Curling Stone

Curling is a sport where two teams take turns sliding circular Granite stones toward a target on a sheet of ice while the other team members sweep the ice in front of the stone to warm the ice and reduce the friction. This sport is named Curling after the unique turning at the end of the stone's path on the ice that requires a lot of strategy and technique; therefore it is also regarded as "Chess on Ice."

Every year when the Winter Olympics start, the one sport that mesmerizes every other person in the audience is none other than Curling. This is so because the movement of the Stone is very unpredictable.

So, to know how the Curling Stone curls, we need to understand the mechanism, factors and learn more about the critical piece of Equipment.

How are Curling Stones made?

The Curling Stone is the most surprising equipment of the sport:

1. It is made of dense polished Granite quarried from Ailsa Craig, an island off Scotland.


2. It weighs between 17.24 to 19.96 kg (38-44 lbs.)


3. Its minimum height is 4.5 inches (11.43 cms.) and a maximum circumference of 36 inches (91.44 cms.).


4. The Curling Stone contains a handle attached to the bolt via a hole present in the center of the stone. The handle aids in suitably gripping and in rotating it on the ice sheet.


5. In addition, the bottom of the stone is concave so that the outer ring, also known as the Running Band, is in contact with the ice. This not only minimizes the friction but also allows the stone to slide farther.


6. The dull grey band present around the greatest circumference of the stone is the Striking Band that is designed to absorb shock when one stone strikes the other.


7. Usually, the makers use red and yellow to differentiate the stones for Olympics.             

History and Evolution of Curling Stones:

Curling is an ancient sport that originated in the 16th century in Scotland. The first official record of Curling Stones was found in 1511 in the Scottish regions of Stirling and Perth. Later in the 1600s, stones with handles were introduced for a smoother playing experience. In 1838, the Grand Caledonian Curling Club drew up the sport's first official rules.

Modern Advancements in Curling Stones:

Nowadays, Electronic Handles are fitted in the stone for quickly detecting hog line violations as a modern advancement.  The Electronic Handles on the stones are known as the "eye on the hog."  It electronically detects any false start by the players and eliminates the risk of any human error.

The Modern Stones have a total circumference of 91.44cm (36 inches) and a height of 11.43cm (4.5 inches), as specified by the World Curling Federation (“WCF”). This advanced technology has made the stone more feasible with a uniform surface.

Where does the Curling Stone for the Olympics come from?

The two major manufacturers of Curling Stones are:

• Kays of Scotland located in Mauchline, Ayrshire, that uses Granite from Ailsa Craig Island off Scotland. They have been manufacturing Curling Stones since 1851.  They have been exclusive providers for Curling Stones to the Olympics since 2006.


• Canada Curling Stone Company that has the exclusive rights to use the Trefor Granite found in Wales, United Kingdom since 1992 and has supplied stones for the 2002 Winter Olympics only.

Moreover, Curling Stones are traditionally made of Granite that is readily available in Italy, USA, India, and China. But, for the Olympics, nothing can replace the unique properties of the Granite from the island of Ailsa Craig off of Scotland.  Ailsa Craig Granite is the most demanding and purest form of Granite found in the world and it has fixed shape despite the low temperature of the Curling surface.

The two types of Ailsa Craig Granites used are:

• The typical Green Ailsa Craig Granite is used to create the body of the Olympic Curling Stone.


• Whereas the Blue hone Granite is another type of Ailsa Craig Granite that is fitted to the body of the Curling Stone to form the Running Band.

Mechanism of Curling Stone:

As we know, the tendency of rotating and moving objects to travel along a curved trajectory is essential in many sports.  In Curling, the main objective is to get the team’s stones into the Bull’s eye (‘House’ at the end of the ice sheet).  However, the game-changer in the Curling sport is the Curling Stone itself.

Let’s understand the mechanism with an example:

Suppose we take a circular object (let it be a bowl) and turn it down. Now, spin the bowl in an anti-clockwise direction and slide it along a surface. We observe:

The bowl curled to its ‘Right’ rather than Curling in the direction of the rotation, which happened because the bowl tips over while moving.  This puts more pressure (Pressure= Force/Area) in the front than the back leading to asymmetric friction that ultimately causes the curl. But, a Curling Stone is an exception to the above phenomenon.

Moreover, the essential characteristic of the game is a curled trajectory of the rotating Curling Stone, and it is well-known among the players that:

• If a small clockwise rotational velocity is imposed on the stone when released, the stone will curl to the right.


• Meanwhile, when a small anti-clockwise rotational velocity is imposed on the stone, it curls to the left.

Because greater the amount of rotational velocity at the time of release, lesser the stone will curl. This type of effect is implemented in the game to reach the target spots and let the stone travel farther.

Factors influencing the Curling of the stone:

Now the question arises - "Why is Curling Stone an exception to the above mechanical phenomenon"?

The Curling Stone is an exception to the theory mentioned above because of the ice prepared for Curling and the environmental factors.

How is the Ice prepared?

The preparation of ice takes place in two steps:

Step 1 - Pebbling:  In this process, deionized water is sprinkled on to the ice sheet with the help of a sprinkler nozzle and is allowed to freeze.  The deionized water is prepared by Reverse Osmosis (It is a water purification method through which unwanted ions/molecules are removed from water by passing it through a partially permeable membrane).  These water droplets freeze and form bumps known as pebbles, which reduce the friction and help the stones to travel farther.

Step 2 - Nipping:  After pebbling, the top of the pebbles is nipped using a metal blade to create a smooth, uniform running surface.

When a Curling Stone passes over the pebbles, the weight of the stone gets distributed equally among the nipped top of the pebbles. This results in increased pressure at the Curling Stone front-end as it is released and leads to melting of the ice. The melted ice acts as a lubricant to the surface and helps in reducing the frictional force that finally lets the Curling Stone curl in the direction of its rotation (Since Pressure is inversely proportional to the Melting point).  Moreover, other than this, the players use the Curling Broom in clearing the ice bumps on the surface by sweeping according to how much they need to redirect the stone.

Environmental Factors- Temperature and Humidity

The temperature and humidity play a vital role in Curling.

The surface of the ice sheet needs to be kept at a constant  5 degrees Celsius (23 degrees Fahrenheit) temperature. Therefore, the humidity of the atmosphere in the rink is kept low to prevent condensation of water on the ice, which, when increased, can adversely affect the performance.

The temperature of the Running Band is kept the same as the ice to ensure fair play because if the Running Band is warmer than the ice surface, then there would be a gradual increase in the molecular forces between the water molecules, increasing the friction with the ice.