist Camper English, did a fascinating series of experiments in which he tried a number of oft-recommended tricks for making clear ice (alcademics.com/ice) – everything from consecutive distilled water to boiling and refreezing, to using carbonated water. In the end, he found that without access to specialized equipment, all homemade ice will trap gas. The trick, then, is to force the gas (and therefore the cloudy part of the ice) all to one area, where it can be isolated and removed from the clear remainder.
To make his ice cubes, Camper used custom molds and an igloo cooler to force long sticks of ice to freeze from the top down. The cooler’s insulation prevented cold air from getting to the sides or bottom of the ice. All the cloudiness was trapped at the bottom of the sticks; a quick melt easily got rid of it.
Option #2: High-temperature freezing
While Camper’s method certainly works, especially for large quantities or unique shapes of ice, it can be a little inconvenient, not to mention all the equipment involved.
A commenter on Camper’s blog turned me on to high-temperature freezing – basically, allowing water to freeze extremely slowly at just below its freezing point, ideally around 30F or -1C.
The method combats all the reasons ice turns cloudy that I talked about above. The slow freezing forces gases out, creates large crystals, and allows density changes to occur with minimal stress.
If you don’t want your fridge at a really warm temperature, I’ve found that the top shelf of my refrigerator sits at about 29.5F when I turn it to its coldest setting. See below.
Stuff that doesn’t work
Search the internet for directions on making clear ice and the most popular recommendations are to boil the water first, use distilled water, and insulate the ice while freezing.
Boiled and distilled water help rid water of impurities, but remember that oxygen dissolves readily in water at room temperature and atmospheric pressure, so any gas you manage to boil out rapidly finds its way back in. More importantly, simply getting dissolved gases out doesn’t help to address expansion or crystal formation.
Insulation works to an extent, but not if your freezer is really cold. My main freezer sits at just below 0F/-18C. I tried a commercial insulated ice cube product, as well as about a dozen variations of DIY-jobs using styrofoam, PVC, water baths, etc. None of these worked in my freezer – all left a cloudy area somewhere in the cube.
Insulation will probably work if you have a warmer freezer – somewhere in the 17-25F range, though I haven’t test this.
Finally, I’ve read reports that if you first freeze ice, then pour boiling hot water on it and refreeze, the whole block will come out clear. I think this does work – but only because the hot water raises the temperature of the freezer.
What would you use fancy ice cubes for?