Curling Ice Has Bumps. That's the Whole Game.

Curling ice is not flat. Right before each match, the ice maker walks the rink with a backpack tank and a wand, spraying a fine mist of warm water across the surface. The droplets freeze in place as small beads, called pebble. The whole sheet ends up looking like an orange peel, and the stone, when it travels down the rink, rides on the tips of those pebbles rather than on the underlying ice.

This is the opposite of the engineering done for hockey or figure skating. There the goal is to make the surface as smooth as possible. In curling the goal is controlled friction. The base of a curling stone is concave, so only a thin running band touches the ice; that band rests on a tiny number of pebble tips at any moment. As the stone passes, its weight melts the tops of those pebbles into a momentary film of water, which is what lets the 40-pound stone glide twenty meters.

The stones themselves are mostly cut from granite quarried on Ailsa Craig, a small uninhabited island in the Firth of Clyde, prized because the rock is dense, water-resistant, and uniform. World Curling Federation rules pin the weight between 17.24 and 19.96 kilograms. The Royal Caledonian Curling Club, founded in Edinburgh in 1838 as the Grand Caledonian, wrote the first standardized rules.

And then there is the part nobody has nailed down. A curling stone given a slow clockwise rotation curls to the right — the opposite of what a spinning glass on a wet table does. Two competing theories, one based on differential friction and one on tiny scratches the stone leaves and follows in the pebble, both fit some of the data and not all of it. A 2022 paper in Tribology Letters offered a new ice-deformation model. The argument is not closed. The sport works fine without it being closed.