Cork is peeled from living oaks that go on growing for centuries
Every wine cork began as bark on a cork oak that was never cut down. Harvesters strip the trunk by hand with a sharp axe, roughly once every nine years, and the tree keeps living, often for about 300 years. Portugal alone supplies around half of the world's annual crop.
The trade relies on patience. A tree must reach about 25 to 30 years old and roughly 60 centimetres around before its first stripping, and that initial virgin cork is poor quality, fit for flooring, shoes or insulation. Only later harvests yield the fine material used for wine and champagne stoppers. Work is limited to early May through late August, when the bark separates without lasting harm. Extractors cut a ring around the trunk and several vertical slits, then lever off planks with the axe handle, taking great care not to damage the living layer beneath, before carrying the planks out by hand.
The material's properties come from its cells, usually five or six sided and filled with air-like gas, so they act as tiny cushions that spring back after squeezing. Cork is roughly 40 percent suberin, a water-repelling substance, which makes it buoyant, nearly impermeable and fire retardant. It also has an almost zero Poisson's ratio: squeeze it lengthwise and its width barely changes. Robert Hooke's microscope studies of cork led him to discover and name the cell.
People have used cork for over 5,000 years, for sealing jars and fishing gear in ancient China, Egypt, Babylon and Persia, and for sandal soles in ancient Greece. As late as the mid-1600s French winemakers still plugged bottles with oily rags. Today about 300,000 tonnes are harvested each year, 49.6 percent from Portugal and 30.5 percent from Spain, and stoppers make up about 60 percent of all cork production.
Cork's reign over wine has been challenged. In 1972 more than half of Australia's bottled wine was spoiled by bad corks, fuelling a turn to aluminium screw caps. Screw caps avoid TCA, the chief cause of cork taint, but let almost no oxygen through, which some say hurts ageing. Industry-funded life-cycle studies argue cork wins on carbon, estimating 1.5 kilograms of CO2 per thousand stoppers against 37 for aluminium caps.
Source: Cork (material)
