The case is a very easy processing of metal - it is 95% easier to process it than to produce a new one.
And all because aluminum is incredibly active, oxidizes right in the air and is always covered with an oxide film. This is the one that all the time is smeared with unprocessed aluminum. For a short time by earthly standards, he goes into an oxide. Aluminum oxide is known as clay, very common in the earth's crust, but also very durable as a compound. By the way, the crystalline form of clay - corundum, the second in hardness the material goes after the diamond. And because of this, because of this, you can’t take anything except an expensive electrolysis, which is aluminum and is produced;)
