I use coal out of the bunker, it doesn't leave as much of a lasting mark, but it doesn't ruin the scoop either.
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As far as firing 464, I enjoy firing her a little more than firing our ten wheeler 152. 464 is fairly easy to fire, partially because we don't work her that hard. She's a bit of a pain to service though. 464 is a little more forgiving as far as the condition of your fire than 152; since 464 has so much more grate area you can get away with a lot of things that will cause you trouble on 152. Of course, I'm a little partial to 464 since that's where I did better than half of my student time and I feel like I know 464 better than 152.