I have noticed that in real steam locomotives, the smoke expelled from the stack is occasionally white, but it is also sometimes black. I am a bit confused because there doesn't always seem to be a strict correlation between the speed of the loco and the color of the smoke. I have also noticed that sometimes the smoke expelled on starting is black, yet it seems that other times this same starting smoke is white. There also doesn't seem to be a strict correlation between the weight (or speed) of the locomotive's consist and this effect. I understand the basic premise of steam locomotion demands that excess steam be expelled through the stack. I also understand that contaminating soot particles from the burning of coal sometimes makes that smoke black. My question is when and why does that happen? Can the engineer or stoker be at fault for this effect? Can the effect be produced deliberately? Can anyone please explain this phenomenon? Thanks.
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