Why does locomotive smoke change color?

Ask Trains from the October 2015 issue
Cass Scenic Railroad’s Heisler No. 6 climbs a grade puffing smoke and steam into a gray cloud.
Steve Sweeney
Q Why is it that one locomotive, on the same day, may have black exhaust, white/light gray, or at times almost invisible exhaust coming from the smoke stack? – Russ Gray, Kernersville, N.C.

A The color of exhaust you see coming out of a steam locomotive’s smoke stack indicates how efficiently it is burning fuel. Darker or blacker smoke is an indication that small fuel particles (coal, wood, fuel oil, etc.) have made it through the firebox unburned and are therefore wasted. Light or nearly invisible exhaust means that the locomotive fuel is mostly burned and transformed into heat, carbon dioxide, water, and trace elements. Factors affecting how efficient a locomotive burns fuel include the skill of the fireman on the engine, the quality of the fuel used, the design of the locomotive itself, and the amount of work the engine is doing. Similar concerns affect the efficiency of internal combustion engine locomotives as well.

Gray clouds trailing a steam locomotive are often a mix of exhaust smoke and steam that appear gray at a distance. On special occasions, train crews on tourist railroads may purposefully create dark smoke plumes so that railfans and observers can take dramatic pictures. – Steve Sweeney
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