English: Formation of crystals of ammonium (di)chromate

Dissolve a spatula full of ammonium chromate in a few ml of water and then pour this solution in a petri dish. The petri dish should be placed in a dry and warm place, and it should be somewhat tilted, such that the solution is at one side of the petri dish.

Let the solution evaporate. This may take between several hours and a few days, depending on temperature and humidity of the air. When all water has evaporated, then a beautiful crystalline structure remains.

There also is a remarkable effect, that the crystals, which are oldest (are dry first) are orange, while the newest crystals are yellow. If one waits too long, then all crystals become orange, so it is important to monitor the petri dish and inspect it every few hours. Ammonium chromate is not air-stable. In contact with air, it slowly looses ammonia and water, leaving ammonium dichromate behind.

Immediately after evaporation of the water, it looks as in the image.