This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
No part of a furnace is of more importance than the grates. The plain grate rotating about a center pin was for a long time the one most commonly used. These grates were usually provided with a clinker door for removing any refuse too large to pass between the grate bars. The action of such grates tends to leave a cone of ashes in the center of the fire causing it to burn more freely around the edges. A better form of grate is the revolving triangular pattern which is now used in many of the leading furnaces. It consists of a series of triangular bars having teeth. The bars are connected by gears and are turned by means of a detachable lever. If properly used this grate will cut a slice of ashes and clinkers from under the entire fire with little, if any loss of unconsumed coal.