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. PREVENTING CARBURIZING BY COPPER-PLATING Copper-plating has been found effective and must have a thickness of 0.0005 in. Less than this does not give a continuous coating. The plating bath used has a temperature of 170 deg.F. A voltage of 4.1 is to be maintained across the terminals. Regions which are to be hardened can be kept free from copper by coating them with paraffin before they enter the plating tank. The operation is as follows: Operation No. Contents of bath Purpose 1 Gasoline To remove grease 2 Sawdust To dry 3 Warm potassium hydroxide solution To remove grease and dirt 4 Warm water To wash 5 Warm sulphuric acid solution To acid clean 6 Warm water To wash 7 Cold water Additional wash 8 Cold potassium cyanide solution Cleanser 9 Cold water To wash 10 Electric cleaner, warm sodium Cleanser to give good hydroxide case-iron anode plating surface 11 Copper plating bath of copper Plating bath sulphate and potassium cyanide solution warm There are also other methods of preventing case-hardening, one being to paint the surface with a special compound prepared for this purpose. In some cases a coating of plastic asbestos is used while in others thin sheet asbestos is wired around the part to be kept soft. PREPARING PARTS FOR LOCAL CASE-HARDENING At the works of the Dayton Engineering Laboratories Company, Dayton, Ohio, they have a large quantity of small shafts, Fig. 40, that are to be case-hardened at _A_ while the ends _B_ and _C_ are to be left soft. Formerly, the part _A_ was brush-coated with melted paraffin but, as there were many shafts, this was tedious and great care was necessary to avoid getting paraffin where it was not wanted. [Illustration: FIG. 40.--Shaft to be coated with paraffin.] To insure uniform coating the device shown in Fig. 41 was made. Melted paraffin is poured in the well _A_ and kept liquid by setting the device on a hot plate, the paraffin being kept high enough to touch the bottoms of the rollers. The shaft to be coated is laid between the rollers with one end against the gage _B_, when a turn or two of the crank _C_ will cause it to be evenly coated. [Illustration: FIG. 41.--Device for coating the shaft.] THE PENETRATION OF CARBON