Why should a magnesium ribbon be cleaned before burning in air?

Explanation

Magnesium ribbon has generally a coating of magnesium oxide deposited on its surface. It is formed when the ribbon is kept in air for some time. The coating or layer prevents the metal from burning when flame is brought in contact with the metal. The surface should be properly cleaned preferably with a sand paper in order to remove the layer of magnesium oxide before burning the ribbon in air.