Ash cloud and polarised sunlight

I heard a suggestion that the ash cloud from the Icelandic volcano might polarise sunlight. Ordinarily, light waves from the sun are unpolarised. This means that some vibrate up and down, some vibrate side to side and others vibrate diagonally. If the ash cloud polarises the light, it would mean that it would vibrate in one direction only. You can check this with a polarising filter. Point it at the sky and rotate it. If it goes alternately light, dark, light, dark every quarter of a turn then the light is polarised. I've just been out to try it and this has turned out to be the case. Of course, I need to try again when the ash cloud has gone to be fair. You could try it if your sunglasses are polarising - some are and others are just thick filters.