The cork tree is a unique species of oak whose bark is so thick that it can actually be stripped from the tree, incurring no damage during the process. Cork grows in many European, Asian and American countries, but not all types of cork and not all locations produce the kind of bark that can be used in quality cork stopper production. The most prolific forests are in Portugal, Spain, Northern Africa and southern France. Cork can be harvested every nine years, and only once the tree has reached a certain level of maturity; otherwise the cork is not dense enough to serve any value.
Cork bark is carefully stripped from the trunk in 10- 12" panels, hauled to the factory and then left to dry for at least six months. After the specified drying period, the cork strips are soaked in a very hot solution for over an hour, purifying and treating it to prevent any bacterial growth. This also increases the flexibility of the bark to facilitate cork production. The treated strips are air-dried, cut into the appropriate thickness, and then punched out into a rough shape. The corks are then pressurized, compacted and molded to exact specifications, separated based on quality, and placed in huge driers to bleach and further prevent bacterial growth. According to orders, the corks are then stamped with a winery's logo and shipped off.
The quality of cork can vary dramatically, as do wine corks, and winemakers order the quality of cork they desire for each product in their line. Of course there is competition among the world's greatest wineries for the best cork, and supply does not always meet demand. The recent worldwide increase in wine supply has exacerbated the problem of availability, tempting producers to turn to lower quality corks or substitutes like coagulated shavings to fulfill the growing demand. Not surprisingly, the incidence of cork taint appears to be rising, fueling the move to other options like synthetic corks and screwcaps. The debate about which type of closure is optimal rages on.