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the cork

As world major cork producer and transformer, Portugal has in this superior product “the jewel of the crown” of its natural resources.

But how is it formed? How is its growth processed and which are the properties that make this product so important?

The Oak (Quercus Súber) is the only forest kind in the world that produces cork with capability for industrial application: all the other kinds produce suberose tissue but with completely different characteristics.

 

Notwithstanding the plastic stoppers competition and the “taste” that sometimes cork seems to give to wine, the best wine producers bet in cork utilization because its impermeability and lightness properties give to its product the best guarantees of quality.

The first cork to be formed is designed as virgin cork. The strong tensions of radial grow give to this sort of cork a very irregular structure, used basically for decoration articles.

It’s the “amadia” cork, extracted from trees with an age usually near or above 40 years that constitutes the basis of industrial transformation.

This cork presents very specific characteristics, having a very high elasticity capacity, compressibility, imputrescibility, isolation capacity, low density and permeability.The period of decortication goes generally from July until September. By legal obligation, the decortication can only be done every 9 years and only when the cork presents a girth superior to 70 cm.

The cork is removed in boards of cork pieces with 1 meter height and about 40 cm in breadth, depending on the cork height and extent – trunk and branches – of the cork removal. This operation, because of the damage caused to the trees, needs special care in order not to injury seriously them, compromising the next cork production.

After being removed from the tree, the cork is placed in piles waiting in six months duration “breaks”, moment in which begins its technological transformation.

Concerning cork, the annual growth cycle comprises two stages: the vegetative activity stage, which goes from spring until the end of fall, and the stage of winter break, which extends throughout the months of November until February. The winter break can be more or less extensive, depending on the environmental conditions.

On the first stage we distinguish two periods: the spring period, with a more active growth, corresponding to 2/3 of the annual suberose growth, in the months of March, April, May and June and the estival autumnal period in which the intensity of growth is much lower and which extends to July, August, September and October.

To these periods of growth correspond two different types of cells, the spring cells, larger and with thinner walls, and the autumn cells, smaller and with larger wall thickness. The gradual increase of the walls thickness and the minimal dimensions of the cells produced in autumn origin different board colorings, which allow to delimitate successive annual layers, that correspond to the darker rings in that period.

We consider as a complete growth the result of the vegetative activity stage, comprehending tissues formed in spring and tissues formed in the summer and autumn. All the growth which doesn´t present cells from both types is considered an incomplete growth and has two distinctive periods, one that goes from the cork removal until the moment in which the vegetative activity ends and the tree enters in its period of winter break, and the other period, which goes from the vegetative activity beginning until the cork removal, at the end of the productive cycle. On the first period of incomplete growth, a thin layer of suberose tissue is produced, characterized by the exclusive presence of summer and autumn cells, whose thickness varies according to the growing intensity of the tree, the removal moment and the beginning of the “breaks” period.

To this suberose extract misses the part formed during spring, which continues adherent to the board, when the extraction. The age determination is made based on complete growths, corresponding to one year, and incomplete growths, corresponding to half a year.

In a way that one can tell the age of the cork, one must add one year to the number of annual complete layers, which represents the sum of two years. Sometimes, the temporary suspension of the activity from the generating basis is caused by the severity of drought (which forces a period of estival break) by the destruction of the leaf by parasites or fire, which forms then, a false autumn zone.

Vieira Natividade (1899-1968)

“Our wealth of cork is not a bottomless bag. It is urgent to protect effectively the properties planted with oaks, if we don´t want that this abundant wealth, which gives us such an important position in the world, to become one more glory of the past, one of the richnesses that we have had in our hands and have allowed to lose as incorrigible prodigals.”

Vieira Natividade

Agronomist engineer and silviculturist he made a distinction in the field of arboriculture and silviculture as a technician and researcher, and published throughout his career more than 300 works. He is considered to be a significant cork specialist.

Vieira Natividade (1899-1968)

“Our wealth of cork is not a bottomless bag. It is urgent to protect effectively the properties planted with oaks, if we don´t want that this abundant wealth, which gives us such an important position in the world, to become one more glory of the past, one of the richnesses that we have had in our hands and have allowed to lose as incorrigible prodigals.”

Vieira Natividade

Agronomist engineer and silviculturist he made a distinction in the field of arboriculture and silviculture as a technician and researcher, and published throughout his career more than 300 works. He is considered to be a significant cork specialist.

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