The operation of the burners was perfectly satisfactory in every respect, and practically smokeless.
The muffle kiln illustrated in Fig. 51 is used by many manufacturers of enamel ware. In this kiln the product is surrounded with a wall that entirely protects it from the direct action of the flame. This type of kiln is necessary where the ware has to be treated with two or more coats of enamel, as the second and third coats must be applied when the ware is at a high temperature. A bathtub is a good ex ample of this process. The tub comes from the foun dry and is cleaned and smoothed. Then a "filler" coat is applied. This is allowed to set a few days until thoroughly dried. Then it is swung into the furnace eans of a "fork," the temperature of the furnace being 1800 degrees F. As soon as it reaches a "heat," it is taken out and treated with powdered material, and replaced in the furnace until the operator knows that it is ready for the second treatment. The average time required to enamel a bath tub is 24 minutes. A typical muffle kiln has a capacity of 120 tubs each 24 hours, on a consumption of 10 bbl. of oil or 3y2 gal. of oil per tub.
The burning of porcelain ware with fuel oil re quires careful attention on the part of the fireman. The temperature must be regulated with precision. A common method of determining the temperature is by means of "cones," which are made from a compo sition of feldspar, kaolnite (china clay), quartz (flint) and calcium carbonate, so proportioned that the cones melt at definite temperatures. These "cones" are put into the "saggers" that are placed opposite small peep holes, making it possible for the operator to regulate the temperature of the kiln.
The average size kiln is 17 ft. in diameter by 18 ft. high and is heated by means of ten furnaces located at equal centers around the bottom of the kiln. The total time necessary to burn the ware first is 56 hours, the water smoking period requiring from 26 to 30 hours, followed by 22 to 26 hours of "high heat," a maximum of 2500 degrees F. being reached during this period. The kiln is then allowed to cool off grad
ually for a few days. This size kiln has an average capacity of 2500 lbs. of ware, and from 72 to 80 bar rels of oil are use at one burning.
After the burning is completed the ware is dipped into an enameling solution, and allowed to air dry. It is then ready for the gloss kiln, which is of the same construction (See Fig. 52). The same method of placing the ware in saggers is used, as well as the "cones" for determining the temperature. Care must be taken with the temperature of the kiln dur ing this process, as a sudden fall causes the ware to "blib," or show a wavy or lumpy appearance on the surface.
The total time required to complete the process of enameling is 36 hours. For 22 hours a "low heat" is required, followed by 14 hours of "high heat," a temperature of 2100 degrees F. being required dur ing the "high heat" period. The oil consumption is about 60 barrels.
"Fire clay" is an indefinite term, applying in general to clays which do not fuse except at high tem peratures. Although no definite limit has ever been set, it may be said that no clay can be called a fire clay of good grade unless it will withstand a temper ature of 3000 degrees F. without sign of softening. When burned these clays are generally white or yel low in color. The value of fire clay can only be de termined definitely by a refractory test.
The high fire resisting quality of a clay is due to the absence of fluxes, and to the fact that its com position approaches more or less closely to that of the ideal clay substance Al. 01 2Si 02 2H10. A clay that contains much above 46.3 per cent silicia, can not, therefore, be a high grade fire clay. When pur chasing fire brick the purpose for which it is to be used must be known, for it is impossible to obtain a fire brick which has great mechanical strength, and at the same time is highly refractory. A fire brick which is coarse, clean, white and flinty, is of good quality, and highly refractory. If the grain is fine and smooth, the quality is apt to be poor.