smoke shelf, and smoke chamber. As its
name implies, the throat is the opening through which smoke, hot
gases, and some flames pass on their way upward. Experts hold that
its correct construction contributes more to the efficiency of a
fireplace than any other feature, save proper flue design. The area of
the throat opening should not be less than that of the flue and its
length must be equal to the width of the fireplace. It should be
located eight inches above the lintel. Under present practice, a
cast-iron throat with a damper which can be opened and closed to
regulate the up-chimney flow is standard. Also, when the fireplace is
not in use, this damper can be closed and so prevent loss of other
heat.

The smoke shelf comes immediately above the throat and is formed by
recessing the brickwork of the back the full width of the chimney for
at least four inches. With very large fireplaces, it may be as much as
twelve inches. The object of this feature is to stop any accidental
draft within the flue from going farther and blowing smoke out into
the room. The area in between this and the flue itself is called the
smoke chamber. Here the walls are drawn in with a gradual upward taper
to the point where the flue lining begins. The chamber so formed can
and does hold accumulated smoke temporarily when a gust of wind across
the chimney top cuts off the draft for a moment.

In building chimneys, the old masons varied their structural ways and
materials according to the part of the country in which they worked.
New England workmen were partial to a central chimney, the core around
which the house was built, and their usual material was stone.
Occasionally brick was used but this material was more in favor with
old houses of the middle states and the South. Here, instead of the
central stack, a chimney was built in each of the two end walls. The
climate was milder and the style of architecture, with central hall
and stairway, made such practice desirable.

The mark of an old chimney is its massive construction. In those of
the central type, it is not uncommon to find a foundation pier of ten
by twelve feet in the cellar. This was laid dry and just below the
level of the first floor, large transverse beams were put in place to
support the hearthstones of the fireplaces above. Here dry work
stopped and, from there to the chimney top, all stones were laid in a
mortar made of lime and sand. At a point above the smoke chambers of