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?
Note- I really couldn't find a way to search this thing so forgive me if it's a duplicate of some sort.
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?
Best Answer
It's probably a mixture of metal oxides and residue from solder vapors. The high heat accelerates oxidization. Melting solder releases vapor and particulates (smoke), and some of it deposits on the iron over time.
Cleaning wouldn't hurt: if it wipes off easily then just wipe it off with a rag. If the iron still works, there's no reason to stop using it.
It's probably some fairly nasty stuff: somewhat corrosive and contaminated with heavy metals. Wash your hands.