I know it's a moot point now, but the comment about cheap soldering iron tips turning black because they're too hot, well I found you can actually reduce the temperature by undoing the grub screw that holds the tip, and feed the bit out an inch or more, so less heat makes it to the tip, so it's cooler. Not long after I discovered this, the element failed. Crappy iron...
Also when I used to have an Antex iron, when the crud on the tip got too hard and thick to wipe off, I used to file it very gently and carefully with a worn needle file. Never damaged the cladding.