When the water is heated and converted into steam, the sediment or suspended dirt remains in the boiler and forms scales. These scales are composed principally of mineral matter and affect the economical generation of steam by preventing the water from coming in contact with the plates and tubes. The latter are then heated to a much higher temperature than would otherwise be necessary and to too high a temperature for the good of the metal. Thick scales on the surface of a boiler cause unequal expansion of the plates and tubes, resulting in leaky tubes and seams, and largely accounting for blisters and bagging.

Various methods have been invented for removing and preventing scale. Kerosene oil removes oil scale very effectually. About half a pint of kerosene oil per day fed continuously into the feed water will be found sufficient to remove scale as fast as it can be taken care of by cleaning the boiler, and without danger of accumulating and causing serious overheating. Scale may be to some extent prevented by the use of a good compound, provided the water has been analyzed and the compound which has been prepared particularly for that water is used. Mechanical boiler cleaners may also be used with good effect, but with any method a boiler should be thoroughly cleaned at regular and frequent intervals. Boiler tubes also should be cleaned often. The soot that collects in them is a non-conductor of heat, and, therefore, when the surface of the tubes is covered with soot only a portion of the heat of the gases passing through them can get to the water surrounding the tubes. The remainder is carried to the chimney. In a boiler tube, a layer of soot 1/8 in. thick will cause as much waste of fuel as 3/32 in. of scale. When burning bituminous coal, soot will collect to the above depth in about ten hours. Therefore, in order to have reasonably clean tubes at all times it is necessary to clean them once each day.