Not quite sure this is the correct subforum to ask this in, but it seemed the most relevant. Please redirect me if necessary.

A friend of mine approached me telling me he had aquired several old prints (about 100 years old he estimates) of old riverboats from his area.
The prints were in somewhat poor condition though and has what he explains as 'they appear to have 80 years of smokers soot on them'.

How would one go about cleaning these prints if at all possible?