Corks

A cork stopper for a wine bottle

Champagne corks

Varnished cork tiles can be used for flooring, as a substitute forlinoleum or tiles.

Simple cork

Cork material is a prime-subset of generic cork tissue, harvested for commercial use primarily from the Cork Oak tree, Quercus suber, with Portugalproducing 50% of cork worldwide. Cork consists mostly of suberin.

Sources and harvesting

There are about 2,200,000 hectares of cork forest worldwide; 33% in Portugal, and 23% in Spain. Annual production is about 340,000 Tons; 52% from Portugal, 32% from Spain, 6% Italy.

Once the trees are about 25 years old the cork is stripped from the trunks every 9 years. The trees live for about 200 years. The first 2 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. However, only two cork companies, Oeneo Bouchage of France, and the Cork Supply Group of Portugal, have conducted and released their complete carbon footprint studies, and these studies are suspected to be self-serving (see below).

Cork oak forests comprise the majority of habitat for the Iberian lynx, one of the most endangered cat species in the world, Its decline has been a function of a loss of habitat due to development of housing as well as the introduction of a virus to control the rabbit population which was the lynx's primary prey. The lynx's survival is claimed by the cork industry and the world wildlife foundation to depend upon the continued viability of cork production.

However, since the cork forests will provide this habitat regardless of if the cork barks are harvested, the connection between cork production and and habitat for the lynx is only indirect, and assumes that the cork companies will deforest their land themselves if uses for cork decline. Many consider this issue to be an instance of greenwashing

Properties and uses

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's low density makes it a suitable material for fishing floats and buoys, as well as handles for fishing rods (as an alternative to neoprene).

Sheets of cork, often the by-product of more lucrative stopper production, are used to make floor tiles and bulletin boards.

Granules of cork can also be mixed into concrete. The composites made by mixing cork granules and cement have low thermal conductivity, low density and good energy absorption. Some of the property ranges of the composites are density (400–1500 kg/m³), compressive strength (1–26 MPa) and flexural strength (0.5–4.0 MPa).

Use for wine bottle closures

As late as the mid 1600s, French vintners did not use cork stoppers, using oil-soaked rags stuffed into the necks of bottles instead.

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.

Cork is an suitable material for use as a bottle stoppers. 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 it 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-1990's, a number of wine brands have switched to alternative wine closures such as synthetic plastic stoppers, screwcaps, or other closures. Screwcaps are often seen as a cheap alternative destined only for the low grade wines. These alternatives to real cork have their own properties, some advantageous and others controversial. For example, while screwtops are generally considered to offer a TCA trichloroanisole 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 (Amorim, Álvaro Coelho & Irmãos, Cork Supply Group and Oeneo) have developed methods that remove most, but not all 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.

A study made public in December 2008 by PricewaterhouseCoopers, commissioned by cork manufacturer Amorim, concluded that cork is the most environmentally responsible stopper, in a one-year life cycle analysis comparison with the plastic stoppers and aluminium screwcaps.

However, this study contains a number of assumptions that cause a bias towards cork, including:

- Taking credit for the positive impacts of the cork forrest. (Something that that a peer reviewer in the study called "abusive")

- Failing to take into account the environmental impact of foil capsules which are normally applied to the bottle when sealed with cork or synthetic cork. Because screwcaps do not require an additional capsule, this omission skews the report in favor of the cork and synthetic cork.

- Using low values for both the expected amount of recycled aluminum that would be used in manufacture of screwcaps, and the amount that would be recycled by the end user.

Other uses

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.

Cork can be used as bricks for the outer walls of houses, as in Portugal's pavilion at Expo 2000.

On November 28, 2007, the Portuguese national postal service CTT issued the world's first postage stamp made of cork.

Cork is used as the core of a baseball. Replacing the interior of a baseball bat with cork—a practice known as corking—was historically a method of cheating at baseball; the efficacy of the practice is now discredited.

Miscellaneous

The Cork Oak is unrelated to the "cork trees" (Phellodendron), which have corky bark but are not used for cork production.

Cork was examined microscopically by Robert Hooke and he discovered and named the cell.