The process of forming crystalline substances from vapour, solutions, or melts. For example, snowflakes are formed from water vapour, ice is formed from water, and alum crystals from a saturated solution. A crystal may form from molecules, atoms, or ions by coming together randomly to form a cluster. An embryo is formed when sufficient particles have come together to form a solid phase. If supersaturation is sufficient, the embryo may grow into a nucleus, which if it gains more particles, will grow into a crystal. The crystallization of petroleum fraction contaminants is used to remove wax and other semi-solid substances from heavier fractions. The removal of wax from lubricating oils involves mixing the oil with solvents and then cooling to a temperature of around −20ºC to cause the wax to crystallize. This is then separated.