Clear Ice in Cook's Illustrated
Matching the Fitness Program to the Liquor Brand

Make Clear Ice Balls Using a Thermos

Most readers of Alcademics are probably familiar with this by now, but I realized that in my many (many, many) ice posts here on Alcademics I don't have a straightforward post listing how I most often make large perfectly clear ice spheres at home. So I made this post to insert into my ice project index. 

6a00e553b3da2088340240a501d5c8200bThis method scales up so if you want to make 10 large clear ice balls at a time you can do so without taking up a lot of room or buying more expensive versions of this set-up. 

The process is based on Directional Freezing (pushing the cloudy part of freezing water in one direction, in this case out the hole in the bottom of the ice ball mold). The particular thermos to use was the suggestion of a reader on Alcademics. You nerds are the best! 


Ice balls molds - You'll want the large, 2.5 inch ice ball molds that are circular. 

Thermos - The most compact shape that fits these large ice sphere molds perfectly is these Thermos Funtainers. (You can use an insulated coffee mug or other thermos-style container if you want.)


  1. Fill the thermos with water to the top. The water doesn't need to be distilled or boiled or any of that. 
  2. Fill the ice ball mold with water.
  3. Hold your thumb over the hole in the ice ball mold and place it upside-down on the thermos. The water will stay in the mold part as well as the thermos.
  4. Put it in the freezer. The ice ball mold part will freeze first because it is not insulated. As it freezes, clear ice forms first and it will push the cloudy part of the ice down out the hole in the bottom (because it's partially insulated sitting on top) and into the thermos. The thermos should only just be starting to freeze when the ice ball on top is done freezing. 
  5. Wait about 24 hours. Try not to let it go for more than a day and half or so, as you don't want the water in the thermos to completely freeze, as it expands and will deform your thermos, or even shatter if it's a glass-insulated thermos. 
  6. The ice ball mold should easily pop off the thermos and be perfectly clear or pretty darn close.

Ice balls in thermos01
Ice balls in thermos01Ice balls in thermos01Ice balls in thermos01

Ice balls in thermos11
Ice balls in thermos11
Ice balls in thermos11
Ice balls in thermos11Ice balls in thermos27
Ice balls in thermos27


  • If your freezer is set on super duper ridiculously cold it may freeze too fast for this to work (it won't push the cloudy part out the bottom but instead into the center of the sphere). You probably don't need your freezer quite that cold!
  • If you're getting more of an egg shape from your ice sphere, try leaving a little space in the mold and/or thermos. 
  • You can also "polish" your egg-shaped ice sphere into a better sphere just using a clean dish cloth, or even just rubbing it in your hands.
  • If you're opening and closing your freezer and generally jostling the set-up, it tends to be cloudier. 


Be sure to check out the many many other things you can do with ice at the Index of Ice Experiments on Alcademics