I think that what you are asking about is probably very near to the beginning of the book. I think that you are referring to the part where Guy Montag first meets Clarisse McClellan.
When they meet, she tells him that she would have known that he was a fireman even if her eyes had been shut. She says that it is because he smells of the kerosene used to start the fires.
At that point, Montag tells her that it never does wash off completely and that his wife is always complaining about the smell.
So I think the answer to your question is kerosene.