Read an Excerpt
From Chapter 2
Why Is Ice Cloudy?
In the introductory sections we addressed directional freezing and how clear ice is made in professional sculpture block machines and at home. We're going to cover the home part again in detail because the rest of the book—and your success at making great ice—depends on it.
Consider a typical ice cube, made in a typical ice cube tray in a typical freezer. The cube is cloudy but not completely. The cloudy white parts are mostly near the center of the cube, while most of the outsides are clear. Why is that? And what's in that cloudy part anyway?
Water doesn't turn solid all at once when it freezes. A lake freezes from the top down because the cold air is at the surface, but in most ice cube trays, cold air surrounds each cube on all sides of the tray. It freezes from the top, the bottom, and the sides in toward the center. When the water freezes, its molecules slow down and lock into place, forming a solid crystal lattice. As that happens, anything that is not water gets pushed away from the point of freezing.
Because the ice cube in the tray is freezing from the outside in, all the nonwater impurities get pushed in toward the center of the cube—the last part to freeze. And that's where the cube is cloudy.
So, what are those impurities? They're the minerals like sodium, calcium, and magnesium in your tap water, possibly chlorine, and any other organic matter. They get pushed toward the center of the cube. But all these impurities are less important than the biggest source of cloudiness: air. Air is naturally dissolved in water, and as the cube freezes, the air is pushed together toward the last part to freeze, where it forms pockets. These little pockets have big ice crystals in them that refract the light passing through the cube, making it look extra white and cloudy instead of clear.
Directional Freezing
In order to make clear ice, we just need to get the trapped air and other impurities out, right? Likely your first thought is to try filtering the water, or using distilled water, to get rid of the minerals and other solids. Then to get rid of the air in the water we need to degas it by boiling the water or sucking it out with a vacuum. That would seem correct. Unfortunately, neither removing minerals nor using any conventional method of degassing the water will make the ice less cloudy.
The way we make clear ice in this book is not by getting rid of impurities or cloudiness. Instead of eliminating cloudiness of ice altogether, we control where the cloudy part of the ice—the last part of the water to freeze—ends up. Rather than freezing an ice cube or ice block from the outside in, we make it freeze from the top down toward the bottom.
"Directional freezing" is a method for making clear ice by controlling the direction in which water freezes.
Consider an ice cube in an ice cube tray again. If we insulate the tray on the bottom and sides but leave the top open, the insulation forces the water to only freeze from the top down rather than from the outside in. Now as the water turns to ice, it pushes trapped air and impurities to the bottom of the ice cube tray rather than into the middle. The last part of the water to freeze is where the ice will be cloudy, which is now at the bottom of the cube.
How to Take Advantage of Directional Freezing
The practical way to take advantage of directional freezing isn't to insulate a small ice cube tray to make ice cubes that are clear except at the bottom. A better and easier way to make clear ice is to use a hard-sided insulated container. For more than ten years now, I've made my beautiful crystal-clear ice using a hard-sided cooler, which is spacious enough to hold a big lunch or a six-pack of soda.
The insulated cooler forces the water to freeze like a pond does. Fill the cooler with water and place it in a freezer with the top off so that the water is exposed to air on the surface. Because the cooler has insulation built into its sides and bottom, the water freezes from the top down rather than from the outside in.
The first part of the water to freeze into ice—the top—is perfectly clear because when it forms its crystal lattice, the trapped air and impurities are pushed away from the point of freezing into the water below. The last part to freeze, the bottom, is where all the cloudy white ice ends up. Then you can chop off the bottom cloudy part of the ice block you've just made and keep only the clear top portion.
Better yet, you can completely avoid making cloudy ice. The trapped air and impurities keep getting pushed away from the point of freezing into the remaining water underneath. As the ice freezes down to near the bottom of the cooler, there is too much air in the water. This is when the cloudy ice forms—but only if you let the water freeze that long.
If you remove the cooler from your freezer before that last bit of water freezes, you will be left with only the clear top slab of ice. My insulated cooler in my freezer at home takes about four days to freeze solid, so I try not to let it stay in there that long. After two days or so, I pull the cooler out of the freezer and turn it over; the block slides out, and then the unfrozen water at the bottom sloshes out all over my kitchen. But the slab of ice that drops out is crystal clear. By allowing just some of the water to freeze into ice, the air and impurities remain in the unfrozen water that can then be discarded.
