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By default, this creates the smoke shelf behind it.

The shape and size of the throat of the fireplace, as well as the design of the smoke shelf, will ensure that all the smoke and hot gases go up the chimney rather than into the room.

If the cast-iron throat is not used, therefore, it will be well to lay an iron plate on the smoke shelf in such a way that it could be drawn forward across the opening to close it.

The second vital consideration is the introduction of what are known as a “smoke shelf” and a “smoke chamber.”

The smoke shelf is built just where it will prevent this action.