The smoke shelf is made by setting the brickwork back at the top
of the throat to the line of the flue wall for the full length of the
throat. Its depth may vary from 6 to 12 inches or more, depending on
the depth, d, of the fireplace.
[Illustration: Figure 37.--Grates of this type are commonly used in
fireplaces for burning coal or coke. This one has a metal back and ends
and is only set in to permit proper circulation of air around it.]
The smoke chamber is the space extending from the top of the
throat, _ee_, up to the bottom of the flue proper, tt 9 and between the
side walls. The walls should be drawn inward 30° to the vertical after
the top of the throat, _ee_, is passed and smoothly plastered with
cement mortar not less than one-half inch thick.