that the high heats necessary to heat water through thick scale will
sometimes actually convert the scale into a species of glass, by
combining the sand, mechanically separated, with the alkaline salts. The
same authority has carefully estimated the non-conducting properties
of such boiler incrustations. On this point he remarks that the evil
effects of the scale are due to the fact that it is relatively a
nonconductor of heat. As compared with iron, its conducting power is
as 1 to 371/2, consequently more fuel is required to heat water in an
incrusted boiler than in the same boiler if clean. Rogers estimates that
a scale 1-16th of an inch thick will require the extra expenditure of
15 per cent. more fuel, and this ratio increases as the scale grows
thicker. Thus, when it is one-quarter of an inch thick, 60 per cent.
more fuel is needed; one-half inch, 112 per cent. more fuel, and so on.

Rogers very forcibly shows the evil consequences to the boiler from the
excessive heating required to raise steam in a badly incrusted boiler,
by the following illustration: To raise steam to a pressure of 90 pounds
the water must be heated to about 320 deg. Fahr. In a clean boiler of
one-quarter inch iron this may be done by heating the external surface
of the shell to about 325 deg. Fahr. If, now, one-half an inch of scale
intervenes between the boiler shell and the water, such is its quality
of resisting the passage of heat that it will be necessary to heat the
fire surface to about 700 deg., almost to a low red heat, to effect the same
result. Now, the higher the temperature at which iron is kept the more
rapidly it oxidizes, and at any heat above 600 deg. it very soon becomes
granular and brittle, and is liable to bulge, crack, or otherwise give
way to the internal pressure. This condition predisposes the boiler to
explosion and makes expensive repairs necessary. The presence of such
scale, also, renders more difficult the raising, maintaining, and
lowering of steam.