Fenner's Complete Formulary - Southwest School of Botanical ...

In filtering a liquid which contains a precipitate (unless the precipitate
is designed to clear the liquid, as magnesia or pumice-stone are used)
the liquid should be poured carefully off and filtered first, the
precipitated portion being added after most of the liquid has passed
through the filter; this makes the process more rapid.
The first portion that passes through the filter should be returned to it
and re-filtered, as, when the filter is dry, it admits of the passage of
small particles which are retained when its fibers have had time to swell
by the absorption of moisture.
In filtering liquids containing albuminous or gummy precipitates, it is
also advantageous to put a coarse cotton cloth strainer on the inside of
the filter paper; this catches the precipitate or albuminous substance,
which may be removed with it, or in which it may be pressed to strain
out the liquid, and thus make the filtration more rapid.
A plaited filter is generally used, except when a filter-rack is employed,
then the ordinary folded (quartered) filter is used.
Besides filtering through paper, other means are often employed by
druggists. Syrups and heavy liquids may be filtered through a flannel
or cotton strainer, or felt filters that are made expressly for this purpose.
Charcoal and sand, in alternate layers, are employed for filtering light
liquids when larger quantities are to be filtered.
A little charcoal in powder, or powdered pumice-stone sprinkled in the
filter, will often assist to clear preparations that are difficult to filter
clear.
Fenner’s Complete Formulary - Part I-II - MISCELLANEOUS FORMULA - Page 34
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