Why not stir the french press using a metal spoon?

Question

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)

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)?

Answer 1

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.

Answer 2

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.