[Illustration: Lunkenheimer Fusible Plug.]
=A Fusible Plug= is a short brass bolt which has a hole running through
its center, filled with a metal that melts at a low temperature. This
plug is screwed into the crown sheet directly over the fire, and as
long as it is covered with water the metal will not melt and run out;
but should the water become low, exposing the crown sheet to the
intense heat of the fire, the metal will run out, and the steam rushing
through the hole puts out the fire and many times saves the crown sheet
from injury.
Q. In what condition would the plug become useless and of no value?
A. By allowing it to become covered on the inner end with scale and
sediment. It should be unscrewed and occasionally examined, at least
two or three times during the season, and all scale and dirt removed
from the end of the plug before replacing. Examine the crown sheet to
see that no scale has formed over the hole to prevent the water from
reaching the plug.
Q. How can a plug that has melted out be refilled?
A. Unscrew the plug from crown sheet and cap one end with clay, then
melt the lead or babbitt metal in a shovel, spoon, or even a piece of
bent sheet iron, and refill the plug. Now, with a light hammer, close
the ends tight, and screw the plug into the crown sheet.