If you're trying directional freezing and still not getting clear ice, here are a few considerations.
Know What's Reasonable.
Directional Freezing doesn't get rid of any cloudiness in ice, it just moves it toward one end of your cube/block. You will always have about 15-30 percent cloudy ice as the last part of the ice freezes (if you allow it to freeze all the way).
Motion or Vibration.
While your ice is freezing are you moving the container around? Does your freezer shake and vibrate? Are you opening and slamming the door shut a lot? Any sort of motion tends to knock air bubbles together, and they float up and stick to the bottom of ice as it's freezing downwards, often leading to little bubble trails and starburst-shaped groups of bubbles. The more vibration, the more bubbles/cloudy bits form.
Super Cold Freezers.
A friend of mine couldn't get his ice balls all the way clear using the thermos method. After much back and forth, we realized that he has a super fancy freezer set to super duper cold. The ice ball froze so quickly from all sides that it didn't have a chance to push the cloudy parts out the bottom of the ice ball mold into the thermos.
In general, the warmer your freezer (but still below freezing, obviously), the clearer the ice - but you still want to be food safe (below 0 Fahrenheit or -18 Celsius). As far as I know, the Igloo cooler method should still work in a very cold freezer, but some thinner vessels will freeze from the outside-in rather than in one direction.
Some Tips to Improve Clarity.
If your ice is pretty clear but you've become obsessed with making it ridiculously clear and minimizing the cloudy part at the end of the block (it happens), a few things you can try are:
- Unscrew the aerator off your sink faucet (and put it back on after; it saves water waste).
- Run it through a water filter. Even though direction freezing acts as a filter, pushing minerals and other impurities into the last part of the ice to freeze, you can reduce the mineral/impurity content by using water from the pitcher.
- The reason I run my water through a filter is to remove chlorine/chloramine tastes in the water. I don't care about the minute improvement, if any, in clarity, but I find that if I use unfiltered water when I pop the block out of the cooler there's a big release of chlorine smell. Blech.
- Boil the water. In my opinion, this isn't worth the effort or energy waste, but it can improve clarity ever so slightly. Boiling water should reduce trapped air.
These tips shouldn't make radical, but small, improvements in your ice's clarity.
To read all the ice posts here on Alcademics, check out the Index of Ice Experiments.