With a share of about 50% Portugal is the largest producer of cork, grown and harvested either in natural forests in the Serra or in plantations. Cork production is generally considered sustainable because the cork tree is not cut down to obtain cork, only the bark is stripped to harvest the cork.
Cork is a one hundred percent natural, recyclable and reusable material with unique and incomparable qualities.:
- very light
- impermeable to liquids and gases
- elastic and compressible
- an excellent thermal and acoustic insulator
- fire retardant
- highly abrasion resistant
Until the beginning of the last Century, the Algarve was the centre of the Portuguese cork industry. Over hundred cork factories processed the cork harvested in the forests of the Serra and contributed significantly to the wealth of communities such as São Brás de Alportel and Silves. Nowadays the cork plantations in northern Portugal and the increasing process of automation have moved the industrial centre to Greater Lisbon. Nevertheless, the cork oaks growing in the natural environment of the Serra de Caldeirão are producing cork of the highest quality, and a handful of traditional cork manufactories are still operational.
Cork oak, (quercus suber L), a noble tree with very special characteristics that grows in Mediterranean regions such as Spain, Italy, France, Morocco, Algeria and, most particularly, in Portugal, where there are more than 720 thousand hectares of cork forests, as well as a cork industry of considerable economic importance.
It is an astonishing tree, very long-lived and with an enormous capacity for regeneration. It can live on average 150 to 200 years, despite its bark being stripped around 16 times during its lifetime, at nine-year intervals. In the untreated forrests of the Serra de Caldeirão and the Serra de Monchique, cork oaks find excellent natural conditions.
A traditional process performed by specialists, requiring manual skill and experience
The life cycle of cork as a raw material starts with the extraction of the bark from cork oaks, the so-called harvesting or stripping which is carried out during the most active stage in the annual growth of the cork, from mid-May or early June to the end of August. It takes 25 years for a cork oak trunk to start to produce cork - it has to reach a circumference of 70 cm when measured at 1.5 metres from the ground.
The first stripping, which is known as "desbóia", produces cork of a very irregular structure which is too hard to be easily handled. This is the so-called virgin cork which will be used for applications other than cork stoppers (flooring, insulation etc.), since its quality is far from that necessary to manufacture stoppers. Nine years later, the second harvest produces material with a regular structure, less hard, but still not suitable for cork stoppers - this is known as secondary cork.
It is from the third and subsequent harvests that the cork is suitable for the production of quality corks, since its structure is regular with a smooth outside and inside. This is the so-called "amadia" or reproduction cork. From then on, the cork oak will supply good quality cork every nine years for around a century and a half, producing, on average, 15 to 16 bark harvests throughout its life.
The stripping of the cork oak is a traditional process that can only be done by specialists, the debarkers, since much manual skill and experience is required in order not to harm the tree. The stripping process consists of five steps:
- Opening: A vertical cut is made in the cork, choosing the deepest crack in the cork bark
- Separating: The plank is then prised off the tree, by inserting the edge of the axe between the strip and the inner bark.
- Dividing: A horizontal cut defines the size of the cork plank to be removed and what is to remain on the tree.
- Extracting: The plank is removed from the tree with care so that it does not split. The larger the planks extracted, the greater their commercial value.
- Removing: After the stripping of the planks, some fragments of cork remain attached at the base of the trunk. To remove any parasites in these "wedges", the debarker gives them a few taps with his axe.
Finally, the tree is marked, using the last number of the year in which the extraction took place.
Today you can find only a handful of traditional cork factories in the Algarve
After the harvest, the cork planks are stacked in piles either in the forest or in yards at a factory. There they remain exposed to sun, wind and rain. During this seasoning period of not less than 6 months, the raw material matures and the cork stabilises.
After the seasoning, the planks are cooked in clean boiling water. They should be boiled for at least one hour. The objectives of the boiling are:
- to clean the cork,
- to extract water-soluble substances,
- to increase thickness and thus reduce density,
- to improve flexibility and elasticity.
Before boiling, the cork cells are collapsed and wrinkled, but during this process the gas in the cells expands. As a result, the structure of the cork becomes more regular and its volume increases by around 20 per cent. The boiling is an operation prescribed by the International Code of Cork Stopper Manufacturing Practice. An operation which, besides improving the internal structure of the cork, also ensures that the microflora is significantly reduced. Several cork stopper manufacturers use complementary procedures to achieve improved disinfection. Some have introduced, for example, controlled boiling processed in a closed environment.
After the boiling, a period of stabilisation of the cork takes place for two to three weeks. Stabilisation serves to flatten the planks and to allow them to rest, in order to reach the necessary consistency for their transformation into cork stoppers. Only after this period the planks are manually selected.
Only cork of high quality can be used to produce cork stoppers for wine and champagne bottles
The manufacturing of cork stoppers is strongly regulated by the International Code of Cork Stopper Manufacturing Practice:
- Slicing: After the stabilisation period, the cork planks are cut into strips slightly wider than the length of the cork stoppers to be produced.
- Punching. Punching is the name of the manual or semi-automatic process of perforating the strips of cork with a drill. A cylindrical stopper is thus obtained within the dimensional limits prescribed.
- Rectification. After the punching, rectification is designed to obtain previously specified final dimensions and to regularise the surface of the stopper.
- Selection is the operation designed to separate the finished stoppers into different grades, which are determined by automatic scanning of their surface. In some cases, the selection is done by visual inspection. During this stage, besides the definition of grades, defective stoppers are also eliminated.
- Washing. After rectification, the stoppers are washed using either hydrogen peroxide or paracetic acid. This cleans and disinfects the stoppers, but there are other methods also used such as microwaves or ozone.
After washing/disinfection, the moisture level is stabilised, thus maximising stopper performance as a seal and simultaneously reducing microbiological contamination.
Cork stoppers of lower quality may also be collimated to seal the surface pores of stoppers (lenticels) with a mixture of cork powder to improve their presentation.
Printing or Branding is carried out according to customers' specifications as to the type of marking to be applied. The methods used are ink printing (with food quality ink) or traditional heat branding. After branding, they are given a paraffin or silicon surface treatment to make them easier to insert and extract from the bottles.
Once their production is finished, the stoppers are packed in plastic bags with SO2 (sulphur dioxide), a gas that blocks microbiological proliferation. Only then will they be delivered to wine or spirits bottlers.