Solid tin has two allotropes at normal pressure. At low temperatures it exists as gray or alpha tin, which has a cubic crystal structure similar to silicon and germanium. When warmed above 13.2 °C it changes into white or beta tin, which is metallic and has a tetragonal structure. It slowly changes back to the gray form when cooled, which is called the tin pest or tin disease. However, this transformation is affected by impurities such as aluminium and zinc and can be prevented from occurring through the addition of antimony or bismuth.