Ice is usually not clear because light rays that pass through ice are scattered by mineral impurities and air bubbles that are trapped within. Light can also be scattered by small ice crystals that results in higher surface area for light scattering. Impurities and air bubbles get trapped because water normally freezes from all directions (heat is removed from all sides), forcing the impurities and air bubbles to be pushed towards the center of the ice. This results in cloudiness in the ice, which is usually present in the middle.

In order to make clear ice, water that is ideally free of impurities is used, and it undergoes what is known as directional freezing, meaning heat is removed from only one side, while the other sides are insulated. Many methods have been devised to refine this process of clear ice making, which is commonly manually done by bartenders to make aesthetic drinks. Currently, there are also many commercial models of clear ice machines on the market, with costs ranging from $300 for home models, to $3000 for industrial models.