The density of the cork is associated with the elasticity of the stopper. Cork for natural corks have a density between 160 kg/m3 and 220 kg/m3. Cork stoppers can, however, be found with lower or higher densities, outside of that range.
The humidity in cork stoppers should be between 4% and 9% in order to maintain the suitable elasticity and reduce the possible risk of microbial development.
There are different products used in the surface treatment of stoppers, with paraffin and silicon most used. Treatments with paraffin are used for sealing and also provide some lubrication. Treatments with silicon are principally for lubrication of the stopper, which facilitates use at the time of bottling and when opening the bottle. There are also other options available on the market which use lubricating and sealant polymers compatible with the foods industry.
The type of treatment applied and dosage depend upon the type of wine, the type of bottle, the maturation period and the bottling machine. For wines that need to be aged in the bottle (more than 18 months), a paraffin surface treatment must be done first followed by silicone treatment. Whatever the treatment used, it must be of the highest quality, as there is no use in having an aesthetically attractive stopper with deficient finishing that may harm the final performance of the stopper.
The force of extraction required for the stopper tends to lessen with time in the bottle. The recommended values are between 20 and 40 kg (24 hours after bottling). The specifications vary according to the market. Nowadays, producers of cork stoppers have the means necessary for the development of surface treatments that meet requested specifications.
The visual class of the stoppers is established on the basis of the quantity and size of holes (lenticels) the surface has.
The size of the batches should always be taken into account when sampling and the standardized sampling tables – NP (Portuguese Standard), NP 2922 or ISO 3951 or ISO 2859, should be followed depending on the applicable mode of quality control.