Within the limits of the present volume it would be impracticable to give a complete treatise on the manufacture of leather; but there are many recipes and processes involved in it which may fitly occupy a place here.
Those desirous of consulting a more elaborate essay will find it in Spoils' 'Encyclopaedia,' pp. 1213-1240, from which most of the following information has been condensed.
This process is as follows: - Selected calf-skins, dried or salted, are the raw material, and after a suitable softening in fresh water, are limed for 2 to 3 weeks, or till the hair goes easily. They are then unhaired and fleshed in the usual manner, pured with a bate of dogs' dung, scudded, and again cleansed with a bran drench. In Germany, the bran drench is used alone, and is composed of 33 lb. bran to 100 medium skins. Before use, the bran should, especially in summer, be well washed to free it from adhering meal. The temperature of the drench should not exceed 100° F. (38° C), and the skins should remain in for 8 to 10 hours. Lactic acid is produced by fermentation; this removes lime, and is itself neutralized by the products of putrid fermentation which succeeds it.
The tanning is accomplished in a drum with a mixture of alum and salt; and after drying, the skins are again moistened, and worked in the drum with a mixture of oil, flour, and egg-yolk. In the German method, these two operations are combined. Eitner, who has written a series of articles on the process, gives 40 lb. flour, 20 lb. alum, 9 lb. salt, 250 eggs or about 11 gal. of egg-yolk, 7/8 pint of olive-oil, and 12 to 16 gal. water, as a suitable mixture. The skins are worked in a drum-tumbler (preferably a square one) for 20 minutes, then allowed to rest 10 minutes, and this process is twice repeated. The temperature must not exceed 100° F. (38° C), and it is said to be important that the drum should be ventilated by holes at the axis.
The skins are allowed to drain, and then rapidly dried at a temperature of 140° to 160° F. (60° to 71° C), and after "samming," or damping with cold water, are staked by drawing them to and fro over a blunt knife fixed on the top of a post. They are then wetted down and shaved, with either the moon-knife or ordinary curriers' shaving-knife, and sometimes receive a second dressing of oil, flour, and egg, to soften them still further.