A. Light firing and occasionally moving the grates lightly is the best
preventive. When once formed, they should be removed if possible by
firing around and burning them out.

47. Q. How can you explain the slower burning of the coke and how
understand the proper manner of supplying fresh coal?

A. The gases of coal are lighter than air and will pass away whether
consumed or not. The slow burning of the coke is due to the fact that it
burns from the outside only. When a fire reaches a white or incandescent
heat it indicates that the gases are burned and a fresh supply of coal
should be added; this is to be done as light as the service performed by
the engine will permit.

48. Q. When and for what purpose is the use of a rake on the fire bed
allowable?

A. The rake should be used on the fire very seldom, because raking the
fire bed tends to form clinkers, especially when the rake is plunged
down through the fire to the grate. It may be used when necessary to
rake the fire lightly when on the road for the purpose of breaking the
crust, which may be found as a consequence of too heavy firing.

49. Q. Within what limits may steam pressure be allowed to vary, and
why?

A. Pressure should not be allowed to vary more than five pounds from the
maximum for the reason that too much expansion and contraction will
take place, which many times is the cause of flues leaking, cracked or
broken side sheets and stay bolts.