Cork
Cork has attracted significant attention over the years, mainly for the wrong reasons. Whilst cork is quite efficient at keeping (most) of your wine in the bottle, it has a few problems.
Cork is a natural product. It is harvested by hand from an oak tree (Quercus suber) and production is centred in Portugal. After harvesting, the cork bark is air dried before being sorted. The cork is then boiled to remove impurities. After stabilisation, the cork is trimmed to length and punched to produce the basic shape. The corks are then polished to the final shape, washed/sterilized in hydrogen peroxide and dried. Amorin then sorts the corks, coats them in paraffin wax or silicone and packs them in bags filled with sulphur dioxide.
There are several grades of natural cork depending on the quality and length of the cork.
The most significant problem with cork is cork taint. Cork taint refers to changes to the aroma and flavour of a wine from the compound 2,4,6-trichloroanisole (TCA). TCA does not naturally occur in cork. It is thought to be the interaction between a fungi and environmental chemicals.
At its mildest, cork taint results in a flattening or dulling of the nose. The wine will lack some of its usual aromas, but no off characters will be evident. Mild taint may also dull the palate. At its worst, cork taint can make wines smell and taste of wet paper bags and render them undrinkable.
The incidence of cork taint is often quoted at around 5%, but in my view it is more like 10%. In a case of wine, I would expect six to be as expected, three to be even better and three to be different. Not all these wines will be bad, but all will not be as good as the next bottle. Wikipedia has a good review of cork taint.
Other issues with cork revolve around the fact that it is a natural product. Some corks will allow wine to leak, and for oxygen to enter the bottle (causing premature ageing). Others will oxidise with no evidence of leakage. Cork also deteriorates over time, loosing its elasticity and becoming crumbly. This makes the cork hard to remove and prone to breaking.
Alternatives to a natural cork include technical corks (such as Twin Tops, Procork and Diam), plastic "corks", screw tops, Vinlock and Zork.
See also Cork, ProCork, Diam, Screw Cap, Vinlock, Zork, Technical Corks, Synthetic Corks, Crown Seal