Quercus suber, commonly called the cork oak, is a medium-sized, evergreen oak tree in the section Quercus sect. Cerris. It is the primary source of cork for wine bottle stoppers and other uses, such as cork flooring and as the cores of cricket balls. It is native to southwest Europe and northwest Africa. In the Mediterranean basin the tree is an ancient species with fossil remnants dating back to the Tertiary period. The cork planks (from the third harvesting) are cooked in clean boiling water after the period of stabilization or rest. The boiling process lasts for at least one hour. The objectives of boiling are:
1. Clean the cork;
2. Extract water-soluble substances;
3. Increase its thickness, thereby reducing density;
4. Make the cork softer and more elastic.
Before boiling, the cork cells are compressed in an irregular fashion, but during this process, the gas contained in the cells expands. As a result, the structure of the cork becomes more regular and its volume increases by around 20 per cent.
Image of 1955, from Mundet of Seixal, onboard “fragatas” the traditional cargo boat of the time, the packs of cork from last year and ready to be transformed, are carried to Lisbon and then shipped worldwide, from the USA to Japan.