The farmer's bathtub finally froze over. The surface had an interesting freezing pattern, but I've photographed similar ones many times before. So I picked up a rock, broke the surface, put a chunk between two crossed polaroid sheets and shot a picture.

The colors arise from the birefringent nature of ice, which means that light can pass through crossed polarizers if ice lies in between. But only some colors can make it through both polarizers.

The color depends on the way the crystal arrangement is tilted and the ice thickness. Usually, ice that freezes from liquid water is actually many small crystals of various tilts that grew together. Wherever one crystal buts up against a crystal of a different tilt, the color changes abruptly. If the crystal thickness changes gradually, the color changes gradually. Once, when I cut a piece of ice from the tub,there were single crystal blades growing down into the water. In the image below, the gradually changing color bands tells us that they are single crystals that taper down in thickness.

On the other hand, the piece of ice below, which came from a puddle in one of the rice plots near the tub (one of my favorite icespotting places), consists of many small crystals.

Last year, I found that I could create some nice color mosaics by misting some cold water on our car windshield. After several misting sessions, I mounted one polarizer on the outside of the windshield, and then brought the other polarizer into the cold car with my camera.

The black regions within colored regions are not devoid of ice. Rather, they are crystals with a special tilt: we are viewing crystals oriented in the same plane as the crystals in Mark's beautiful photos of snow stars (dendritic crystals). The clear or white snow star is black between polarizers. Speaking of black, Craig Bohren once pointed out that not only is snow one of the whitest materials we know, it is also one of the blackest materials we know: It is white in visible light and black in infrared light. And between polarizers, every color of the rainbow.