Principles of Growing Clear Ice
Ice can be almost as clear as glass -- but it usually isn't. A close look at an ice cube or a piece of natural ice will usually disclose air bubbles and particles, which scatter light and make the ice look cloudy. Water containing dissolved salts will not produce clear ice, either.
In order to grow a large, clear block of ice such as those used for ice sculpture, the first step is to start with clear, particle-free water and a clean, covered container. A fine filter should remove particles, and most tap water should not even need this treatment.
The next step is to be sure the water is free of dissolved materials. Distilled or at least deionized water makes the clearest ice. But why?
Ice is constructed from water molecules that contain one oxygen and two hydrogen molecules each. These molecules fit together in a precise hexagonal framework by sharing their hydrogen atoms. The resulting structure has no room for other molecules, so dissolved impurities are rejected from the growing ice.
Since the impurities are released next to the growing ice face, they tend to accumulate there and produce a concentrated solution. Ice can grow faster in cleaner water, so any little projection on the ice, which can extend into purer water away from the interface, will grow faster than the interface itself. The projections rapidly grow into platelets up to a twenty-fifth of an inch in thickness and a half inch or so across. The tangled forest of platelets traps any air bubbles present, as well as small amounts of the concentrated solution, inside the growing ice. Both the air bubbles and the solution pockets (which have a lower freezing temperature than the pure water) scatter light, making the ice cloudy. Even a little salt will make the ice less than perfectly clear.
Finally, air bubbles must be eliminated. Water normally contains quite a lot of dissolved gasses, mostly air. (City water may also contain dissolved chlorine, and, judging from the odor, some well water in permafrost areas may contain hydrogen sulfide -- rotten egg gas.) The hexagonal ice structure has no more room for gas molecules than it does for dissolved solids, so gasses are forced out of the growing ice into bubbles at the interface.
Gasses can be driven out of water by heating it -- that's why warm carbonated drinks lose their fizz faster than cold ones. But air tends to get back into boiled water fairly quickly. If boiling is used to remove air, the water must then be frozen in a tightly sealed, air-free container or under a vacuum.
If the water is free of dissolved salts, so that the growing surface is not a tangle of crystals, the simplest way to grow bubble-free ice is to remove the bubbles from the ice-water interface before they are surrounded by more ice. Since air is lighter than water, it helps to grow the ice from the bottom up, so the air bubbles will tend to rise away from the ice surface instead of being trapped under it. Stirring the water gently or having it flow over the growing ice will then wash the bubbles from the ice. Stirring and growing the ice slowly will also help keep high concentrations of any dissolved impurities present from building up next to the growing ice face.
One of the authors has grown a cubic foot of clear ice by applying these principles. A container (in this case, a wastebasket), a gentle heat source and some insulation for the top of the container, a mechanical stirrer, and a cold place were all that was required.
Growing large blocks of clear ice would make an interesting high school science project. Why not have a competition to determine who could grow the clearest ice, or the largest piece of clear ice?
