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I have a small French Press (roughly 220ml) that has the following written on it:

Always stir before pressing down, using a plastic or wooden spoon, not metal.

A metal spoon does not really damage the glass, so I assume there are other phsyics related reasons.

Symbolic picture demonstrating the stirring:

Source: https://nmpinoncoffee.com/brew-guides/french-press (Source: https://nmpinoncoffee.com/brew-guides/french-press)

Note:

  • The carafe is made of non-stain, heat-resistant borosilicate glass
  • The brew is to be stirred after at least 4 minutes

Why no metal spoon?

Is the metal having an effect on the coffe brew, like silver is having effects on some materials (like on sulfur)?

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    Perhaps you should rethink if a metal spoon could damage glass - it only takes a small defect to dramatically increase the possibility of brittle failure, particularly when subjecting glass to large temperature swings (i.e. room temperature to boiling in a moment).
    – Jon Custer
    Aug 25, 2021 at 12:55
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    It could also be that metal is a good heat conductor. When I stir, the spoon often becomes uncomfortably hot fast. (I have spent far too much time thinking about the physics of French presses myself, but not this aspect).
    – Anders Sandberg
    Aug 25, 2021 at 13:29
  • @AndersSandberg It wouldn't suprise me at all if "french-press" got its own tag on PSE :D I agree to both of you, probably even by rare chance, a glass containing boiled water breaking right in your hands is harmful enough to have a notice about it.
    – Daniel W.
    Aug 25, 2021 at 13:50
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    Can you elaborate on what is being stirred? Given that most of the press' parts are metal, it's clearly not a "taste" issue. I would be shocked if the glass used in any reputable press were not annealed, Pyrex-style, to avoid the risk of accidental fracture. Aug 25, 2021 at 14:24
  • I've had 2 of those glass models where the glass has spontaneously cracked. I now have a double skin metal press - not only is it larger than the standard size, it keeps my coffee warm and is unbreakable :) Aug 25, 2021 at 19:50

2 Answers 2

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To expand on a couple of the comments:

A french press is a coffee system used to filter a suspension of coffee grounds out of (very hot) water and contain them at the bottom of the canister. In many cases the canister is made of glass. As the linked article says:

French presses with a glass carafe are an extremely fragile type of coffee maker, quicker to chip or break than most. In fact, brand instructions typically advise against using metal spoons when stirring so as not to damage the glass.

A metal spoon used without caution could easily cause minor chipping or scratching in the glass canister. While this might not seem like a problem, a french press is subjected to large temperature swings when the water is poured in. Temperature increases on glass cause thermal expansion of the glass, which subjects it to stresses, particularly where there is a temperature differential between different parts of the pot (e.g. hot bottom, cool top). These can cause minor chips and scratches to catastrophically fail.

In addition to this, during normal usage, the filter part of the press is plunged to collect and remove the coffee grounds from suspension. This results in some pressure being applied to the filter, especially if too much grounds are in the canister to be easily filtered out. The pressure on the filter is transferred to the liquid component. Liquids, are incompressible, which means that any pressure applied to them is directly transmitted to anything containing them. Under such pressures the chips/scratches could also cause failure of the glass.

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I stir after a minute of steeping. CO2 released from the grounds has raised them to the surface snd stirring gets them down in the water. I stir with a wooden chopstick because I’ve cracked a number of the glass cylinders when using metal… early morning clumsiness.

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    Your last sentence pretty much sums up why the manufacturer says not to use metal.
    – Stephie
    Aug 27, 2021 at 8:28

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