Crystal maze: Swetha Sivakumar writes on the mystery of clear ice

It’s the movement, in a sense, that makes ice cloudy. How should freezing occur, in order to render clear ice? How can one make that happen at home? Take a look

When enjoying a finely made cocktail or sipping on premium whiskey, the little details make all the difference: the aroma of the ingredients added, the feel of the glass in one’s hand.

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One of the most-overlooked components? The ice.

I mean, it’s just water, frozen: you might be thinking. What more is there to say? Well, regular ice is just water placed in a freezer and allowed to set. Which is why it is often clouded with air bubbles and trapped minerals. Clear ice is like a polished gemstone in one’s glass, and it adds no tinge of mineral flavour to a drink.

I am not suggesting this is in any way an issue. It just interested me to see the differences — and I recently managed to make clear ice at home. So here’s a bit of what I have learnt lately.

Most ice turns out cloudy because, as water starts to freeze in a regular tray, dissolved gases such as oxygen and nitrogen, and minerals such as calcium and fluoride, are pushed inward and trapped. This concentration creates the cloudy, sometimes opaque effect.

Clear ice is made using a method called directional freezing, in which freezing occurs from a single direction (usually top-down), so that impurities are pushed away from the freezing front, into what remains a body of water below.

In nature, we see this most commonly in frozen ponds: the top layer is often crystal-clear, with sediments and gases trapped further below. Here, the pond’s natural insulation (its banks) causes it to freeze slowly and evenly, from the top down. For this reason, in fact, early ice-sculptors used blocks from the tops of frozen lakes, because this ice was naturally clear.

As with the lakes, a key element in directional freezing is insulation on all but one side of any given body of water. This helps slow the rate of freezing, which is vital too.

Why? Well, because of the way water transitions from liquid to solid.

Ice starts forming at a nucleation site, a tiny point where water molecules first begin to rearrange themselves into a solid structure. Once a stable nucleus forms, it acts like a magnet. Other molecules prefer to attach to it rather than start a new structure. Starting a new nucleus takes more energy, because it requires a precise, ordered arrangement built from scratch. Adding to an existing one is easier.

When water freezes gradually, there’s just enough energy available to support a few nucleation events. This makes for clearer cubes, as the cold spreads more evenly, steadily pushing impurities away.

Incidentally, this kind of structure is exactly the opposite of what food companies aim for in frozen foods. When freezing vegetables or meat, faster is better. A rapid temperature drop triggers many nucleation points at once. Crystals start forming everywhere, and stay small; they aren’t given time to grow. This preserves the cell structure (and therefore texture) of the original food. Ice crystals that get too big rupture cell walls; when this happens, and the food eventually thaws, it ends up mushy.

Now to how I made clear ice at home.

The most common method involves a small, insulated cooler. Fill it with a few inches of water, leave the lid off, and place it in the freezer. The water will freeze unidirectionally, yielding a thick, clear layer of ice on top, and unfrozen, less-crystal-clear water below. One can now chip the ice clear of the edges, slide it out and shape it with a serrated knife. This is low-cost, effective, but requires a fair bit of hands-on work.

Bars and restaurants use clear-ice machines, which are expensive. Or they simply buy clear ice. Manufacturers create large blocks of the stuff, weighing about a tonne. These are then cut and shaped into cubes, or sold to artists and event managers for use in ice sculptures.

Clear-ice moulds with silicone insulation offer a good middle ground. They are designed with sieves at the bottom that allow impurities to be pushed out, into a separate level of the tray. The ice cubes emerge reasonably clear, and can be embellished by dropping an edible flower or other decoration in before placing the moulds in the freezer.

My DIY hack: fill a South Indian coffee filter with water and place it in an insulated flask. The top section has holes (to allow the coffee decoction to pass through), so the impurities get pushed out, leaving clear ice forming on top. Once frozen, this ice simply slides out (no chipping needed).

Toying with my coffee filter, it struck me how pretty water is in all its forms: ice, rivers, snowflakes, waterfalls. Isn’t it a bit magical how much beauty can come simply from molecules rearranging themselves in different ways?

(To reach Swetha Sivakumar with questions or feedback, email upgrademyfood@gmail.com)