transferred to a furnace which is directly in front of the table and which has been heated to the temperature of the glass. This is called the annealing oven. When the plate has been transferred to the annealing oven it is closed and the burners are extinguished and the plate is allowed to cool for a number of days very slowly. Upon again removing from the annealing oven, the plate is uneven and rough. It is placed cn a table and heavy cast. iron rubbers slide over its surface with a whirling motion while water and coarse sand are sprinkled on it. About half the thickness of the plate is cut away during grinding and polishing.

Window glass is always blown. After the refining, the glass is allowed to become pasty and then the blower begins his work, as shown in fig. 92. The pipe of the glass blower is a straight piece of iron tubing 4 or 5 feet long. A lump gathers on the end of the pipe and by blowing through it while whirling it between the hands, the blower forms a hollow globe of glass. The hollow globe is again heated in the furnace called the glory hole (see fig. 93) and when soft is rolled on a flat surface and then swung in a vertical circle. In order to allow room for vertical swinging, the blower stands on a plank or bridge placed across a deep pit. While swinging, the blower occasionally blows through the pipe until the globe becomes a hollow cylinder closed at one end and opening into the pipe at the other. The closed end of the cylinder is re-heated until soft and then blows out. A hollow cylinder open at both ends is thus formed and with a diamond is cut lengthwise and put into the flattening furnace which maintains a temperature sufficient to soften the glass. The cylinder slowly opens and spreads out on the floor of the furnace in a flat sheet.

The regenerative tank furnace is particularly adapted to the manufacture of bottle glass. A typical furnace of this kind may be 75 feet long, 16 feet wide, and the depth to the level of the door may be 5 feet. As the glass comes from the doors it is taken by the bottle machine and made into bottles which are then passed through the annealing furnace. They are carried by an endless chain gear into the furnace to a revolving table and are conveyed into the hottest part of the furnace, which is usually at a temperature of about 100° F. At this degree the bottles come in direct contact with the flame and then pass out the door by another endless chain gear. The entire operation requires about 36 hours.

a. Outlines of Industrial Chemistry, Thorp.

Bottle glass is melted and refined in this tank furnace at a consumption of 140 gallons of oil per ton of glass.

Fuel oil is very generally used now by the large glass factories because the flame can come in direct contact with the glass without producing any discolorization or injuring the glass in any way. In addition, the temperature control is perfect and many articles that were formerly ruined by fluctuations in temperature can now go through the process without injury, due to the maintenance of an absolutely uniform temperature.

CHAPTER XVI

FUEL OIL IN CERAMIC INDUSTRIES

In the manufacture of clay products any fuel which causes discoloration by uneven heating, soot or smoke, is undesirable and unprofitable. Coal is out of the running in the manufacture of ceramic products, such as vases and dishes, and oil is the preferable fuel even in manufacturing enameled, vitrified, fire and common brick.

Many enamel ware manufacturers use the muffle kiln. Whenever it is necessary to treat the ware with two or more coats of enamel, it is necessary to apply all but the first coat at a higher temperature. In burning common brick about five days are required to water smoke and burn and 35 to 50 gallons of crude oil per thousand bricks are required. A longer time, higher temperature and greater consumption of oil per thousand bricks are necessary in the burning of fire bricks, but the process is similar to that used for common brick.

In burning brick with oil the amount of fuel required varies with the quality of clay or shale used. One large plant in Kansas is burning brick using 100 gallons of oil per 1,000 brick. This includes fuel for running their boilers to operate the plant. The presence of carbon in clay is always a serious problem where coal is the fuel, because in case the carbon is ignited and burns freely, the fires in the furnace have to be drawn, all air supply shut off and the carbon allowed to smolder until completely burned out. In pulling a coal fire, the doors must be open and an excess of air rushes into the kiln before it can be daubed, not only checking the ware but supplying large quantities of oxygen for combustion of the carbon in the clay which might overburn the entire kiln. An oil fire does away with these dangers. It can be instantly turned off or turned down and the air inlets closed without loss of fuel or danger to kilns. Fig. 94 shows an oil-burning brick kiln of a capacity of 500,000 brick.

Limestone as quarried is calcium carbonate, and its composition expressed chemically is CaCo,. To make quicklime, which is CaO, it is necessary that the carbon dioxide, CO2, be driven off by heat. Carbon dioxide begins to come off at a tem

perature of about 750 degrees F., but a temperature of over 1,300 degrees is required to completely reduce the stone to calcium oxide. There are always some impurities present in the original limestone and the actual yield of quicklime varies from 30 to 55 percent of the limestone. The different quarries produce limestone of different densities and consequently the difficulty of reducing the stone to quicklime varies. The dense and compact stones yield the best quality of lime.

The old method of burning lime in the "periodic" kilns is wasteful of fuel and time. This type of kiln is shown in fig. 95.

The kiln is made of large blocks of limestone or of brick. Two or three feet from the ground an arch (A) of large blocks of limestone is turned. The fire is built under the arch and the limestone is piled on top of the arch, the lumps varying in size from that of a cocoanut just above the arch to that of a goose egg at the top of the kiln. After the fire is started, the temperature is raised very slowly for six or eight hours to prevent the limestone arch from crumbling. After this interval the temperature is kept at a full red heat for two days or more when the fire is allowed to burn out and the kiln cools. During the time of cooling, discharging and recharging, the kiln is idle and much time is lost. Moreover a large amount of fuel is wasted in heating the walls of the kiln after each recharging.

When fuel oil is used in burning lime all the disadvantages of the periodic kiln are eliminated because the production is continuous. Furthermore, oil burned lime commands a ready market because of its greater cleanliness. There are two types of kiln

which have proven remarkably successful with fuel oil. One, the "continuous" kiln, is vertical and this kiln should be charged with lumps of stone about the size of a man's head. If the temperature is too low or the lumps too large, the stone will not be calcined to the center and the lumps will not slake. At the burning zone the width of the kiln should not exceed eight feet, because with a

greater width the heat may not penetrate to the center of the charge. The combustion chamber should be large enough to allow combustion to take place before the oil enters the kiln, thus insuring a soft, long flame and permitting the gases to pass readily to the center of the ki. A low pressure air burner is preferable