I opened up an old soldering iron and when I took out the tip, I found this substance underneath that even comes on my hand if I touch it. What is this? Is this still suitable for use?
Addition : I removed the tip and saw this - Is this still good enough to use or should I replace the tip? The tip heats up but solder won't stick and tip is just black. I could clean it upto this level - Why does it have a bluish tinge @ some places?