Up to perhaps 150 years ago, no attempt was made to leach the tanning material, which was simply strewed in layers between the hides, and moistened with water. Leaching originated in England, and was first applied merely to complete the exhaustion of the material which had been already used for layers; but the use of even weak liquors instead of water in the layers was found so advantageous, that new material was soon applied to make stronger infusions. The earliest form of leach was simply a pit with a perforated wooden “eye” or shaft down one corner, in which a pump could be placed to remove the liquor without being choked with solid matter. This was considerably improved by the addition of a perforated “false bottom” to the pit, with which the eye communicated. The perforations of the latter were found unnecessary, and it now serves simply for pumping through, or for the manipulation of a plug in a hole communicating with an underground “trunk” leading into a pump-well. The false-bottom is best made of laths about 1 inch thick and 2 inches wide, cut slanting so as to be wider on the upper than the lower surface, which makes the spaces between them less liable to choke. The laths are nailed on cross-battens with copper nails, which should be long enough to clinch, ¹⁄₄-inch to ¹⁄₂-inch spaces being allowed between the laths according to the fineness of the ground material. The lattice-bottom should be in at least two sections, so as to allow of its easy removal for cleaning, and should rest on detached blocks, which are best nailed to the underside of the battens. A space of 2 inches to 3 inches below the false bottom will prove sufficient if it is cleared every time the pit is emptied, but not otherwise. Clearness from obstruction both below the bottom and between the laths themselves is very important in securing free running in the “press leach” system about to be described. A section of the latticed bottom is shown in Fig. 77. The laths are easily cut by employing a circular saw with a tilted table, and turning the board at each cut. No advantage is gained by planing them. [Illustration: FIG. 77.--Section of Leach-Bottom.] As a strong liquor cannot be made by the use of a single leaching-pit, a series of pits are now always employed, and it is the leaching, systematic or otherwise, which determines how much of the total tannin will be thrown away and lost in the “spent tan.” In the case of properly extracted materials the “spent tan” will not contain more than one per cent. of tanning matter, but the degree of extraction which is profitable is dependent on the tanning material employed and the class of leather to be produced. The system of leaches now considered to be the best is based on the “continuous” process of extraction. Of its different forms, the “press-leach” is the simplest and in most cases is all that is required. [Illustration: FIG. 78.--Plan and Section of Battery of Press-Leaches.]