Smoke is a byproduct of the combustion of fuel, and is invariably the result of incomplete combustion. It is composed chiefly of minute particles of carbon and steam, and is due largely to an excess of air admitted to the fire, although in a few cases the production of smoke is due to an insufficient supply of air. If the boiler is not crowded and the draught is good, the volume of smoke will be reduced by first allowing the coal to coke in front of the grates and by then pushing it back over the bright coals. The hollow bridge wall, with suitable means for regulating the supply of air, also gives good results where there is a strong draught. A small grate area and a very hot fire will reduce the volume of smoke, as will a very large grate area and a slow fire, although the former arrangement is the more economical.
An economical manner of banking fires is to push the live coals back against the bridge wall, leaving the forward part of the grates covered with ashes only, then covering the live coals with a moderately thick layer of fresh coal. Fine coal is preferable as the air does not readily pass through it, especially when the draught is diminished by closing the damper; this should be done just before covering the fire with fresh coal. The damper should be left open a very little to avoid the accumulation of gas in the furnace and the possibility of an explosion. This method of banking fires saves much time when preparing to start again. The grates may be quite thoroughly cleaned without disturbing the low fire at the bridge wall.
In case the water level becomes dangerously low, the fire should be drawn immediately. The engine should continue to run, and water should not enter the boiler in any quantity. When the furnace has cooled down to about the same temperature as the boiler, the water level may be raised very gradually until water appears in the glass. The boiler may then be filled more rapidly and 'the fire started.