Did you know that cork comes from the outer bark of the Cork Oak (Quercus suber L.), which is a small to medium size tree found in the Mediterranean area? It is estimated that the Cork Oak grows in about 5 million acres of forest, 2/3 of which are in Portugal and it is Portugal that produces about 70% of the world's supply. Other important producers include Algeria, Spain, Morocco, Tunisia, France, and Italy.
The cork cambium forms a continuous layer over the entire trunk and produces a great number of cork cells towards the outside each year. The first time a tree is "stripped," carefully done with long-handled axes or crescent-shaped saws, "virgin" cork is removed. Portuguese law requires trees to be at least 5" in diameter, when they're usually about 20 years old. "Virgin" cork is coarser and of lower quality than cork of subsequent peelings.
Strippings are usually done on a nine-year rotation and even the bark of large branches can yield thin but finely textured cork. Usually though, the bark is harvested by two cuts around the tree low to the ground and below the main branches. Thickness of the cork varies from 1/2" - 3" and the average productive age of a tree is 100 years.
Depending on their sites, trees yield anywhere from 40 - 200 lbs., although some have produced up to 500 lbs. Harvest occurs between May and August but is sometimes postponed during days when the hot "sirocco" winds blow, which can excessively dry out the newly exposed cork cambium.
After stripping, the bark is left in the woods to dry for a few days and then is boiled to remove tannins and make it possible to flatten and bundle. Later it is shipped to central stations, where it is sorted and graded. There are over 25 grades of cork depending on porosity and elasticity. Specs for the finer grades and certain industrial uses are so exacting that a grader must be a specialist with years of experience.