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 best white filtering paper is composed of flax fibres, and its value depends upon the condition of the latter. Some have cotton mixed with the flax.

The gray filtering paper contains a large quantity of wool, most of which is colored; it also contains jute and esparto grass, generally in an unbleached state.