In order to form the throat of the chimney, the courses are "gathered" over, each projecting 1 1/8 inch or so over the last, until the opening is narrowed to the required dimensions. The exact projection depends of course upon the curve required. The narrowest part or throat should be immediately over the centre of the fireplace. Above the throat, the flue ascends vertically for a short distance, then gathers again to the right or left, as shown in Fig. 63.
1 Sc. Vent-linings.
2 Sc. Stalks.
Fig. 63 is an enlarged section of the flues contained in the chimney breast just above the floor, C D, in Fig. 54. It shows the method of gathering over for the flue of a small fireplace, and also the arrangement of the bricks in forming the withes, etc., for the flues from the stories below.
Fig. 64 is an elevation, and Fig. 65 a section, of a fireplace, showing the rough arch supported by a "turning bar," 1 T T, of which a sketch is given in Fig. 66. The bricks next to the skew-backs are often laid as headers.
This bar is from ½ to ¾ inch thick, and about 3 inches wide. It has a bearing of 4½ inches on each jamb, and beyond the bearing portions, ends about 3 inches long. These ends are sometimes split longitudinally, and corked,2 i.e. turned in opposite directions, up and down, as shown in Fig. 66. Very frequently the ends are turned either up or down without being split, and this is a better plan than that shown, for it renders it unnecessary to cut bricks.
The bar is curved to fit the soffit of the arch, and in order to prevent it from straightening under the thrust a small bolt is sometimes passed through it and secured to a plate on the crown of the arch.
Flat turning bars have been advocated as tending to draw the jambs together instead of thrusting them out, but they are seldom if ever adopted.
The interior of the jambs of chimney breasts should always be filled in solid.
Hearths are stone flags about 2½ inches thick, placed so as to catch the droppings from the grate. The back hearth, bh Fig. 65, covers the space between the jambs of the chimney breast.3 The front hearth, fh, rests upon the trimmer arch described in Part I.
It has already been mentioned that the external walls of chimneys should be 9 inches thick, at least until the shaft has passed through the roof; they are better if built in cement.
1 Or Chimney-bar.
2 Or Caulked.
3 Solid concrete hearths are frequently used instead of stone hearths on brick trimmer arches.
It is, moreover, an advantage to have a thick wall round the chimney shaft, even in the open air, as it tends to keep the flue warm. A thin wall is soon partially destroyed by the weather, and admits cold air to the flue, causing it to smoke.