Way back there, Susan-Marie asked:

(didn't old English use f's for s's and vice versa?)

You're talking about the "medial s", which is comes out of printing with separate-letter hand-set type. The "medial s" is a form of s which was only used in the middle of a word, so would not be used for "soot". "Medial" like in "median", "medium" etc.--that is, "in the middle".

And not "vice versa"--that is, s for f--either. The medial s sort of looks like a form of f, but if you look closely you will see that it looks like an italic f, even when used with non-italic typefaces, and that it lacks the cross-bar of an f.

Dave Oesterreich