290 SORGHUM. 



quently cleansed, and the straw, hay, sand, and gravel, should be 

 either replaced by iresh material, or thoroughly washed with water, to 

 which a little lime has been added. 



Where large quantities of juice are to to be filtered, a large number 

 of bag filters are so arranged as to be fed from a common tank, and 

 filtering into a common receptacle. 



Bags of suitable texture are woven seamless for such purpose. 

 Filter presses of many different constructions have been devised, the 

 general principle being to present large surfaces of the filter to the 

 juice. For a lull description of these, reference is made to the more 

 elaborate works which treat of the manufacture and refiuing of sugar. 

 Besides the several varieties of filters already described, all of which 

 depend upon the removal of impurities through their meclianical en- 

 tanglements in the material of the filter, there is in very extensive 

 use, for the removal of other impurities, filters of charcoal and bone- 

 black, i. e., an animal charcoal prepared by burning bones in ovens or 

 retorts, with the exclusion of the air, in a manner similar to the pro- 

 duction of ordinary wood charcoal. 



It is found by filtering juice or semi-syrup through a layer of bone- 

 black of sufficient depth, that not only are the mechanical impurities 

 removed, as would be expected, but all the coloring matter also, so 

 that the juice or syrup becomes not only clear, but colorless as pure water. 

 It is also found that the bone-black retains other of the impurities of 

 the juice or syrup, as the albumenoid or nitrogenous substances, gum, 

 etc., but it is found that the bone-black soon becomes so charged with 

 these impurities, that it ceases to have any eflTect, and must then be 

 washed, dried, and reburned before it is again fit for use. 



In the process of refiuing the raw sugar, filters varying in length 

 from 10 to 50 feet are used, and it is found that generally about one ton 

 of fresh bone-black is needed for each ton of sugar refined ; but, since 

 it may be used over and over again, (mly needing this revivification, 

 as it is termed, with the addition of so much as may be lost in the 

 operation of washing, drying, and burning, the expense attending it is 

 largely the original cost of the plant and material, as also the additional 

 labor involved in its use. 



The use of bone-black is only practicable with large central factories 

 working up very large quantities of cane, or refineries receiving the 

 product of many smaller manufacturers. It is not within the limits 

 of the farmer or sugar planter; and, although as invaluable to the re- 

 finer as it is indispensable, its economical use involves the investment 

 of a large capital. 



