Tempering chocolate is a precise process that is actually quite scientific. It involves manipulating the fat molecules in the cocoa butter into crystals to achieve the desired smooth, glossy finish and prevent pesky side effects like bloom or a product that melts in the hands. Heating chocolate breaks down the crystals, while slowly cooling it rebuilds them. The rebuilt crystals can arrange themselves in several forms, each producing a different end result. Too many of one type of crystal arrangement could mean a matte finish instead of glossy, while too many of another type of crystal arrangement could equate to chocolate that easily crumbles or melts.
Bloom occurs when the temperature fluctuations cause the cocoa butter to separate and rise to the surface, which creates an unsightly white or speckled film on the surface. A key way to avoid this is to ensure that the chocolate is gradually — rather than abruptly — cooled and reheated. Rapid temperature shifts speed up the separation of cocoa butter, resulting in bloom.