Cake can be cut cleanly with a sharp blade, scraped with a dull blade or sanded with a strip of fine grit wrapped on a piece of dowel. There are about 100,000 versions of commercially made reamers available, too - there isn't exactly one type that suits every pipe but most fit many. I favor a Senior Reamer for taking care of lots of pipes, estate refurbing, etc. For one pipe? Sandpaper and a dowel or, if you are careful, an Exact-o knife or small paring knife with a blunt tip.
You can remove as much (or little) cake from a pipe as you wish. There is no reason to ream a pipe down to bare wood. Leave about a dime's thickness to handle heat and moisture. Removing cake makes a small mess. Don't do it on a white tablecloth.
Charring, when talking about pipes, invariably refers to charred wood. Sandpaper works for charring, too.
Black crud all over a pipe rim isn't usually cake or charring. It is what we call black crud all over the pipe rim. It mostly comes off with a spit/paper towel rub-down.
And so on.
See the "
Ask an Old Fart" thread for many "how to" Q&A's.