368 FILTRATION 



so short, as to render the result inconsiderable. Animal carbon, when properly pro- 

 pared and mixed with the above hardening composition, is capable of giving hardness 

 to the surface even of an iron file. 



This carbonaceous matter may be readily obtained from any of the soft parts of 

 animals, or from blood. For this purpose, however, the refuse of shoemakers and 

 curriers is the most convenient. After the volatile parts have been distilled over, 

 from an iron still, a bright shining coal is left behind, which, when reduced to powder, 

 is fit to mix with the salt. Let about equal parts, by bulk, of this powder, and 

 chloride of sodium be ground together, and brought to the consistence of cream by the 

 addition of water. Or mix the powdered carbon with a saturated solution of the salt, 

 till it becomes of the above consistence. Files which are intended to bo very hard, 

 should be covered with this composition, previous to hardening. All files intended 

 to file iron or steel, particularly saw files, should be hardened with the aid of this 

 mixture, in preference to that with the flour or grounds. Indeed, it is probable, that 

 the carbonaceous powder might bo used by itself, in point of economy, since the 

 ammonia or hartshorn, obtained by distillation, would be of such value as to render 

 the coal of no expense. By means of this method the files made of iron, which in 

 itself is unsusceptible of hardening, acquire a superficial hardness sufficient for any 

 file whatever. Such files may, at the same time, be bent into any form ; and, in con- 

 sequence, are particularly useful for sculptors and die-sinkers. 



The next point to be considered is the best method of heating the file for hard- 

 ening. For this purpose a fire, similar to the common smith's fire, is generally 

 employed. The file is held in a pair of tongs by the tang, and introduced into 

 the fire, consisting of very small cokes, pushing it more or less into the fire for 

 the purpose of heating it regularly. It must frequently be withdrawn with a view 

 of observing that it is not too hot in any part. When it is uniformly heated, from 

 the tang to the point, of a cherry-red colour, it is fit to quench in the water. At 

 present an oven, formed of fire-bricks, is used for the larger files, into which the 

 blast of the bellows is directed, being open at one end, for the purpose of introducing 

 the files and the fuel. Near to the top of the oven are placed two cross-bars on which 

 a few files are placed, to be partially heating. In the hardening of heavy files, this 

 contrivance affords a considerable saving, in point of time, while it permits them also 

 to be more uniformly and thoroughly heated. 



After the file is properly heated for the purpose of hardening, in order to produce 

 the greatest possible hardness, it should be cooled as soon as possible. The most 

 common method of effecting this is by quenching it in the coldest water. Some file- 

 makers have been in the habit of putting different substances in the water, with a ; 

 view to increase its hardening property. The addition of sulphuric acid to the water 

 was long held a great secret in the hardening of saw-files. After all, however, it will 

 be found that clear spring water, free from animal and vegetable matter, and as cold 

 as possible, is the best calculated for hardening files of every description. 



In quenching the files in water, some caution must be observed. All files, except the 

 half-round, should be immersed perpendicularly, as quickly as possible, so that the 

 xipper part shall not cool. This management prevents the file from warping. The 

 half-round file must be quenched in the same steady manner ; but, at the same time 

 that it is kept perpendicular to the surface of the water, it must be moved a little 

 horizontally, in the direction of the round side, otherwise it will become crooked 

 backwards. ; 



After the files are hardened, they are brushed over with water and powdered cokes, 

 when the surface becomes perfectly clean and metallic. They ought also to be washed 

 well in two or three clean waters for the purpose of carrying off all the salt, which, if 

 allowed to remain, would be liable to rust the file. They should moreover be dipped 

 into lime-water, and rapidly dried before the fire, after being oiled with olive oil, 

 containing a little oil of turpentine, while still warm. They are then finished. 



FILIGREE (Filigranc, Fr. ; Filigran, or Feine Drahtgcflecht, Ger.) is, as the 

 last term justly expresses it, intertwisted fine wire, used for ornamenting gold and silv.-r 

 trinkets. The wire is seldom drawn round, but generally flat or angular ; and sol- 

 dered by gold or silver solder with borax and the blowpipe. The Italian word, 

 filigrana, is compounded of filum and ffranum, or granular net-work ; because tho 

 Italians, who first introduced this stylo of work, placed small beads upon it. 



FILTRATION (Eng. and Fr. ; Filtriren, Ger.) is a process purely mechanical, 

 for separating a liquid from the undissolved particles floating in it, which liquid may 

 be either the useful part, as in vegetable infusions, or of no use, as the washings of 

 mineral precipitates. The filtering substance may consist of any porous mutter in a 

 solid, foliated, or pulverulent form; as porous earthenware, unsixnl ]>aj..T. cloth of 

 many kinds, or sand. The white blotting-pai-iT sold by tho stationers 

 tremcly well for filters in chemical experiments, provided it be previously washed with 



