The sound of crackling ice is synonymous with cool refreshment. On a literal level, it’s the sound of structural damage creating smaller pieces of ice with more surface area that will melt faster. That’s not necessarily a problem, but sometimes you want ice that keeps your drink colder longer, retains its shape, and dilutes slowly. For that you need to make perfectly clear ice.

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. 

Why does my ice look cloudy?

When you fill an ice cube tray and pop it in the freezer, it sits in an uninsulated container that’s exposed to cold air from all directions, and begins to freeze from the outside in. If you look at an ice cube halfway through the freezing process, when its core is still liquid, it will likely look crystal-clear. “Crystal-clear” is an apt term because at this stage, the crystal structure of the ice will still be consistent and strong in the outer layers. 

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.

[Related: The scientifically best way to pack a cooler]

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.

Method 2: an insulated ice tray

Get a directional freezing ice mold for cubes (or spheres if you’re feeling fancy). It will be insulated on all sides but the top, and the ice cube tray itself should have a small hole in the bottom of the ice chamber for the air to pass through. There will be an extra chamber beneath the cube mold for the air and cloudy ice to get trapped in, so everything that comes out of the mold itself will be clear. Like the cooler method, this works best if you take it out of the freezer before it has frozen all the way to the bottom, ensuring cloudy ice hasn’t had a chance to form.

Method 3: buy a big machine

A decent countertop clear ice machine will run you about $150 to $250, but if your demand for ice is high enough that you’re routinely buying bags of it, it may be a worthwhile investment. These machines will result in smaller pieces of ice, but will save you time and effort—as machines are supposed to. Most home clear ice machines work by dripping a constant flow of recirculated water over an extremely chilled metal lattice with an open front, so the ice crystals form back to front, and there’s never enough water freezing at once for air to get trapped. The Sentern portable countertop clear ice maker and the NewAir ClearIce40 both work on this principle and are well-reviewed, but you’ll need to immediately use or freeze the ice they make, or—as you may have guessed—your pristine cubes will melt.