Creosote is the result of condensation in the chimney, and trouble from
this source is best avoided by preventing creosote formation. It is more
likely to form when wood is used for fuel than when coal is burned and
is more likely to form in cold than in mild climates. Green wood may
contain as high as 40 percent water, and dry wood 15 to 20 percent. When
wood is slowly burned, it gives off acetic and pyroligneous acid, which
in combination with water or moisture form creosote. When the draft is
strong and an active fire is maintained, much of the creosote is carried
off into the atmosphere. The trouble is aggravated when the fire does not
burn briskly and when an outside flue is subjected to chilling blasts.
The walls of the chimney, being comparatively cool, cause condensation
of the vapors contained in the smoke. Thus the creosote condenses and
runs down the flue, finding its way out of any joints that are not
perfectly tight. The formation of creosote is unusual in chimneys that
are surrounded by warm rooms. The outer walls of a chimney in an outside
wall should be at least two bricks thick and the chimney should have a
good flue lining.
Creosote is difficult to remove and when it ignites makes a very hot fire
that is likely to crack the masonry and char adjacent timbers. The only
safe method of removal is to chip it from the masonry with a blade or
straightened-out hoe attached to a pipe or handle. A heavy chain drawn up
and down the flue walls is sometimes effective. However, when creosote is
removed, care is necessary not to knock out mortar joints or to break the
flue lining.
Large quantities of salt thrown on the fire in the grate or fireplace
will extinguish a chimney fire. A fire in a fireplace flue can be checked
in its intensity and frequently extinguished by first quenching the fire
on the hearth and then holding a wet rug or blanket over the opening so
as to shut off the air. When this is done, the soot and creosote are
likely to slide from the flue walls and drop into the fireplace. Before
extinguishing a fire in a flue, cover openings into the rooms, so that
the soot will not spread over furnishings.