Are you thinking of something like this?
http://www.rtico.com/vt3100.html
Archivists use tape cleaning equipment for truly dirty tapes. Some archives have custom-built machines for open-reel 1/2" or 2", some modified from equipment originally built for computer tape.
http://www.experimentaltvcenter.org/...2&id2=5&id3=12
They're expensive. And honestly--any archivist who's honest will tell you we have no idea what's really coming off in those machines. The key thing archivists will tell you is make sure the *equipment* is clean and well-maintained.