CARBURIZING BY GAS
The process of carburizing by gas, briefly mentioned on page 88, consists of having a slowly revolving, properly heated, cylindrical retort into which illuminating gas (a mixture of various hydrocarbons) is continuously injected under pressure. The spent gases are vented to insure the greatest speed in carbonizing. The work is constantly and uniformly exposed to a clean carbonizing atmosphere instead of partially spent carbonaceous solids which may give off very complex compounds of phosphorus, sulphur, carbon and nitrogen.
Originally this process was thought to require a gas generator but it has been discovered that city gas works all right. The gas consists of vapors derived from petroleum or bituminous coal. Sometimes the gas supply is diluted by air, to reduce the speed of carburization and increase the depth.