What Is Cork and How Is It Produced?
Cork is an extremely versatile natural material that is made from the bark of the cork oak tree. It is composed of dead cells that accumulate on the tree’s outer surface. Cork has a unique set of properties not found in any other naturally existing material. It is lightweight, resistant to fire, rot and insects, impermeable to gas and liquid, soft, and highly buoyant because of its low density. Because of these unique properties, cork has been used in various ways since time immemorial.
Cork’s elasticity combined with its near-impermeability makes it suitable as a material for bottle stoppers, especially for wine bottles. Cork stoppers represent about 60% of all cork based production. Cork bottle stoppers have been found in Egyptian tombs dating back thousands of years.
The ancient Greeks used cork to make fishing net floats, sandals, and bottle stoppers. For centuries, Mediterranean cottages have been built with cork roofs and floors to keep out heat and cold and provide a soft walking surface. Worldwide, cork is used to make a variety of products ranging from life preservers and buoys to shoe insoles and cores for golf balls and baseballs.
Cork was examined microscopically by Robert Hooke, which led to his discovery and naming of the cell. Cork oak trees grow primarily in the Mediterranean coastal region. Cork is extracted only from early May to late August, when the cork can be separated from the tree without causing permanent damage. Cork is harvested at 9 to 10 year intervals, when the cork layer reaches a thickness of 2 to 5 cm.
A mature tree can yield up to 225 kg of cork. There are about 2,200,000 hectares of cork forest worldwide; 34% in Portugal and 27% in Spain. Annual production is about 200,000 tons; 49.6% from Portugal, 30.5% from Spain, 5.8% from Morocco, 4.9% from Algeria, 3.5% from Tunisia, 3.1% Italy, and 2.6% from France. Once the trees are about 25 years old the cork is traditionally stripped from the trunks every nine years, with the first two harvests generally producing lower quality cork. The trees live for about 300 years.
The workers who specialize in removing the cork are known as extractors. An extractor uses a very sharp axe to make two types of cuts on the tree: one horizontal cut around the plant, called a crown or necklace, at a height of about 2-3 times the circumference of the tree, and several vertical cuts called rulers or openings. This is the most delicate phase of the work because, even though cutting the cork requires significant force, the extractor must not damage the underlying phellogen or the tree will be harmed.
To free the cork from the tree, the extractor pushes the handle of the axe into the rulers. A good extractor needs to use a firm but precise touch in order to free a large amount of cork without damaging the product or tree. These freed portions of the cork are called planks. The planks are usually carried off by hand since cork forests are rarely accessible to vehicles. The cork is stacked in piles in the forest or in yards at a factory, and traditionally, left to dry, after which it can be loaded onto a truck and shipped to a processor.
The cork industry is generally regarded as environmentally friendly. Cork production is generally considered sustainable because the cork tree is not cut down to obtain cork; only the bark is stripped to harvest the cork. The tree continues to live and grow. The sustainability of production and the easy recycling of cork products and by-products are two of its most distinctive aspects. Cork Oak forests also prevent desertification and are a particular habitat in the Iberian Peninsula and the refuge of various endangered species.