What is Directional Freezing?

Directional Freezing is a simple method to make crystal clear ice by controlling the direction that water freezes. It was first explained here on Alcademics.com by Camper English (me) in December 2009 after months of experiments.  The method has been written about in books, used in commercial products, and is employed in many small cocktail bars around the world. 

The Directional Freezing method is: Allow water to freeze into ice from only one direction (one side of a container) and the ice will be clear until the very last part to freeze. The last part to freeze (if allowed to freeze at all) will be cloudy.

The simplest (and original) way to make a clear ice block by directional freezing is to fill a hard-sided picnic cooler with water, place it in a freezer, and allow it to freeze with the cooler's top off. The water will only freeze into ice from the top-down, and only the last 25% or so of the ice block that forms will be cloudy. If the block is removed from the freezer before this point, one will have a perfectly clear slab of ice. Otherwise, the bottom cloudy portion of the ice block can be cut off from the clear part.