According to J. David Conrad's paper on "Displayed Locomotives" 34.5 years ago:
For a graphite colored smoke box, mix 1 part number 2 flake graphite with 5 parts boiled linseed oil. This must be painted on by hand.
http://members.aol.com/belpaire/steamcm.htm
This is about the formula we've used at the Huckleberry RR. If you accidentally make it thicker than you like, add a little diesel fuel to thin it. The consistency should be thicker than most paints, not pastey, and should probably not make a sloshing sound if agitating it in an open container (like a can or cup).