Pebbling: why curling ice is bumpy

Why curling ice isn't smooth

Anyone stepping onto a curling sheet for the first time is surprised: the ice does not feel glassy but slightly rough. That is intentional. Before every game, the ice technician sprays fine water droplets onto the sheet. They freeze instantly into tiny bumps, the pebble.

A curling stone does not rest on its whole underside but only on a narrow running band. On smooth ice, this ring would quickly slow down due to high friction; the stone would practically stick. Thanks to the pebble, the running band only touches the tips of the bumps. The contact area is tiny and the stone glides many metres.

Pebble and the stone's curve

The name curling comes from "to curl". If the player gives the stone a slight rotation at release, it follows a curve on its way to the house. The pebble plays a central role, because the interaction between running band and ice bumps makes the stone move sideways. The exact physics is still debated by scientists today.

Players talk of "swingy" ice when stones curl a lot and "straight" ice when they hardly bend. Over the course of a game the bumps wear down. The ice changes and teams have to adjust their shots.

How pebbling is done

1. The ice is first cleaned and scraped with a wide blade to remove old pebble.
2. The ice technician walks backwards down the sheet, swinging a watering can with a sprinkler head evenly from side to side.
3. The water is usually especially pure (often demineralised) and slightly warmed so that the droplets form evenly.
4. Depending on the rink, one or two passes with different droplet sizes follow.
5. Finally, the pebble is often lightly shaved ("nipped") so that the tips are the same height.

How much pebble a sheet needs depends on air temperature, humidity and the number of spectators. Full rinks warm the air and change the ice. That is why the work of curling ice technicians is regarded as an art in itself (more under ice technician).

Why recreational curling on ice hockey ice is difficult

In many German rinks, curling is played on ice hockey surfaces. The ice there is optimised for skates: colder, harder and cut up by blades. Good curling needs an especially level surface, a suitable temperature and fresh pebble. How ice is made and tempered for the different sports is explained in the article making ice & ice temperature.

Pebble during a game

At the start of a game the pebble is fresh, and stones often run a little slower and curl differently than later. With every stone played and swept, the bumps along the most-used paths wear away. Experienced skips remember which lines have become "faster" and adjust their weight calls. At longer tournaments the ice is re-pebbled between games.