How Do Brushes Affect the Stone’s Path in Olympic Curling?

Three weeks before the Milano-Cortina Winter Olympics opened, the World Curling Federation banned a stroke. Not a broom. Not a brush head material. A stroke — a specific motion, performed with the intent to slow a moving stone rather than extend its travel — was now against the rules.

That decision, issued in January 2026, was not an administrative footnote. It was the latest move in a decade of escalating regulatory battles over how much influence a brush should have over a curling stone’s path. The fact that those rules were still being rewritten weeks before the biggest event in the sport tells you something important: the answer to how sweeping changes where a stone goes is still being contested, by athletes, physicists, and administrators, simultaneously.

The Core Answer When a sweeper brushes the ice in front of a moving stone, friction between the brush head and the pebbled surface generates heat. That heat fractionally raises the temperature of the ice’s raised contact points, dropping their friction coefficient. The stone travels further and curls less. By brushing on one specific side of the stone’s path rather than across its full width, a sweeper can also steer the stone in a chosen direction.

The Ice Surface That Makes Sweeping Matter

Curling is not played on hockey ice. Before every game, technicians walk the sheet with a pebbling device, spraying a fine mist of water that freezes into tiny raised bumps across the entire surface — typically 1 to 3 millimetres in diameter.

There is no standardisation across sheets. Every game carries a unique surface condition, which is why sweepers read the ice in real time rather than working from a fixed script.

The stone itself adds another layer. Its underside is concave, meaning only the outer rim, called the running band, contacts the ice. Research published in Scientific Reports (Nature) puts this ring at approximately 13 centimetres in diameter and just 6 millimetres wide. It is made from Ailsa Craig Blue Hone granite, a harder variety sourced from a small island off Scotland’s southwest coast, pressed into the body of the stone as a separate insert.

The practical result: a stone weighing up to 19.96 kilograms is touching almost nothing as it travels 28.35 metres to the target. Those pebble peaks are the only contact points between granite and ice, and they are exactly what the brush is working on.

What the Brush Is Actually Doing to the Ice

When a sweeper moves the brush head rapidly across the pebble surface in front of a moving stone, friction between the brush and those tiny peaks generates heat. Battelle, a scientific research and development organisation, has described the effect precisely: sweeping does not fully melt the ice surface. Instead, it raises the surface temperature toward the boundary between solid ice and liquid water, dropping the friction coefficient and fracture strength at those contact points enough that the stone’s behaviour changes.

Ice surface temperature at an Olympic competition sits between roughly minus 3.5°C and minus 2.5°C, based on instrumented peer-reviewed measurements. Sweeping nudges that surface temperature fractionally toward zero across the strip of ice the stone is about to cross.

The effect is real and quantifiable. In January 2026, Matt Dunstone of the World Curling Athlete Commission put a number to it: skilled sweeping with current equipment can extend a stone’s travel by 10 to 15 feet. Battelle’s independent research places the figure at approximately 10 feet. The range reflects different levels of technique and ice conditions rather than a disagreement about the underlying physics.

Distance, Direction, and the Skip’s Call

Extending travel distance is only one part of what sweeping controls. The degree to which the stone curls — the lateral arc it takes across the sheet — is also a variable the brush can change.

A 2022 study by physicist Jiro Murata at Rikkyo University in Tokyo, published in Scientific Reports, found that sweeping on the forward-right region of a stone’s path causes leftward curling, because it reduces friction specifically on the right side of the running band’s contact area. A 2024 paper in Sports Engineering (Springer) confirmed this asymmetric sweeping effect with 95% reliability across controlled tests.

In practical terms, the brush produces three distinct outcomes depending on how it is used:

• Full-path sweeping extends distance and reduces curl, keeping the stone on a straighter line
• One-sided sweeping reduces friction on that specific side, steering the stone toward the unswept side
• No sweeping allows friction to build back, slowing the stone and increasing curl

This is what the skip manages in real time from the far end of the sheet. “Hurry” or “sweep hard” means more distance and a straighter line are needed. “Off” or “whoa” means let friction return, bring the curl back, and let the stone slow.

Why Does a Curling Stone Curl?

Before a brush can steer a stone, the stone has to be curling. A player releases it with a slight clockwise or counterclockwise rotation, and on a pebbled surface, the stone curves gradually in the direction of that spin.

Why that happens is a question physicists have been working on for roughly a century. No fully established explanation exists.

The leading theories are:

• The scratch-groove model: the running band leaves microscopic grooves on pebble tops as it passes, and the rear of the stone follows those grooves, redirecting gradually across the full travel distance
• The asymmetric friction model: velocity differences between the leading and trailing edges of the rotating running band create uneven lateral friction
• The slow-side pivot model (Murata, Scientific Reports, 2022): the stone swings around slower-moving friction points across thousands of pebble contacts, accumulating lateral displacement over the full sheet

What every model agrees on: a clockwise spin curls the stone right. More rotation does not produce more curl. A stone with excessive spin may not curl at all. And a stone with almost no rotation behaves unpredictably — Murata compared it to a baseball knuckleball, where the absence of spin makes the ball’s path erratic rather than controlled.

The Techniques Used at the Elite Level

Two-time world champion Craig Savill, speaking to the World Curling Federation in early 2024, described the shift in competitive sweeping plainly: “now teams have actual sweeping training and know much more about what is the best sweeping angle, how to manage the stone along the sheet, and when to put the right amount of pressure on the brush.”

Three techniques dominate at the top level today.

Pressing. The sweeper pushes body weight down through the broom handle without running a full lateral stroke in front of the stone. Sweden’s Olympic champion Rasmus Wranää describes it as “a big energy saver” that moves the stone further and straighter with less physical output.

Knifing. The sweeper angles the brush head to use its thin edge, with the stated goal of adding curl. Switzerland’s six-time world champion Carole Howald has noted the results are inconsistent: “sometimes it does look like it works and sometimes it looks like it did not do as much.” A 2025 University of Saskatchewan study commissioned by the WCF added a complication — knifing generates more surface heat than standard brushing, even though its stated purpose is directional rather than speed-related. Why that additional heat does not simply reduce curl remains an open question.

Single-sweeper direction work. When a team wants to steer rather than extend, one sweeper brushes while the other stands off. The sweeper closest to the stone carries significantly more directional influence. Some teams, including Switzerland’s Team Schwaller, have taken this further by designating a single specialist sweeper for the majority of shots.

From Corn Brooms to Frankenbrooms to Foam Bans

Curling brushes were corn-straw household tools in competitive use through the mid-1950s. These gave way to synthetic push brooms with carbon fibre or fibreglass handles, which gave sweepers more control over downward force with less equipment weight.

The crisis point came in 2015. A set of brooms using stiff directional materials appeared at elite competitions. These Frankenbrooms, as they became known, left grooves in the pebbled ice that gave sweepers a level of stone control the sport had never experienced. Brad Gushue’s team in Canada filmed a practice session demonstrating how dramatically these brooms could redirect a stone mid-travel. The reaction from players and officials was swift. The World Curling Federation banned them in 2016 and mandated a single approved brush head material: smooth 0420 Denier 100% Nylon fabric.

That standard held until the 2024-2025 season, when component-style brush heads using foam inserts began raising the same concerns. In June 2025, the WCF updated equipment regulations for the Olympic season, removing the following configurations from approved play:

• BalancePlus RS with Firm 2.0 foam
• Goldline Impact with Evader foam
• Goldline Impact with Pursuer foam
• Hardline Ice Pad with Competitive foam

All traditional brushes with fabric stapled directly to the head remained approved.

The Rules Written Weeks Before Milano-Cortina 2026

The June 2025 equipment changes had direct scientific support. In August 2025, the WCF published findings from a University of Saskatchewan study conducted that spring at Nutana Curling Club in Saskatoon, led by Dr. Sean Maw, Dr. Glenn Paulley, and former world senior champion Eugene Hritzuk. Using infrared videography and visual photography, the team examined individual pebbles before and after brushing with different foam types and techniques. Rigid black foams were physically restructuring pebble tops during sweeping — flattening them rather than warming them. Softer white foams produced measurably less change.

In January 2026, the WCF went further with the Sweeping Technique Policy. Specific stroke motions used with the intent to slow a stone were banned, including single push-stroke sweeping and certain knifing applications. Game umpires received authority to remove stones from play following an official warning.

Dunstone explained what was behind the urgency: “If we’re able to drag a rock 10 to 15 feet as is… but we’re also able to begin to slow rocks down, we’re talking 15 to 18 feet of variance. That’s a lot.”

At the Games, the main controversy on the ice had nothing to do with brush technique. Sweden’s Oskar Eriksson accused Canada’s Marc Kennedy of touching the stone again after releasing it — a double touch, which under the rules results in the stone’s removal. Curling does not use video replay to overturn umpire decisions. The allegation ran through the tournament and has continued surfacing in the months since. Canada’s men, skipped by Brad Jacobs, won gold 9-6 over Great Britain. Sweden’s women, skipped by Anna Hasselborg, took gold 6-5 over Switzerland.

The WCF has stated publicly that the 2025-2026 equipment changes are an interim step, with longer-term materials testing underway with academics and industry specialists. How a brush shapes a curling stone’s path has occupied physicists, engineers, world champions, and governing bodies for decades — and none of them would claim the question is fully answered.

Sources: World Curling Federation (official news, equipment regulations, sweeping study publications), Scientific Reports — Nature (2020, 2022), Sports Engineering — Springer (2024), McGill University Office for Science and Society, University of Saskatchewan and WCF commissioned study (August 2025), PubMed / PMC, Battelle Research, NBC Olympics, CBC Sports, Grand Slam of Curling.