As the ice increases in thickness it begins to corral all of the impurities into the center of the block. Dissolved air and unfiltered water will cause a cloudy core, but impurities are not the main reason why ice cubes are rarely crystal clear.
This expansion pressure is what makes ice cloudy in the center, not minerals and other debris. The visual flaws are caused by fractures in the ice when the last remaining liquid water in the center of the cube freezes and exerts a massive amount of force on the surrounding ice. The cube is basically cracked from the inside out.
This is in line with what I discovered when I tried all sorts of ways to get the air and impurities out of the ice- in the end it just didn't matter that much.
So in his second experiment, Darcy set about making ice freeze from the bottom up, so that the last part to freeze would be the top. Thus there would be no pressure cracking of the ice. His equipment list was:
- 6 ft of copper pipe
- my wife's cookie sheet
- a bag of nails
- a garbage bag
- an aquarium pump (powerhead)
- a square garbage can
- a coat hanger
- two Canadian winter days and nights at roughly -10°C
As he noted,
The process works, but it isn’t very efficient nor is it practical.
Anyway, there is still more work to be done. Icesperiments will continue!