The Assyrians, Egyptians and Greeks already knew the cork in early antiquity. In some cases, cork stoppers were also used to seal vessels such as amphorae. Mostly, however, stoppers made of terracotta (clay) were in use, which were fastened with string and then sealed with varnish, clay or pitch. The Roman author Cato the Elder (234-149 B.C.) wrote that wine jugs had to be sealed with cork and pitch after fermentation. The Romans already knew this type of closure, but it was forgotten again with the fall of the Roman Empire. This was probably due to the reason that the Iberian Peninsula as the main source of the cork bark was conquered by the Moors in the 8th century and dominated for a long time. Until the late Middle Ages, vessels were sealed with wooden plugs dipped in oil and wrapped in hemp, pitch or wax.
With the development of glass bottles, at first mainly glass stoppers were used, but at the beginning of the 17th century the cork was rediscovered. The monk and alleged inventor of champagne Dom Pérignon (1638-1715) also experimented with it. The corks initially had a conical shape due to different bottle neck sizes and were also only half sunk for easier removal. Only through the development of usable corkscrews did they acquire a cylindrical shape and were now driven fully into the bottle neck. From the middle of the 17th century onwards, the cork became the dominant type of closure for bottles, which led to a rapid boom in the cork industry.
Today, natural cork is mainly made from the thick, outer bark of the most suitable cork oak "Quercus suber". More than half of the world production comes from Portugal, other important countries are Spain, Algeria, Italy and Morocco. The trees grow to a height of ten metres and can be used for about 200 years. They can only be peeled for industrial use for the first time when they are 25 years old; for bottle corks, the bark is only suitable from the age of 45. Further times this is done at intervals of 9 to 12 years; a tree can be stripped of bark about 15 times. The bark is stored for at least one year, then boiled in water, pressed, cut into plates and sorted by quality.
Strips are cut out of it, from which the cylindrical stoppers are punched out. They are produced in a length of 38 to 60 mm; longer corks usually also mean a higher wine quality. The blanks are smoothed at the front faces and ground round. They are then bleached and impregnated with a wax-like substance to make the cork smooth. Finally, the cork is fired, today this is usually an imprint. When packing the corks, sulphur is often added for preservation.
A natural cork is an almost ideal and undisputedly aesthetically pleasing closure for wine and sparkling wine bottles. It is light, clean, relatively insensitive to different temperatures, is rarely attacked by rot, is impermeable to air when in healthy condition, is extremely elastic and has a long service life of usually 10 to 20, in exceptional cases 50 years and more. The cork cells are impermeable to gases and water due to the inclusion of suberin (waxy pulp). However, in every bark (bark) there are places with so-called lenticels (for gas and water exchange). For the quality of a cork, as few of these as possible are beneficial. A normal wine bottle cork has a diameter of 24 mm and is pressed together to a bottle neck diameter of 18 mm. However, even after years of bottle storage, the cork returns to its original size after only 24 hours after being pulled.
The air humidity must be high enough so that the cork does not release moisture to the environment (ideal is 75% at 10°C). However, like all organic materials, a cork loses its suppleness and thus its ability to close with age. It is therefore advisable to drink a bottle with a defective cork in time or to cork it again. A few wineries offer their customers the special service of a new corking for very old bottles of their best products. In case of extreme temperature fluctuations, the cork may leak; this is prevented by even temperature. Tartar crystals can be deposited on the cork, which can lead to a certain impairment in terms of tightness.
A question often expressed and discussed is whether the wine "breathes" through the cork during storage or whether and to what extent oxygen is required for the development of the wine during bottle ageing or for its shelf life. It is possible that the small amount in the neck of the bottle is sufficient, but today this space is usually filled with inert gas or a vacuum is created when the bottle is closed. Natural corks are up to a factor of three or four more permeable than other alternative closures such as conventional plastic corks or screw caps. In any case, recent research at the AWRI (Australian Wine Research Institute) has shown that a controlled supply in very small quantities, i.e. at a uniform OTR (Oxygen Transfer Rate), can have a very positive effect on the aroma and colour of red wines in particular. However, the required quantity depends on the grape variety, among other things. So this would speak for the natural cork, but in the meantime there are alternative closures with which the oxygen transfer rate can be individually controlled.
Pressed corks (also composite corks or agglomerate corks of agglomerate = agglomeration) are not cut out of the cork oak in one piece, but are granulated cork bound by resin or glue. These are considerably cheaper than natural corks. As a rule, a piece of two to three mm length of normal natural cork is applied to both sides so that the wine does not come into direct contact with the glue. Such types are called "1+1 corks" or "2-slice corks". Press corks are mostly used for wines that are to be enjoyed young. They are considered to be less storable because there is also a risk of crumbling.
For reasons of cost, a sparkling wine cork is often split in two. While the upper part (head) consists of pressed corks, two slices of natural cork are glued on the bottom, which are in direct contact with the sparkling wine. Due to the much higher pressure inside a sparkling wine bottle compared to a still wine, there are some special features to ensure the fixation of the cork. Sparkling wine corks are also cylindrical, but much thicker. The diameter of an uncorked sparkling wine cork is usually 30.5 mm and its length 48 mm. When inserted into the bottle neck, the cork is compressed to 19 mm, which results in a much stronger seal and the typical mushroom shape of the upper part protruding from the bottle neck. The cork is additionally fixed with an agraffe (wire mesh), whereby a small metal capsule (plaque) protects the cork from being cut by the wire.
In the case of champagne, the cork must bear the text "Champagne", and in the case of millésime (vintage champagne) the year must also appear on the cork. After removing the cork from the bottle, its shape also reveals something about the length of storage (as a rule, sparkling wine should be enjoyed as soon as possible). If it goes in width with the lower part (foot), it is called "juponne" (jupon = petticoat) and the bottle was probably corked less than a year before. If the foot is narrow, it is a "Cheville" (cone), which means corked a long time ago.
Today, due to the enormous demand, corks of increasingly poor quality (large-pored) are coming onto the market. Either they are leaking or the porosity leads to oxidative processes and in extreme cases to leakage of the wine. The unclean cork production can also cause the cork tasting. The closures that are therefore actually ideal would therefore be alternatives such as the screw cap. However, quite a few people mock it, miss the "plop" and complain about a loss of culture, but probably this development cannot be stopped for ecological and economic reasons. Furthermore, the often used pro-argument for the "plop" as a sensual pleasure is not valid, because a bottle should be opened noiselessly. Currently, almost 60% of wine bottles worldwide are closed with natural corks. See also under closures.