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Feb. 20, 2014

Getting to the Core of Olympic Curling Stones

by Julie Leibach

Click to enlarge images
Ten miles off the west coast of Scotland, the Firth of Clyde flows around a small island shaped like a sugar loaf called Ailsa Craig. In breadth, it’s shy of a mile but rises steeply to more than 1,100 feet, carpeted with grass and heather. An abandoned watchtower recalls a time when people once lived on the island, though today it’s completely uninhabited. Devoid of rats but full of rabbits, the outcrop is also a bird sanctuary where species such as gannets, guillemots, and puffins breed on rocky cliffs during the spring. The island is for sale. Its owner—the Marquess of Ailsa—is asking 1.5 million pounds, or just under $2.5 million. Whoever buys it will have the distinct honor of sharing a part of Winter Olympics history.
 
Ailsa Craig has been a source of granite used in every official Olympic curling event. Kay’s of Scotland, a family-owned curling stone business that traces its origins back to 1851, has exclusive access to the island’s quarries and makes the Olympic stones, according to Mark Callan, the company's field sales and service manager. For this winter’s games, Kay’s—which corners about 65 percent of the new curling stone market—crafted 80 stones and picked the best 66 to go to Sochi.
 
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Just why Ailsa Craig granite is preferable for Olympic curling stones—as well as stones used in different competitions—over other materials seems to be a story steeped in tradition, necessity, and geological happenstance.
 
About 61 million years ago, volcanoes started to spew as the supercontinent joining North America, Greenland, and Europe broke up, making way for the Atlantic Ocean. The magma deep beneath one of those volcanoes cooled and crystallized to create the granite that forms Ailsa Craig.
 
More precisely, Ailsa Craig granite is a microgranite, so-called for its fine-grained crystals, according to Phil Stone, an honorary research associate with the British Geological Survey who co-authored a 1987 study on the island. Among its predominant minerals—quartz and feldspar, which comprise every granite—Ailsa Craig granite also contains a few other rare minerals that bestow a speckled, blue-green appearance to an otherwise creamy-gray stone.
 
The degree of speckling relates to a distinction made between two “types” of Ailsa Craig granite, says Stone—“blue hone” and “common green.” In Olympic curling stones, the former comprises the running band—the only part that touches the ice—while the body of the stone is fashioned from common green. But “in terms of the chemical composition, in terms of the minerals present, in terms of the grain size, [Ailsa Craig microgranite is] all exactly the same, except that in some parts of the rock body there are more or less of these little clusters of blue-green minerals,” says Stone, and the rock types get their names depending on the proportion of those flecks. 
 
Speckling aside, where utility is concerned, at least part of Ailsa Craig granite’s appeal lies in its resilience. Curlers need a stone that withstands impact without chipping or fracturing, and the island rock happens to be excellent at taking whacks. “The stone is very densely constructed; the crystals fit together very well internally," says Godfrey Fitton, professor of igneous petrology at the University of Edinburgh, who has studied Ailsa Craig granite under the microscope. “The feldspars are elongate, and they form a kind of mesh with the quartz crystals in between.” This interlocking construction gives Ailsa Craig granite strength, he says. “You can hit it with a hammer, and it's a bit of a devil to break it.”
 
“It’s very, very uniform,” adds Stone. “There are no planes of weakness within the rock, there are no little micro fractures, there are no little veins cutting through it,” whereas most rocks have these kinds of imperfections, he says, and are prone to fracturing upon a hard impact.
 
Another property that might also make Ailsa Craig granite desirable for curling stones is its well-known sliding behavior. Curling stone running bands aren’t completely smooth—ice technicians or craftsmen scratch them with little grooves, according to Staffan Jacobson, a materials scientist and one of the authors of a recent study on the mechanism behind why curling stones curl, appearing in the journal Wear. The nature of the roughness that results from scratching affects the stone's friction against the ice and the degree it curls, he says. The same scoring technique used on another stone might result in a roughness of a different character—and the desired friction properties might not be achieved, suggests Jacobson. “It’s a good material, and it has tradition,” he says.
 
Ailsa Craig might have also been appealing as a source of granite for practical reasons, says Stone—specifically, ease of quarrying. Millions of years ago, as the granite cooled and hardened underground, it was under considerable pressure. Erosion relieved that pressure, and as the rock relaxed, cracks called joints formed along long planes. The result is a series of columnar rock faces that, coincidentally, break off into lumps roughly the size of curling stones. “Perhaps the industry started way back when simply because it was a reasonable type of rock to make a curling stone,” suggests Stone. In the mid-20th century, Kay’s quarried the rock with the aid of dynamite, but now the company collects leftovers from the blasts, as well as other stones that have naturally cleaved and tumbled down the island, according to Callan. (The company harvests about once every 10 years, and after the seabird breeding season.)
 
Ailsa Craig granite isn’t the only game on the market, however. Canada Curling Stone Co., another manufacturer, uses a Welsh granite—referred to as “blue trefor”—for the body of the stone (though it uses Ailsa Craig granite for the running band, repurposed from older stones), according to Kimberly Tuck, who works for the company and is a curler. Kay’s of Scotland has also used blue trefor in the past for the body of its Olympic curling stones, but supply problems, in part, led to a switch to the Ailsa Craig rock, according to Callan. Meanwhile, “there are several other Scottish rock types which are reputedly well-suited for curling stones,” says Stone. One is Essexite, another igneous rock that, while different compositionally, shares similar physical properties to Ailsa Craig granite in that it’s very hard and polishes well, according to Stone.
 
Still, there’s something aesthetically pleasing about a stone made entirely of Ailsa Craig granite. “From a little chunk of rock coming in, to see this beautiful polished thing coming out, ready to be played at something like the Winter Olympics is really quite impressive,” says Callan. “It’s quite a process.”
 
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About Julie Leibach

Julie is the managing editor of ScienceFriday.com. She is a huge fan of sleep and chocolate. Follow her @julieleibach.

The views expressed are those of the author and are not necessarily those of Science Friday.

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