The filter decides what ends up in the cup. Metal has enormous pores: fast flow, almost no retention of oils or fines, body and sediment. Paper slows the flow, absorbs a share of the oils and a share of the migrating fines, depending on its thickness; the pores of cone papers resemble one another, with a peak at 45 to 50 microns, unbleached papers having more uniform pores but a cardboard taste that must be rinsed out at length (5 ppm of solids released by a natural Hario against 0 for a bleached Hario, in Gagné's measurement).

Gagné 2021, pp. 105-125; Chemex, About; Wikipedia, Cafestol

Bleaching is done with chlorine or oxygen. Cloth, common on siphons and in Tokyo cafés, gives a cup that is fast and cloudy like metal but absorbs the oils, which go rancid; Gagné judges it “a nightmare” to maintain: immediate washing, freezing, boiling in unscented detergent then in clear water, hanging to dry. The useful indices of a paper are its hydraulic resistance, its capacity to load fines before clogging (the Cafec “dark roast” papers are among the quickest to clog), its uniformity and its wet strength.

Gagné 2021, pp. 105-125 (Rao 2010)

The idea is not new. In 1915, at the request of Ukers, the Mellon Institute of Pittsburgh compares nine appliances: the boiled pot gives the most “caffetannic acid” and caffeine, the French drip the least, and the best cup is obtained between 195 and 200 °F (90 to 93 °C) rather than at boiling. In 1916, the University of Kansas concludes that filtration gives the best brew. The first committees of American roasters recommended keeping muslin bags “in cold water” between uses, because drying makes them break down.

Ukers 1922, ch. XXXVI (Bacon, Mellon Institute, 1915; Home Economics Laboratories, Kansas, 1916; The Coffee Book, 1915)