How to make clear ice

If you’re trying to perfect your presentation of cocktails, you might want to make perfectly clear ice. While a lot of people will tell you to boil the water first — and it does help a certain amount — it may not give you quite the perfection you’re looking for, depending on the quality of your water. So we can take advantage of chemistry!

When water freezes, it “wants” to make as perfect a crystal as it can, so it pushes dust particles and dissolved air away as the crystals form. However, they have to go somewhere.

In a traditional ice cube tray, where you want the water to freeze as quickly as possible, the walls of each cell conduct enough heat that the water freezes more-or-less evenly from all sides. The result is that the outer portion of each cube is clear, but as you go inward, you get the cloudy inclusions, because there’s nowhere else for the trapped air to go.

The guy who makes YouTube videos under the name “Cocktail Chemistry” revealed the simple solution in this video: use a cooler, to force the ice to freeze in one direction only; that is, from the surface.

The only problem with his solution was, after several attempts, I simply could not get a clean enough cut to make nice ice cubes. I got very pretty and clear irregular chips, which would be great in some drinks or in the shaker, but still not quite what I wanted. So, I refined further, and I found a winner.

The instructions I’m providing are complete, but you may still want to watch Cocktail Chemistry’s video (if only for the relaxing elevator music) as an illustration.

First, get a small cooler which can fit into your freezer, and into which the ice cube tray can fit (oriented flat as normal).  Depending on the size of the trays, you might figure out a more efficient way to get them to fit, say by cutting a row or column off of one or both trays.  In my example here, I cut one row off each and trimmed the wide edge of the trays; then they fit neatly, as you’ll see, and I’m getting the most cubes out of a batch.

We had trays I liked. I placed them in the cooler to figure out the optimum number of cubes I could get in a batch.
We had trays I liked. I placed them in the cooler to figure out the optimum number of cubes I could get in a batch.

Bore some small holes into the bottom of each cell of the ice cube tray.  If you’re using a plastic tray and don’t mind a little bit of the smell, you can take a metal skewer or knitting needle, heat one end in the flame of a gas stove, and while it’s hot, melt the holes into the bottom of the tray.

If you’re using silicone, the heat you’d need for that plan is prohibitive, and simply pricking a hole with a tack or even a nail won’t give you a good clean hole.  It’ll still leak, of course, but it may not be as efficient for the purification and your cubes may still be cloudy.  So I will show you in a separate post how to solve this problem!

Now put something underneath the tray to support it so that it stays just at the surface of the water.

I got these copper pipe couplings at the local big box hardware store. They are solid metal (well, their walls are) and so will not float and ruin the process.
I got these copper pipe couplings at the local big box hardware store. They are solid metal (well, their walls are) and so will not float and ruin the process.
Here's the copper couplings resting in the bottom of the cooler. They don't need to be that neat.
Here’s the copper couplings resting in the bottom of the cooler. They don’t need to be that neat.

Put the tray(s) in the cooler and fill the cooler until the water fills the space below the trays as well as the trays themselves.  Since you bored holes in the tray, you can fill the whole assembly through the trays.

Here's one tray placed in the cooler ...
Here’s one tray placed in the cooler …
... and the other. Note that I had to cut off one row of cubes from each to get them to fit, but that's fine.
… and the other. Note that I had to cut off one row of cubes from each to get them to fit, but that’s fine.
Filling the cooler. The water will flow through the bored holes and fill the "impurities" space first, then the trays.
Filling the cooler. The water will flow through the bored holes and fill the “impurities” space first, then the trays.
The water fills the trays as normal.
The water fills the trays as normal.

Put it in the freezer. Do not put the lid on the cooler.

The cooler in my chest freezer. While you're waiting, you could bake that delicious Trader Joe's™ pizza. Mmm, pizza.
The cooler in my chest freezer. While you’re waiting, you could bake that delicious Trader Joe’s™ pizza. Mmm, pizza.

Wait until the water has frozen all the way through the ice tray, but not entirely all the way through the cooler. (You’ll have to experiment with your cooler and freezer to figure out how long this takes. I have a Coleman FlipLid 6 Personal Cooler and in my basement chest freezer, 12 hours is plenty.)

IMG_2223
After freezing, the cooler is removed and placed in my kitchen sink.

Take the frozen mass out of the cooler.

I have a red plastic cutting board in the sink.  The trays have been slid out.  You can see the ice clinging to the sides of the trays and how it extended down below the trays along the sides of the cooler.  If I allowed it to freeze a few more hours, the bottom of the cooler would have started to freeze too, and I would have a big block of beautifully transparent ice with an unfrozen water core.
I have a red plastic cutting board in the sink. The trays have been slid out. You can see the ice clinging to the sides of the trays and how it extended down below the trays along the sides of the cooler. If I allowed it to freeze a few more hours, the bottom of the cooler would have started to freeze too, and I would have a big block of beautifully transparent ice with an unfrozen water core.

With warm water, chisel, ice pick, or whatever tool you like, detach the ice tray from the rest of the ice which has frozen below the tray. (Generally, the water at the sides of the cooler will have just begun to freeze, so under the tray you’ll have a “hollow” block of ice still filled with unfrozen water. That water is where all the invisible impurities have been pushed by the crystallization process.)

Pop the cubes from the tray and enjoy!

The clear ice cubes in a bowl.
The clear ice cubes in a bowl.
The cubes in a glass.
The cubes in a glass.
The cubes in water.  There are no less than six cubes in there.
The cubes in water. There are no less than six cubes in there.

Additional notes:

  • If the ice cube tray isn’t just at the surface of the water, you’ll have headaches, because there will still be a solid mass of ice to which the cubes are frozen, and you’ll still be trying to chip them away.  So you really want it as precise as possible.
  • The silicone trays I photographed do not float, because they’re a solid piece of silicone which traps no air between the cells.  If you get molded plastic trays, you may find that they have a small air pocket between each cell which makes the tray float.  That saves you having to put supports in the cooler and you can just sorta fill the cooler until there’s “enough” water under the tray.  Also, it will automatically stay at the surface.  (Again, you may have to experiment to find what depth works given your water, freezer, cooler, tray, etc.)
  • Remember, you can’t just leave the tray at the bottom of the cooler and fill it, because you need the big water “gap” into which the impurities get pushed.
  • If you really hate wasting the water below your tray, there’s no reason you can’t save that water if you pour it out carefully, and then put it back into the cooler for the next batch of cubes! The freezing process will purify it as well.  You’re going to have to add more water for the second batch, but at least you don’t have to fill the cooler completely from empty.

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