Cork and cork products


Cork is an impermeable, buoyant material, a prime-subset of bark tissue that is harvested for commercial use primarily from Quercus suber (the Cork Oak), which is endemic to southwest Europe and northwest Africa. Cork is composed of suberin, a hydrophobic substance, and because of its impermeability, buoyancy, elasticity, and fire resistance, it is used in a variety of products, the most common of which is for wine stoppers. The montado landscape of Portugal produces approximately 50% of cork harvested annually worldwide.

There are about 2,200,000 hectares of cork forest worldwide; 32.4% in Portugal, and 22.2% in Spain. Annual production is about 300,000 tons; 61.3% from Portugal, 29.5% from Spain, 5.5% Italy.

Once the trees are about 25 years old the cork is traditionally stripped from the trunks every nine years. The trees live for about 200 years. The first two harvests produce poorer quality cork.

The cork industry is generally regarded as environmentally friendly. 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.
Cork is extracted only from early May to late August, when the cork can be separated from the tree without causing permanent damage. When the tree reaches 25-30 years of age and about 60cm in circumference, the cork can be removed for the first time. However, this first harvest almost always produces poor quality or “male” cork. Subsequent extractions usually occur at intervals of 9 years, though it can take up to 13 for the cork to reach an acceptable size. If the product is of high quality it is known as “gentle” cork, and, ideally, is used to make stoppers for wine and champagne bottles.

The workers who specialize in removing the cork are known as extractors. Extractors use 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 quite a bit of strength, the extractor must not damage the underlying phellogen or the tree will die.

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 usually have to be carried off by hand since cork forests are rarely accessible for vehicles. Finally, the cork is stacked and traditionally left to dry, from where it can be loaded onto a truck and shipped to a processor.


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.

As late as the mid-17th century, French vintners did not use cork stoppers, using oil-soaked rags stuffed into the necks of bottles instead.

Wine corks can be made of either a single piece of cork, or composed of particles, as in champagne corks; corks made of granular particles are called “technical corks”.

Natural cork closures are used for about 80% of the 20 billion bottles of wine produced each year. After a decline in use as wine-stoppers due to the increase in the use of cheaper synthetic alternatives, cork wine-stoppers are making a comeback and currently represent approximately 60% of wine-stoppers today. High-speed air-gap flash image of a champagne bottle being uncorked.

Because of the cellular structure of cork, it is easily compressed upon insertion into a bottle and will expand to form a tight seal. The interior diameter of the neck of glass bottles tends to be inconsistent, making this ability to seal through variable contraction and expansion an important attribute. However, unavoidable natural flaws, channels, and cracks in the bark make the cork itself highly inconsistent. In a 2005 closure study 45% of corks showed gas leakage during pressure testing both from the sides of the cork as well as through the cork body itself.

Since the mid-1990s, a number of wine brands have switched to alternative wine closures such as synthetic plastic stoppers, screw caps, or other closures. In some countries, screw caps are often seen as a cheap alternative destined only for the low grade wines; however, in Australia, for example, the majority of non-sparkling wine production now uses these caps as a cork alternative. These alternatives to real cork have their own properties, some advantageous and others controversial. For example, while screwtops are generally considered to offer a trichloroanisole (TCA) free seal they reduce the oxygen transfer rate to almost zero, which can lead to reductive qualities in the wine. TCA is one of the primary causes of cork taint in wine. However, in recent years major cork producers) have developed methods that remove most TCA from natural wine corks. Natural cork stoppers are important because they allow oxygen to interact with wine for proper aging, and are best suited for bold red wines purchased with the intent to age.

The study “Analysis of the life cycle of Cork, Aluminum and Plastic Wine Closures,” commissioned by cork manufacturer Amorim and made public in December 2008, concluded that cork is the most environmentally responsible stopper, in a one-year life cycle analysis comparison with the plastic stoppers and aluminum screw caps.

Cork is used in musical instruments, particularly woodwind instruments, where it is used to fasten together segments of the instrument, making the seams airtight. Conducting baton handles are also often made out of cork. It is also used in shoes, for acoustic and thermal insulation in house walls, floors, ceilings and facades, floor and wall tiles, fishing floats and buoys (as an alternative to neoprene), in spacecraft heat shields, inside footwear to improve climate control and comfort.

Shipment / Storage / Risk factors

Cork and cork products are shipped from Portugal, Spain and certain areas of Northern America in a number of different forms. From a shipping insurance point of view these need to be considered separately.

Raw Cork (Bark or waste)
For grinding or further processing in the destination country, is shipped in compressed bales wrapped in hessian or polypropylene and banded or wired. These bales should be shipped with a moisture content not exceeding 14%, which is the normal basis on which they are purchased, but frequently have a higher moisture content due to rain at the point of origin or during transportation to the ship. Are susceptible to further absorption of moisture in transit and should never be shipped as deck cargo. If severely wetted by sea water, even after drying, residual salt is deleterious to the end products.

Cork Grain
Shipped in bales as above and also sometimes in sacks. Under temperature conditions cork grain reaches equilibrium at between 5% and 7%, although 6% is normally specified in purchase contracts as a maximum. It is, however, very susceptible to the effect of humid conditions and will quickly absorb moisture from the atmosphere. Grain wetted by sea water is unusable, even after drying in the manufacture of composition cork, for which the majority of it is used, because of the effect of the residual salt.

Composition Cork and Insulation Corkboard
Composition Cork is shipped in the form of blocks of several inches thick and of varying overall dimensions, from 600 mm x 900 mm to 1,000 mm x 1.500 mm, and also in the form of sheets of thicknesses varying upwards from 0,8 mm in the same overall sizes as the blocks. Blocks and sheets are normally either packed in cartons and strapped on to pallets or loaded directly on to a pallet with an overall protective covering of cardboard and/or waterproof paper. Insulation Corkboard is shipped almost always in cartons, but sometimes just shrink-wrapped in packs on pallets. Despite special protection sometimes provided, corners can become damaged in transit due to mishandling. The consignment does not usually suffer a total loss on account of this as part-sheets can be used, but there are occasions when only full sheets will suffice. Excess atmosphere humidity can be absorbed by sheets and boards which might involve drying at destination and this can cause buckling, which may render them unusable. Humidity and heat can also cause mould growth and, if this occurs, the cork must be burnt quickly before the infection spreads. Sea water wetting would render sheets, blocks and boards unusable in most cases.

Cork Discs
Discs damaged by water are valueless and these corks could not be used for any other purpose, and definitely not in the food trade because of the possibility of contamination, which would normally contravene any local Food and Drug Act.

Cork shavings
Shipped in bags or bales. When shipped in bales may be subject to loss in weight due to drying out. Loss may result from chafage and seepage from bags.

Cork becomes brittle when cold. Due to the variety of uses to which the end product is put, it should not be stowed in the same compartment or container as highly odorous cargoes. Dry cargo. Do not permit slinging by the bands. Cork board in cartons stows at 9.75 m3/tonne. Cork blocks, Cork discs and cork (agglomerated), all in cartons, stow at 4.18 m3/tonne.