How is cork fabric made?
Cork naturally grows on cork oak trees and requires skill and knowledge to harvest it from the bark of the tree. The European Cork Federation has set up many rules and guidelines for cork extraction so that the best qualities of cork can be extracted.
The harvesting process of cork starts when the cork oak tree reaches 20 years of age. A skilled cork harvester uses a machine such as a machete or hatchet to slice the cork layer from the trunk of the tree. Special care is being taken so that the cut made is not too deep to damage the living layer of the trunk of the tree. Horizontal and vertical cuts are made to cut the bark in sections of the required size. In some trees of bigger size, the cork is also removed from the lower part of the branches. The trees are marked with numbers after the cork is harvested from them so that it will not be harvested again anytime before 10 years.
• Stabilizing or curing:
After the cork planks are extracted from the tree, they are stacked in piles in an outdoor area either in the forest or the factory for a duration of 6 months. The cork stacks are laid on some inert materials to prevent its direct contact with the soil. This stacking of cork should be done according to the specific rules and regulations made by ICCSMP – International Code of Cork Stopper Manufacturing Practices.
During the period of six months, the cork gets exposed to sun, wind and rain which aids in the chemical changes and helps to improve the cork quality. The plants of cork get stabilized, flattens out and lose 20% of the original moisture content.
After the stabilization period or curing period, the next step in the pre-processing phase is to treat the cork with heat and water. This process also has strict rules of ICCSMP, which need to be followed in order to get certifications for good quality of cork. Cork plants are boiled for around 1 hour in order to clean, remove dirt and extract water-soluble substances like tannin. This results in increasing the thickness and makes the cork more smooth, elastic and less dense. The boiling process of cork is done in large copper vessels which contains fungicide mixed in water. The boiling of cork is done in a controlled and closed environment to trap volatile substances. This is done by putting weights over the cork to keep it submerged in the boiling water.
After boiling the cork planks are removed from the vat. The poor quality outer layer of cork is scraped off from the surface using a hoe-shaped knife. This consists of only 2 percent of the whole plank volume but the weight is around 20 percent. After this under controlled temperature and humidity these planks are stacked, dried and cured for a period of 3 weeks. After this phase, the size of the cork material is stabilized and attains consistency.
After the selection phase, the cork planks are cut or trimmed into uniform strips of rectangular shape. After cutting these cork materials are sorted as per the different quality. Natural cork products such as cork stoppers for wine bottles are manufactured from fine or good quality cork. The cork with poor quality is powdered and then used to make agglomerated cork.
This step is, particularly for manufacturing bottle stoppers. The cork stripes are perforated with a drill either manually or semi-automatically to obtain the stoppers of the desired size and cylindrical shape.
The cork material is recyclable and biodegradable in nature hence the by-products left after the punching stage are used for cork granulate. This granulated cork material is used for manufacturing technical stoppers and other cork products used for insulations, coverings, flooring etc.
• Making textiles
For textile manufacturing, the cork planks after the slicing process are sliced into three layers. The inner and outer layers of cork planks are used for other products while the middle layer is used for making textiles. This is done because the densest and best quality cork is found in the middle of cork planks.
Again the middle layer is sliced into layers of 0.3 to 0.4 mm to obtain the thickness of textiles. Then the slices are glued together to form one block with the help of heat. The cork materials contain natural glue called “resin” hence only heat is required and no extra glue is required.
These thin slices are very weak hence they need a backing fabric to provide elasticity, strength, and stability. This backing can be made from cotton fabric, polyester fabric, polyamide or leather. As per the design the backing is attached like patchwork, like cork-board or in thin lines. The dyeing process of these layers can be done either before gluing them or even after gluing the backing layers together to create a variety in its appearance.
And the last step is applying a coating of non-toxic sealant to make the final fabric free from dirt. This step can be skipped because the cork fabric is naturally water- and dust-resistant.