Why is the soldering iron tip not tinned?

A: The soldering temperature is too high, such as more than 500 degrees above will cause the tin surface spatter.
Second, the use of the process, the soldering iron head cleaning method is not appropriate.
Third, not before the use of tin-plated surface eating tin
Fourth, not often clean up the oxidized layer of the iron head
Fifth, the plating layer is thick or the use of low temperature