Most ice you make at home or purchase from a store is at least a bit cloudy. While this cloudiness is often attributed to vague “impurities,” the cleanliness of your starting water has little to do with it. Of course, you should always start with clean water, but even an ice tray full of filtered, boiled water will still produce cubes with cloudy centers. Within those outer layers of your half-frozen cubes, there will be a bubble of air just waiting to cloud up your ice. When the expanding ice crystal matrix closes in on that bubble, the trapped air will disrupt the clear crystal formation and create that fuzzy white web we’re all so used to. In more scientific terms, the trapped air has a lower thermal load than water and expands differently when the temperature changes. This causes differential expansion rates between the inner and outer layers of your ice when it touches liquid, resulting in internal fissures—the crackling you hear in your glass.
How to make clear ice
If you want beautiful ice and all the benefits that come with it, there are three ways to make it at home. The first method is the cheapest but most labor-intensive, the second method is faster but more expensive, and the third is just buying a big machine to do it for you. All three rely on controlling the direction and speed of ice crystal formation.
Method 1: a cooler and a serrated knife
The most important factor in making clear ice is directional freezing, by which I mean ensuring that the ice forms in one direction rather than from all sides. In practice, this means using a container that’s insulated everywhere but on top, such as a cooler. Find a cooler that fits in your freezer and fill it with clean water, leaving about an inch empty on top to allow room for expansion. Err on the side of caution with more empty space than you think you need when you first try it, as the alternative is cracking your cooler. Pop it in the freezer and wait 24 hours. The water will freeze from the top down, and the trapped air will all get pushed to the bottom. When time’s up, remove the cooler and invert it onto a surface you’re not afraid to get wet. There will likely still be a layer of liquid water and trapped air at the very bottom, and all that’s going to come pouring out. This also means the air won’t have had a chance to cloud your ice, and you will have a nice, clear slab. Now, cut the ice to the desired size. Using a sturdy serrated knife, score grooves into the ice, about 1 centimeter deep, and then tap the back of the blade with a hammer to break through the rest of the way. Repeat this process and you’ll have perfectly clear, solid ice cubes of custom size.
