How can I get a fire going in the convention room?

If you talk to Thomas Jefferson, you should know that he won't give up his
log just on Hancock's opinion.  Listen to what he says, and you'll find that
there's a better judge of the temperature.

From someone who spent a long time in a cold place.

George Washington needs to be cold, or at least look cold.

Chattering teeth are usually a sign of being cold.

And Bernard can get a set from his time.  (See the appropriate hints if you
don't know how to get them.)

Now all you need to do is convince George to give up the teeth he has.