cloud in the cube

I. The freeze front

Take a cube of household ice from the tray and hold it up to the light. Clear at the corners. Clouded toward the middle, sometimes as a single soft opacity at the geometric center, as if a thumbprint had been sealed in glass.

The cube freezes from the outside in. The tray pulls heat through its walls; the water nearest each wall reaches 0°C first and freezes first. A thin shell of ice forms, then thickens, advancing inward as a front. Behind the front there is ice. Ahead of it, still liquid.

II. What the ice doesn’t want

Tap water carries a lot of dissolved air. It carries calcium, magnesium, fluoride, the chlorine compounds the utility added, trace minerals from whatever happened to be present that day. Ice doesn’t want any of it. The crystal lattice will only accept water molecules. So the advancing front pushes everything else ahead of itself, concentrating the dissolved load into whatever liquid hasn’t yet frozen.

III. The center

The last place to freeze is the place farthest from every wall. By the time the front reaches it, the volume of liquid is small and the load is high — supersaturated with the air and minerals the rest of the cube spent freezing to reject. The gas comes out of solution as microbubbles; the minerals precipitate. The front closes over them all at once and they are sealed where they ended up. The cloud at the heart of the cube is everything the ice refused.

IV. Directional

A bartender’s trick: freeze a tall insulated cooler of water with the lid off, sitting inside a deep freezer. Only the top of the water is exposed to cold. The front advances downward in one direction and the dissolved load is pushed ahead of it, all the way to the bottom, where it concentrates into a layer of opaque mud that you saw off and discard. What’s left above is a slab of clear ice the whole way through. Given somewhere to go, the rejected things go there. Household ice is cloudy because the front closes from every side at once.