A few days after my encounter with the pawprints and grey muck, one of the ponds did freeze over. After me and my camera spent about 30 minutes admiring this rare event, I went and ruined the complete glaze job by punching a hole in it. Although the top surface had lots of interesting curves and shallow grooves, the underside, only about 8-mm below, was flat and featureless. I suppose this is because the surface marking the melt-line (i.e., 0 degrees C) is flat. But when I put a piece of pond glaze between two polaroid sheets, with one polaroid 'crossed' to the other, I saw an odd sight.

The large black "X" that appears here is sometimes called the "Maltese cross". Elizabeth Wood, in her classic little book "Crystals and Light", calls it the "black cross". And when I looked up some cross shapes online, I thought the above figure looked more like the German "iron cross". Anyway, whatever you call it, it doesn't seem to fit the scene.

Also strange is the colored circle surrounding the center of the cross. These are interference colors. Both the cross and the colors can appear in other crystals. How did they get there? The ice by itself shows neither.

I know of no quick way to explain them. Wood's book devotes chapter 8 to both phenomena, though one might have difficulty with it without reading some of the earlier chapters. Briefly, both phenomona can occur only when the ice slab consists of either a single crystal or crystals that are all nearly alligned with each other. But this condition is not sufficient; two other conditions must be met. One is that we must view the ice along the axis of six-fold symmetry, that is, along the same direction that we view Mark's stellar crystals. We call this direction in ice the 'optic axis'. The other condition, at least for the colors, is that the ice cannot be too thick or too thin.

Ice and other crystals that show the cross are called uniaxial. Uniaxial means that a light beam passing through ice splits up into two beams, superimposed (so we can't usually distinguish them) but polarized perpendicular to each other, and one faster than the other. Without ice between the polaroids, light that makes it through the first polaroid, being polarized along the first polaroid axis, gets blocked by the second polaroid because the second polaroid axis is perpendicular to the first. The uniaxial nature of ice rotates the polarization so that some light can make it through the second polaroid.

However, in some viewing directions, the ice doesn't rotate the polarization, making the region black. One happens when we view the light along the ice's optic axis. This produced the center black spot. The other happens when we look off the optic axis along one of the polaroid axes. As there are two polaroids with perpendicular axes, this case produces the two black lines that cross in the center - the lines of the Maltese cross tell us the polarization directions of the polaroids. As each slab of pond glaze I looked at had all or part of the cross, I learned that the pond ice grew with a basal orientation, that is like the floating star I showed in my previous post "They came from out of the tub". Anyway, that is the basic picture of the Maltese cross phenomenon. It isn't an easy thing to understand - I certainly had to struggle with it. And I am perplexed at what I saw when I removed only one polaroid.

Though I suspect the faint interference rings arise because the skylight is partly polarized, and thus functions as the first polaroid, the four black spots are a mystery. Elizabeth Wood's book shows various types of black-cross patterns from different crystal orientations and polarizer configurations, but none of them look like this.

In the ice slab below, you can see part of one cross, and lots of abrupt changes of color.

These changes of color mark where one crystal ends and another one starts. You ordinarily can't see these boundaries without the polaroids. Well, if you look closely at ice that is melting, you might see faint boundaries. But they are a lot prettier in color.

-JN