Shock and awe (there are no lubricants or solvents in this particular 'ible).
Shock is provided with a hammer. Awe comes from the awesome power of hellfire... or a little propane torch, pick one.
This technique has resurrected many rusty bikes in salty places like Montreal.
Remember to vote for me if you find this knowledge useful.
So many people have spoken of their success with various penetrating oils, I can't even keep track of them anymore. Please view the comments at the end.
Some people have kindly spoken in favour of penetrating lubricants as well. Whyteboar, Cross and toad all advise using PB Blaster and letting it soak in repeatedly. mastershake916 is partial "Yield" by Chemsearch and dlaninga1 suggests JB80 metal conditioner. rimar2000 says "You MUST try using liquid phosphating. You can dip the pieces many hours or days into normal liquid phosphating. It does not attack hands or skin."
Also, mastershake916 asked what I wanted the cog for. It was going to go on a Stokemonkey electric bike motor using an adapter but I changed my mind. You see, I'm building a electric/solar/pedal car in an attempt to merge the superb efficiency and light weight of an electric bicycle with some of the features of a car.
Kevanf1 had a neat idea: "...putting the lot into a freezer for 24 hours. Then try heating. But this time you will not have to bring the metal up to red hot and thus not risk changing the crystalline composition of the metal". Someone buy this dude a beer.