Growing cyrstal gardens

A fun project to do in class over the period of a week or so is grow crystals. Here's one of the "classic" recipes:

• substrate.
• 4 tbsp. bluing (found in grocery stores)
• 4 tbsp. salt (any kind)
  
• 1 tbsp. ammonia
  
• 4 tbsp. water
  
• food coloring
  
• plastic bowl or plate
  

Place small pieces of porous rock (some lava rocks are ideal), pieces of broken clay flowerpot basins, pieces of brick, or even charcoal bricks (or paper towels, if nothing else is available) in a shallow plastic plate. Soak the substrate for 15 minutes, dot with food coloring, and pour the crystal mix over the substrate. Make sure there's enough of the mix to soak/float the substrate in.

Crystals will begin to grow in four hours and can continue growing for up to three days, depending on other conditions. Add more solution to keep them growing for a bit longer.