How do borax crystals form?
Borax has a solubility of 5.8 grams per 100 milliliters of solution or 5.8 percent at room temperature. However at an elevated water temperature, more borax will dissolve than at room temperature. As the hot borax solution cools to room temperature, it becomes supersaturated. Supersaturated solutions are unstable and the excess borax will crystallize on any rough surface.