There are two types of wool that can be used to help keep your soldering tips clean. You will either use inox wool, or brass wool.

Brass wool is the less abrasive of the two, and has become industry standard, adopted by many vendors. It can be used to remove both dirt and other contaminants that collect on your soldering tip.

The other type is Inox wool. This provides a more aggressive clean and must be used with care from the operator. Inox is also more durable and flexible, which results in less broken particles and contamination.

In terms of which wool to select, this will depend on a couple of factors, mainly the type of solder wire and flux that you are using. As a good rule of thumb, it is always best to start with the least abrasive brass first. If you find that this is not working then you can look at switching to Inox wool to see if this works more efficiently.