As you use your soldering iron you may find that the tip starts to adhere less well to the solder or it starts to show little black sports or discoloration perhaps even a rusty appearance.
When that occurs the tin coating on the tip is beginning to deteriorate and you need to refresh it. We do so by tinning the tip.
In order to do so we have to break up whatever kind of oxide coating is forming on the tip. We use different types of materials. This is a block of sal ammoniac which is going to generate ammonia vapors which will allow us to disintegrate the oxide layers and reapply tin to the tip.
We begin by heating the block of ammoniac and then doing so once it begins to generate vapors we flow some solder to the tip and this will wear away any of the oxides that have formed and when we finish I wipe the iron on the wet sponge.
Now we have a nice, brand new, shiny tip.