The cork consists of dead, air-filled, thin-walled cells and contains cellulose and suberin. Cork is heat and sound insulating, the suberine gives it water-repellent properties. The cork layer is replicated by the cork-producing phellogen and can therefore be harvested repeatedly without damaging the tree too much. The first harvest usually takes place after about 25 years with a trunk diameter of (DBH), though new techniques (such as better irrigation systems) could shorten it to only 8 to 10 years. The first cork layer is called “male cork” or “virgin cork”, is still not very elastic and cracked and is only used for insulating mats. The second harvested cork (known as "secundeira"), has a more regular structure and is softer, but is still only used for insulation and in decorative objects. Only the following cork harvests deliver a higher quality cork, the "female cork", which can be used commercially in full. The best quality cork is obtained from the third and fourth harvest. Cork harvesting takes place every nine to twelve years when a layer thickness of is reached. Under favorable (warm) conditions, the harvest can take place every eight years, in North Africa every seven years. A cork oak can be harvested five to seventeen times in total. In order to minimize the damage to the trunk surface, harvesting can be carried out every three years, whereby only a third of the usable surface is removed. An important maintenance measure is pruning, which begins around the age of ten at a height of about . Some sources say an oak can provide around of cork over its lifespan, and one hectare around per year while others suggest a single tree can produce on average of cork per harvest, a comparatively higher value, as cork oaks can live more than 200 years in good conditions.