The alum crystals are slowly getting bigger. But a lot of the alum in solution in the cups crystalizes out into separate tiny little crystals. Whenever there are too many little crystals in the cup, we decant the solution into a clean cup. And I put the little crystals into a third cup I started.
At first nothing grew. I figured there wasn't enough alum in the solution. But little by little I added more crystals and let the water in the cup evaporate.
Then the other day, I glanced in the cup and saw these, lying on the bottom:
Without the food coloring, they look like diamonds waiting to be put in rings. So cool! All the crystals are flat on the bottom, though. I tied one on a string to see if it would grow, and if it would even out on the flat side. Updates in a few days.