Every cupped, rocking, half-binned beer mat you have ever seen is the same piece of physics happening on a bar. It is worth two minutes.

Absorption is one-sided

A glass sits on the top face, condensation runs down onto the top face, and spills land on the top face. The underside spends the whole evening against a dry bar.

Pulpboard fibres swell as they take up water. One side of the sheet gets longer; the other does not.

A sheet that is longer on one side is a dish

There is nowhere else for the extra length to go. The mat cups, lifting its edges, and now it sits on three points instead of its whole face.

That is when it starts sliding under a glass, and that is when bar staff start throwing it away.

What actually controls it

Board choice first: how much fibre there is, how compressed it is, and how fast it takes water. Then grain direction, which decides which axis the sheet prefers to move on — run it right and the cupping is far less pronounced.

Both are decisions somebody makes at the mill and at the press. Ours are written on the quote; most are not written anywhere.

The ten-minute test

Wet the top of a mat, set it down, and walk away. Come back in ten minutes and look at the corners.

Flat means somebody specified the board. A shallow dish means you bought whatever was on the shelf, and it will be in a bin by ten o’clock.