The following procedure produces reasonably pure carbon.
Hold a clean tin can upside down over a candle flame and collect a heavy layer of soot. Tap the soot into another can with just a tiny hole in the top. Heat it strongly several times over a gas flame to drive off the impurities (but not so strongly that you burn up all the carbon). You should have reasonably pure carbon in the end.
Author: Fred Senese senese@antoine.frostburg.edu