What about the tin on the soldering iron?
The electric iron head, with the file file out the copper color, and then plug in electric heating, the temperature rise after the sticky rosin tin can.
First polish the iron head, shine, smooth, and show the true color of copper. Then, the electric heating, before the solder has reached the melting point, it should be frequently dipped in rosin, to protect the fresh surface, to prevent oxidation. When the temperature reaches the point of the solder, dip the rosin into it and rub it back on the solder so that the soldering head is soldered. Repeat it several times to get good soldering.
Use sand paper to make a new copper, tin, or replace the iron head
