Expert: Dr. Henry Boyter, Jr. - 4/23/2005

Question
Background for question: Despite my physics degree, I don't understand what causes this difference, yet TV shows, news reporters, auto mechanics, Roman Cardinals, and at least one of my coworkers seem to understand this well.  I just wanted to rectify that deficit in my knowledge.

In one detective show, a policeman knew that evidence (clothes) were being burned in an incinerator instead of leaves b/c the smoke was black instead of white.  Here at work in Jersey City we have a good view of a manufacturing sector.  A few years ago there were several local factory fires.  But a coworker pointed out that one suspicious looking billow of smoke couldn't be a fire b/c the smoke was white instead of black.  I've looked this up on Google, but get mere correlations, not explanations.  As far as I've gotten is that black smoke means particulates are stuck to large globules of unburned fuel, and white smoke is from low-temperature combustion.  I really don't see the common thread in all of this.  So if fuel is burning efficiently, then it doesn't produce smoke?  If fuel is not burning completely then it's not being turned into carbon and CO2, I believe.  So if the fuel's not burning completely then hydrocarbons are being sent upward as smoke.  A reason for the fuel's not being burned completely is that an engine isn't hot enough to fully combust the fuel.  But this doesn't help me with the difference between leaves and clothes, or the difference between a burning building or intended factory fuel use.  If this is more of an engineering question, let me know and I'll redirect it.  Thanks in advance for your help.

Answer
White is USUALLY a signal that water vapor is present.  Darker smoke is USUALLY a signal that organics with no water is present.  You can have everything in between also.  There are no hard and fast rules.  For example, the leaves versus clothing.  Dry leaves and cotton fabric would look the same.  Wet leaves versus polyester fabric would give what you described.  On the white smoke not being a fire.  It depends.  Small fire with water being sprayed on it would be white.  Also, acid fires can be white for another example.  A boiler can give both.  Black if the boiler fire is not optimal.  White or almost no color if balanced (close to 100% combustion).  Steam trap releases on the boiler - white.