In order that oil-lamps may produce a brilliant light free from smoke, combustion must be complete.  The correct quantity of oil must be fed to the burner and it must be properly vaporized by heat.  If insufficient oil is fed, the intensity of the light is diminished and if too much is available at the burner, smoke and other products of incomplete combustion will be emitted.  The wick is an important factor, for, through capillarity, it feeds oil forcefully to the burner against the action of gravity.  This action of a wick is commonly looked upon with indifference but in reality it is caused by an interesting and really wonderful phenomenon.  Wicks are usually made of high-grade cotton fiber loosely spun into coarse threads and these are woven into a loose plait.  The wick must be dry before being inserted into the burner; and it is desirable that it be considerably longer than is necessary merely to reach the bottom of the reservoir.  A flame burning in the open will smoke because insufficient oxygen is brought in contact with it.  The injurious products of this incomplete combustion are carbon monoxide and oil vapors, which are a menace to health.

To supply the necessary amount of oxygen (air) to the flame, a forced draft is produced.  Chimneys are simple means of accomplishing this, and this is their function whether on oil-lamps or factories.  Other means of forced draft have been used, such as small fans or compressed air.  In the railway locomotive the short smoke-stack is insufficient for supplying large quantities of air to the fire-box so the exhausted steam is allowed to escape into the stack.  With each noisy puff of smoke a quantity of air is forcibly drawn into the fire-box through the burning fuel.  In the modern oil-lamp the rush of air due to the “pull” of the chimney is broken and the air is diffused by the wire gauze or holes at the base of the burner.  These metal parts, being hot, also serve to warm the oil before it reaches the burning end of the wick, thus serving to aid vaporization and combustion.

The consumption of oil per candle-power per hour varies considerably with the kind of lamp and with the character of the oil.  The average consumption of oil-lamps burning a mineral oil of about 0.80 specific gravity and a rather high flashing-point is about 50 to 60 grams of oil per candle-power per hour for well-designed flame-lamps.  Kerosene weighs about 6.6 pounds per gallon; therefore, about 800 candle-power hours per gallon are obtained from modern lamps employing wicks.  Kerosene lamps are usually of 10 to 20 candle-power, although they are made up to 100 candle-power.  These luminous intensities refer to the maximum horizontal candle-power.  The best practice now deals with the total light output, which is expressed in lumens, and on this basis a consumption of one gallon of kerosene per hour would yield about 8000 lumens.