What and Why Is Smoke?
Smoke is produced only when something which is being burned is burning imperfectly. If we were to put anything burnable into the fire and establish just the right amount of draft, and knew how to build our fires properly, there would be no smoke and very little ashes.
In the case of the black coal smoke which we think of mostly when we think of smoke at all, the black portion is principally little unburned particles of coal which pass up the chimney with the gases which are thrown off when the coal is being burned. These gases would be invisible—they really are invisible—if it were not for the little particles of coal which are drawn up the chimney with them. If you look at the chimney from which a wood fire expels the gases you find the smoke very light in color—showing that not so much unburned matter is being thrown off. A charcoal fire makes no smoke, because the charcoal has had the unburnable things taken out of it beforehand, and the charcoal stove is almost perfect in construction from the standpoint of combustion.
Of course, the thickness of the smoke from a coal fire is often increased by the fact that there are unburnable things mixed in with the coal, some of which also pass off through the chimney.