How to make clear ice

De-ionized water from the store, ice cube tray, fridge or a wintery back yard.

Clear ice is beautiful and mesmerizing in cocktails.

Ice, the final frontier ... what better time to start thinking about ice but when the days grow short and darkness falls across the lands.

Ice is one of the essential ingredients for most cocktails, with its purpose being manifold. It can be used for dilution, cooling or simply for its optical appeal. The latter is particularly intriguing if the ice is clear and free of 'white mist'. Making mist-free ice at home, however, can be a challenge.

White discoloration of ice is due to impurity-inclusions such as gas-bubbles or small particles. Both scatter photons and give the ice 'color' instead of letting light pass through unobstructed. The challenge is thus to avoid the incorporation of those impurities in ice during the freezing process.

Fortunately, nature is willing to help us by letting pure water freeze first and impurity-rich water later. In geek-talk, this effect is called freezing-point depression which can be attributed to a decrease of the 'chemical potential' of liquid water in the presence of impurities (gas molecules or ions from dissolved salts/minerals). Impure water melts at lower temperatures which is also why salt helps to keep wintery roads un-frozen. During the freezing process, this effect also prevents impurities from being incorporated into newly formed ice. Instead, gas and minerals are pushed into the remaining liquid, thus increasing its impurity concentration. Chemists use this phenomenon in a related purification process referred to as 'recrystallization'.

Now, the mist in water-ice develops either when water freezes to fast to allow impurities to escape from the boundary-region between frozen and liquid water, or if the concentration of impurities in the remaining liquid exceeds their solubility product, at which point impurities begin to precipitate, gas in the form of bubbles and ions in the form of mineral floc.

This suggests that there are two possible remedies.

• Using mineral-depleted (de-ionized) water and

• reducing the cooling rate.

The photo above shows ice cubes from our tap water (left) and from deionized water (right). These were either frozen in non-insulated silicone ice cube trays (top) or with styrofoam insulation to slow down the cooling rate (bottom). It is clear how both, using de-ionized water and - to some degree - slowing the cooling rate help to reduce white mist.

'Directional freezing', as the technique of thermally insulating an ice-cube tray is sometimes called, helps because it slows the freezing process. Water can thus freeze in its natural direction (top-down) and impurities have time to gather in the last remaining liquid which, if the ice-tray is prematurely removed from the freezer, can easily be discarded.