In the modern fat tannages drums are used to give the necessary mechanical working to the goods. The raw hide leather produced in the U.S.A. is made by drumming the nearly delimed goods with tallow and neatsfoot oil. In this country the fat tannages have been typified by the "Crown" and "Helvetia" leathers. The hides are thoroughly limed in mellow limes, and after the beam work are delimed by drenching, scudded, and sometimes fleshed again, and then coloured off in tan liquor. After partial drying, they are drummed warm for some hours to ensure isolation of the fibres. After further drying they are coated with the tanning paste, which consists essentially of soft fats and flour to produce partial emulsification. Equal parts of soft fats and of flour may be used, to which may be added smaller proportions of degras, cod oil, mutton tallow, salt, together with about 25 per cent. water. The goods are coated with this mixture, drummed, and dried further, and this routine repeated as often as necessary to fill the interstices thoroughly with fat. The temperature in the drum may reach 95° F. In finishing an attempt is made to stuff further with grease. The goods are thoroughly set out, dried a little, and coated again, flesh and grain, with a mixture of tallow, cod oil, glycerine and degras, and dried further. The excess grease is slicked off and the goods again set out and grained. They are then dried out. REFERENCES. Bennett, "Manufacture of Leather," pp. 245, 246 and 376. Procter, "Principles of Leather Manufacture," p. 378. SECTION III.--OIL TANNAGES There are very obvious analogies between the fat tannages discussed in Section II. and the oil tannages now to be dealt with, but there is nevertheless a distinct departure in principle involved. In the oil tannages the mechanical treatment is generally more vigorous, and the "drying" process is conducted at a much higher temperature, with the result that there is a vigorous oxidation of the oil. This results in the formation of insoluble oxidation products which coat the fibre and play an essential part in the production of a permanent leather. Pungent vapours are evolved in the drying operations, amongst which is acrolein and probably also other aldehydes, and it is thought by Procter that these aldehydes also are essential tanning agents and typical of the process (cf. Section IV.). Fahrion considers that the tanning action is due solely to unsaturated fatty acids with more than one double linkage. Garelli and Apostolo, however, believe that the tannage is due to a coating of fatty acid whether saturated or not. These observers made leather with stearic and palmatic acids in colloidal aqueous solution.