When growing Epsom salt crystals, the first step involves dissolving salt in hot water. Hot water is important because the temperature of the water has a direct effect on the amount of salt that can be dissolved in it. Heat increases the amount of space available between molecules and, as a result, hot water will hold more salt than cold water.
Dissolving is not a chemical change either: water is simply pulling the salt atoms apart and not changing the actual salt molecules themselves.
When no more salt can dissolve in water, the solution is considered saturated because all the available molecular space has been filled. When the solution begins to cool, the space between the molecules decreases and the salt is slowly pushed out as a solid, which is also the beginning of crystal formation.
Furthermore, the evaporation of water hastens the crystal formation process. Evaporation is the process in which water becomes a gas and rises. Epsom salt, which does not change states as easily as water does, remains behind in its solid form, resulting in long needle-like formations.
TIPS: IF you want to speed up the process try using a hair dryer. Also can add food coloring to the solution and put smaller amounts of water with each color on the glass.