BURNING.
The muffle terra-cotta kilns, as used by the New Jersey com panies, are extremely simple in their construction, of which Fig. 188 gives a correct elevation ; see also frontispiece.
The kilns are up-draft in principle, and provided with short high furnaces arranged for anthracite coal as fuel. There is a double door, the upper one resting upon piers, thereby form ing alleys and flues. In the centre is a communicating chimney that connects with the hollow floor space, carrying heat from the furnace under the floor and up through the center of kiln, whence it escapes through the center hole in main crown into dome, not to be utilized any further.
The principal heat rises directly from the furnaces into the flue surrounding the muffle, then under the main crown over the muffle crown to the center outlet into the dome.
The regulation of the heat under the floor and through the center of the kiln is by closing and opening more or less the flue spaces leading from the furnaces to underneath the floor and center chimney. The draft of kiln is effected by regulat ing the opening in main crown in dome by means of a fire clay tile.
The kilns are invariably round in shape. The inside meas urements are 12 feet diameter and 12 and 20 feet high. The muffle flues are 6 inches, muffle tiles being inches thick, Some of the furnaces have open fires, as shown in Fig. 189, while others are provided with doors, as in Fig. 188. The burning quality of the kilns is good, but they are wasteful of fuel.
The kilns as built by the two works in Boston differ some what in construction, as will be seen in elevation and section of Figs. 189 and 19o. Some of them are built round and some oblong. Their size Jo to 12 and 13 feet in diameter, by 9 to 17 feet in height. The muffle flues are from to 5X inches spacing ; thickness of muffle varies from two to three inches. In circulation of heat they are up and down draft ; the heat rising upward into the muffle flue, over the muffle crown, and downward through the centre flue (or chimney), then under the floor, and from there the heat passes upward in flues built between the furnaces in the muffIc space, then leads into the dome on side of kiln. The number of furnaces to a kiln are six and seven. All the kilns are built with projecting top feeders, arranged for anthracite coal, which is being used exclusively as fuel.
These kilns do good work, and are more economical in the consumption of fuel than the direct up-draft as used by the New Jersey companies.
The muffle kilns adopted by western works differ in many ways from those of eastern manufacturers, ranging from 12 to 24 feet in diameter, and from 9 to 22 feet in height, and are, without question, superior to those used in the east.
The Northwestern Terra-Cotta Company of Chicago adopted two styles of muffle kilns—direct down-draft and up-and-down draft. They claim that the latter are giving better results in even burning and a slight saving of fuel. The fuel used is Indiana block coal, also called Brazil block. This coal is very rich in carbon and very free burning, much resembling Scotch machine coal, except that it is harder and can be cut and split like slate. It makes a clear, mild fire, like that of oak wood. Fuel, of course, has much to do with the good working of a kiln. It is reasonably certain that with an anthracite coal that lies quiet in the furnace, producing a less lively fire, a less per fect circulation or distribution of heat will be effected than by using a coal carrying a decided flame, thereby having a farther calorific effect. Of course, in building kilns, the requirements of mechanical science are far different than for building a brick house. Some masons think that in bricklaying all brick bonds are the same. This may do for the inexperienced, but it can be readily understood that in a wall that is exposed to a con tinuous action such as expansion and contraction, the wear and tear is very destructive in comparison with the undisturbed masonry in a building of different character and purpose. In a good kiln three qualifications are essential : good combus tion, even circulation of heat, and durability of construction. The building of kilns should not be left to inexperienced per sons, as they represent the principal part of the capital of a well established works.
One of the hardest colors to obtain uniformity in the tint is the elegant buff, and to secure this rich, pleasing color in terra cotta, requires long burning, and a highly experimental knowl edge of firing, as well as a thorough acquaintance with the clay, and its behavior in the kiln.