Chimney breasts in stone buildings are very often built with bricks, which are better adapted than stone for forming the thin withes and walls required, and generally less expensive than sound masonry.
The chimney breasts and flues are, however, frequently built in rubble.
When the chimney passes above the roof it is of course necessary that, for the sake of appearance, it should be of the same material as the walls of the building generally.
Figs. 73 to 75 show the plan and elevations of a chimney in cut stone, of a form frequently used.
The cap is supported by blocks, d d, and surmounted by semicircular " terminals," T, T, which are intended to prevent down-draughts, and to protect each flue from the action of those adjacent to it.
Fig. 75. End Elevation.
Fig. 74. Side Elevation.
Chimney Flues, especially those in masonry, are frequently formed with earthenware pipes, which afford but little resistance to the smoke, are free from the objectionable corners of brick flues, do not collect the soot, and are easily kept of uniform section throughout; on the other hand, if the internal surface is too smooth, the soot is apt to collect and fall in lumps.
The flues may be rendered inside with Portland cement.
The ordinary method is, however, to plaster the inside of the flue over with a mixture of one part of lime with three of cow dung; this forms a tough lining with a smooth surface, and not so liable to crack as ordinary mortar.
While a chimney flue is being built, it is advisable to keep within it a bundle of rags or shavings called a "sweep," in order to prevent mortar from falling upon its sides; and after the flue is finished, a wire brush or core should be passed through it to clear away small irregularities, and to detect any obstruction that there may be in the flue.
Chimney Pots1 are frequently placed over flues, to prevent the eddy of wind that would be caused by a flat surface at the top of the chimney.
1 Sc. Chimney cam.