Coffee filter

In 1932,[12] Melitta originally introduced the so called Schnellfilter (English: rapid filter), a cone-shaped filter holder with circular bottom with either 4 or 8 holes suitable for use with squarish sheets of filter paper, which still had to be pressed into shape through a metal cone. These rapid filter holders were manufactured of porcelain or metal, available in sizes "100", "101", "102", and "103". This system was available up to 1939.[13]

In 1936, Melitta introduced the Filtertüte (English: filter bag) in various sizes. The cone-shaped filter holders were refined to get a slot-shaped bottom (originally with 4 holes) more suitable for the filter bags, now looking more fan- or boat-shaped. Over the years the system was expanded to eventually consist of filter bag sizes "100" (for 1–2 cups à 1⁄6[14]–1⁄8 litre[15][16][14]), "101" (for 2–3[15] or 2–4 cups[16][14][17]), "102" (for 3–6,[15] 4–6[18] or 4–8 cups[16][14]), "103" (for 6–15,[15] 8–15 cups[16][14][19] or 10–15[20]), "104" (for 15–25[16][14] or 15–30 cups[15]), "105" (for 25–50[16][14][21] or 30–60 cups[15]), "106" (for 50–80[16][14] or 60–100 cups[22]), "112" (for 2 cups, with pot mount[23][24][25]) and "123"[26] (for 6–10 cups[27]). The system also included special types like tea filters "401" (1–6 cups,[28] compatible with "101"[26][14]) and "402" (for 3–9 cups,[29] compatible with "102"[30]) and the miniature filter "801" (for 1–2 or 1–3[31] small cups for children, or 1 normal cup[31]). Brigitta once marketted a fan- or boat-shaped filter size "502".[32][33] A disadvantage of the system was that one had to pour water continuously or several times while the proper amount of necessary water could only be guessed.

Therefore, in 1965[34] Melitta developed a new fan- or boat-shaped filter system with corresponding "1×" nomenclature: In this system the filters are sized big enough so that the whole amount of water (except for the water needed for blooming) can be poured in one go. Consequently, the filter sizes "1×2", "1×4", "1×6" and "1×10"[35] result in 2, 4,[36] 6, and 10 cups of coffee when filling the filter once. Since these filters only differ in height and have otherwise the exact same geometry, bottom width (about 49 mm) and angle (about 54°), the filter bags are interchangeable between filter holders of different sizes.

Both systems are still in use today in principle, but the sizes "103", "104", "105", "106", "112", "123", "401", "402", ("502",) "801" and "1×10" are no longer manufactured.

Common in the US are fan- or boat-shaped filters "#0" (similar to "100"), "#1" (similar to "101"),[nb 1] "#2" (similar to "102"),[nb 2] "#4" (similar to "1×4"),[nb 3] and "#6" (similar to "1×6"),[nb 4][nb 5] with "#2", "#4" and "#6" being particularly popular, as well as basket-shaped filters in an 8–12 cup home size and larger restaurant sizes.

Since 2004, Hario has cone-shaped paper filter bag sizes "01" (for 1 cup), "02" (for 1–4 cups) and "03" (for 1–6 cups) for their "vector 60" V60 filter holders (with 60° angle).[37][38]

A basket-type coffee filter, here made of bleached paper

Other Melitta filter sizes include the pyramid filters "202s", "203", "206(G)", "220(G)", "240(G)" and "270(G)", round filter disks "1" (94 mm), "1a" (60 mm), "2" and "2b", circle filter rings (for percolators) "3 1⁄2 in." (89 mm),[39] "164mm", "190mm", "203mm", "235mm", "240mm", "244mm", "256mm", "260mm", "290mm", "330mm", "400mm" and "440mm", basket filters "(A)250/90" and "(A)250/110", roll filters "2004" as well as wrap filters.[40][41] While some of them are still available today, most of them have fallen out of use for long.

A squarish pyramid filter Filtra "602" was available as well.[42]

The six conical filter holder sizes for the Chemex coffee maker (originally introduced in 1941) and the Funnex utilize two different sizes of paper filters. A half-moon shaped filter paper (bleached: FP-2, unbleached: FP-2N) is used for the 3-cup holders (CM-1, CM-1C, CM-1GH) and the Funnex (CM-FNX), which must be folded before use. The larger holders for 5 (CM-2), 6 (CM-6A, CM-6GH), 8 (CM-3, CM-8A, CM-8GH), 10 (CM-10A, CM-10GH) and 13 cups (CM-4) can alternatively use prefolded square sheets (bleached: FS-100, unbleached: FSU-100), prefolded circle filters (bleached: FC-100) or unfolded circle filters (bleached: FP-1). The paper is 20–30% thicker than regular paper filters.

Other important coffee filter paper parameters are strength, compatibility, efficiency and capacity.

If a coffee filter is not strong enough, it will tear or rupture, allowing coffee grains through to the coffee pot. Compatibility describes a filter medium's resistance to degradation by heat and chemical attack; a filter that is not compatible with the liquid passing through it is likely to break down, losing strength (structural failure). Efficiency is the retention of particles in a target (size) category. The efficiency is dictated by the particles or substances to be removed. A large-mesh filter may be efficient at retaining large particles but inefficient at retaining small particles. Capacity is the ability to "hold" previously removed particles while allowing further flow. A very efficient filter may show poor capacity, causing increased resistance to flow or other problems as it plugging up prematurely and increasing resistance or flow problems. A balance between particle capture and flow requirements must be met while ensuring integrity.