Why Do Coffee Grounds Float in a Cafetière — Then Suddenly Sink?

Add hot water to coffee in a cafetière and you will notice that the grounds surge upwards, bubbles gather, a dark crust forms across the surface, and, after a while, individual particles drift down again.

It can look unpredictable, but the movement follows a simple physical story involving gas, water, density and gravity, and understanding it helps explain why a cafetière brew may not be uniform from top to bottom — and why forceful stirring is not always necessary.

Roasted coffee is full of tiny spaces
A coffee ground is not a solid, polished pebble. Roasting creates a porous structure containing countless microscopic spaces, and some of those spaces hold gas, including carbon dioxide produced during roasting.

When hot water first reaches the dry grounds, it does not instantly replace all that gas. Bubbles escape, but gas remains trapped inside and around many particles, which lowers their effective density relative to the water and helps them rise.

The result is the familiar floating layer known as the crust, made from grounds, foam and gas at the boundary between liquid and air.

Why the grounds begin to sink
As brewing continues, water moves into the pores, more gas escapes and the balance changes, the particles become wetter and effectively denser, so gravity starts pulling them down through the drink.

This does not happen at the same moment for every ground - particle size, pore structure, roast level, trapped gas and how evenly the water was poured can all influence the movement.

Should you stir the crust?
A gentle stir can help wet dry pockets and bring floating grounds into contact with water. Aggressive stirring, however, may create extra turbulence and move fine particles throughout the brew.

Why plunging slowly matters
The mesh plunger separates much of the spent coffee from the drink, but it does not make the liquid perfectly clear. Pressing hard or fast can disturb the settled layer and push more fine material through or around the filter.

A slow plunge gives the bed more time to compress without a sudden rush. It’s a good idea to stop if the resistance becomes unusually high, as forcing it can lead to splashes and an unsettled cup. The resistance may indicate a grind that is too fine, too much coffee, or a plunger moving faster than the liquid can pass through the mesh.

Once the plunger is down, pour the coffee rather than leaving it sitting over the grounds for a long period. This makes the serving more consistent and reduces continuing contact between liquid and coffee.


Grind size changes the experience
Cafetière brewing usually suits a medium-coarse grind: very fine coffee creates more suspended material and can make the plunger harder to press, while extremely coarse particles may extract more slowly.

If you are choosing coffee specifically for this method, our coffees for cafetières come pre-ground in an appropriate grind.

Whatever coffee you use, change one variable at a time: if the cup tastes thin, try a slightly higher dose or a little more time before immediately reaching for a much finer grind; if it tastes harsh or silty, check the grind and plunge technique first.