By Mark Jones

Last month, I got caught up in the Olympic mixed doubles curling. At the start of the competition, I think it is safe to say no one guessed a team would go undefeated on the way to a gold medal. The gold-medal winning Italians were young and from a country with very few curlers. It was one of the biggest surprises of the recent Olympics.

I wasn’t expecting sustainability to creep into the curling commentary, but it did, at least once. Play-by-play in curling is an interesting artform, with a lot of time to fill between shots that take mere seconds. Curlers are on-the-clock now, time-limited to ensure brisk play, yet curling remains a sport with more dead time than action. While watching one of the full, unedited games, sustainability crept into the commentary, in a discussion about the rocks.

Curling stones are granite — and not just any granite. As the commentators pointed out, Olympic curling stones are made with Ailsa Craig granite. Ailsa Craig is a volcanic island off the coast of Scotland notable for its unique granites. The Olympic stones are actually multiple pieces, with the bulk of the 20 kg stone being Ailsa Craig Common Green. The running surface is an insert made of higher quality, less abundant Ailsa Craig Blue Hone. Thanks to a grain structure and composition found only on Ailsa Craig, both are extremely durable, non-porous, and shatter-resistant. They are perfect for curling stones.

Ailsa Craig granite is a finite resource only occurring on a small island, a true dot on the map. Granite comes out of Ailsa Craig, is carved into a curling stone, used for curling and disposed of when wear renders it unplayable. There is loss during the carving process as chunks, chips, and powdered material is removed. When stones are no longer playable, they are down-cycled to other uses, most commonly as a unique decorations. There is no circular path to recycle stones at the end of their useful playing career back to playable stones. There is no way to reprocess the used stones or manufacturing scraps back into granite. Today’s curlers are robbing future generations of Ailsa Craig granite.

Andrew Kay & Co. Ltd., commonly called Kay’s Curling, owns the sole rights to harvest Ailsa Craig granite, and with it, a monopoly on curling stones used in competitions like the Olympics. I can’t find corporate sustainability goals for Kay’s Curling, which is not too surprising. ESG reporting is the realm of big companies.

Granite is an igneous rock, the solidified remains of volcanic activity. It is a mixture of crystalized minerals, where domains of the different minerals determine the appearance and physical properties. Quartz, feldspar, and mica are the three most common minerals in granite and separate domains of each creates the distinctive appearance of granite. The Ailsa Craig granites coveted for curling stones are microgranites, with especially fine grains. Finer grains are the result of faster cooling. While the elemental composition may not be unique, the combination of composition and morphology is quite unique.

Metals are the poster children for circularity. Recycling rates for iron, both in percentage and absolute amount, are the highest of any material. Aluminum, copper, gold, and platinum-group metals all put plastic recycling rates to shame. The metals have a huge advantage because they are recycled as the elements. Oxidation state doesn’t matter. Morphology doesn’t matter. Infinite recycling is possible. As a result, metals will still be here for future generations. They won’t be in the ground as the ores, but the useful metals will remain in the economy.

In materials where chemical composition and morphology matters, recycling is more difficult. The transformations done on the raw materials used to make curling stones and plastics are not easily undone. Recycling granite and recycling plastics share composition and morphology challenges. Curling stones and petrochemical plastics both begin in the ground, a finite geologic resource. Work done on those extracted resources wastes some of the material. Stone dust will never be granite again. Forming a precise heterogeneous mixture like granite is daunting. Transforming CO₂back into plastics is similarly challenging. The heterogenous nature of many plastics recycling streams creates challenges. Controlling polymer properties when composition varies is nearly impossible. Mechanical recycling of plastics will always face this hurdle. Getting back to monomers is one way to tackle the problem. Chemical recycling converts plastic waste back to feedstocks for plastic production. Once back at feedstocks, the controlled synthesis that allows production of composition and morphology can be done again.

Research into recycling granite is being done, though it is more appropriately labeled down-cycling. Granite wastes are not returned as high-quality granite in any of the papers I found; rather, the dust and scraps were being used as components of other materials. There is no like-for-like recycling. The case is very different for plastics recycling. Plastics R&D for both mechanical and chemical recycling is now coming out weekly. Efforts to do like-for-like recycling continue to grow, in addition to the down-cycling that characterizes much of past and active recycling efforts.

Curling stones won’t be part of the circular economy in the foreseeable future. Plastics, thanks to many ongoing efforts, are becoming more circular. Strides are being made in both mechanical and chemical recycling. It is a time of great promise.