The filtration of small quantities of liquid, as in chemical experiments, may often be conveniently performed by merely placing the paper on the circular top of a recipient (see engr.), or on a ring of glass or earthenware laid on the top of any suitable vessel. A filter of this kind that will hold one fluid ounce will filter many ounces of some liquids in an hour.
Good filtering paper should contain no soluble matter, and should not give more than 1⁄250 to 1⁄230 of its weight of ashes. The soluble matter may be removed by washing it, first, with very dilute hydrochloric acid, and secondly, with distilled water.
The ‘Munktell’ Swedish filtering paper[301] is composed of flax fibres very much crushed and broken, and owes its value to the broken pieces of the fibres filling up the pores, and thus preventing solids from passing through the paper. Rhenish filtering paper is also made from flax, but in consequence of the more perfect condition of its fibres, is more porous than Munktell’s, and therefore inferior to it for filtering purposes. Another kind of Rhenish paper, also of flax, in which the fibres are much torn, is manufactured and is said to be a useful article, and to allow the rapid passage of fluids through it. The white filtering papers of English make have a small quantity of cotton mixed with the flax; and the fibres are much torn and crushed; hence they make serviceable filters.
[301] Dr F. Mohr says that Swedish filtering paper is now undeserving its traditional reputation, and that it contains soluble alumina.
The grey, circular cut filtering paper of varying sizes, of foreign make, as well as the grey sheet filtering paper of Dutch and English manufacture, contains a large quantity of wool, much of which is coloured; as well as jute and esparto grass, both of these latter in an unbleached state. The amount of ash in the Munktell paper has of late increased in quality.[302]
[302] Greenish.
For filtering a larger quantity of a liquid than can be conveniently managed with a funnel, and also for substances that are either too viscid or too much loaded with feculence to allow them to pass freely through paper, conical bags made of flannel, felt, tweeled cotton cloth or Canton flannel, linen or calico, and suspended to iron-hooks by rings or tapes, are commonly employed. The first two of the above substances are preferable for saccharine, mucilaginous, and acidulous liquors; the third for oily ones; and the remainder for tinctures, weak alkaline lyes, and similar solutions. These bags have the disadvantage of sucking up a considerable quantity of the fluid poured into them, and are therefore objectionable, except for large quantities, or when they are to be continued in actual use as filters for some time. On the large scale, a number of them are usually worked together, and are generally enclosed in cases to prevent evaporation, and to exclude dirt from the filtered liquor that trickles down their sides. These arrangements will be noticed further on.
A simple mode of filtering aqueous fluids, which are not injured by exposure to the air, is to draw them off from one vessel to another, by means of a number of threads of loosely twisted cotton or worsted, arranged in the form of a syphon. (See engr.) The little cotton rope at once performs the operations of decantation and filtration. This method is often convenient for sucking off the water from a small quantity of a precipitate.