There are several kinds of cork stoppers that are commonly manufactured, ranging from premium natural cork to agglomerated cork and mixtures of the two. Different usages can be better suited to particular kinds of corks and the pricing can be dramatically different so it is worth understanding what your options are. Here is a brief summary of the kinds of cork stoppers that are available.
Natural Cork Closures
Natural cork closures are made from punching through a piece of cork strip, like a biscuit cutter through dough. The natural structure of cork is perfect for ageing wine as it allows just the right amount of oxygen to come in contact with the wine to allow it to mature. Many wines are not intended to mature in the bottle of course, so only those higher quality wines are suited to using natural corks.
Cork’s natural compressibility and elasticity allows it to adapt to the internal irregularities of the bottleneck, guaranteeing a perfect closure during storage, even when expansion or contraction of the glass occurs due to temperature variation. A natural cork will continue to perform its function for decades as long as it is of high quality and temperature and humidity levels are controlled; furthermore a cork stopper should be 6mm longer than the bottleneck and should not be compressed more than 33% by diameter when being inserted into the bottle.
Natural cork closures are categorized according to their quality as follows:
Flor, Extra, Superior, 1st, 2nd, 3rd, 4th and 5th. This categorization is made by various tests on the corks, including visual inspection of each cork and prices will vary dramatically according to category.
Multi-Piece Natural Cork Stoppers
The multi-piece natural cork stopper is manufactured made from two or more pieces of natural cork which are glued together using an approved food contact glue (FDA). These corks are made from thinner strips of cork that are too thin to be used for natural on piece cork stoppers, but may still be from good quality cork. Multi-piece cork stoppers may also be used for oversized bottles that require corks too large to be made from one piece corks.
Colmated Natural Cork Stoppers
Colmated corks are natural cork stoppers that have had their pores (lenticels) sealed with cork dust, in order to rectify irregularities in the natural cork structure which might otherwise lead to an imperfect seal and also make the cork look less attractive. The cork dust is adhered to the cork using FDA grade natural resin, rubber glues or water-based glues.
Colmated cork stopper will always have a homogenous visual appearance and consistent mechanical characteristics, as natural variations in the natural cork are compensated for by the cork dust.
Colmated corks are generally categorized as A, B or C (or sometimes I, II or III) according to the level of colmation used.
Champagne and Sparkling Wine Cork Closures
Champagne/sparkling wine cork closures are a form of technical cork closures as they integrate an agglomerated cork granule body to which natural cork disks are glued on the end.
These corks have a larger diameter than for normal wine corks as they need to retain the high internal pressures generated by gasified wine and they are generally categorized as Extra, Superior, 1st and 2nd .
Technical Cork Stoppers
Technical corks are used for wines that are destined to be consumed within a period of two to three years. They consist of a dense agglomerated cork body with natural cork disks glued on one or both ends, to provide a closure that is chemically very stable and mechanically very strong.
Technical corks with one disk glued on both ends are known as 1+1 technical corks, with two natural cork disks, 2+2 technical corks and with two disks on one end, 2+0 technical corks. An approved food contact glue (FDA) is used to attach the disks to the agglomerated cork body.
Technical corks are generally classified as A, B or C quality.
Agglomerated Cork Stoppers
Agglomerated cork stoppers are made from granulated cork, which is a by-product from the manufacture of natural cork closures. Agglomerated cork stoppers can be manufactured by individual moulding or by extrusion, using approved food contact glue (FDA) to bind the cork granules together.
Agglomerated corks are a low cost option, generally used for wine destined to be consumed within 12 months. As they are manufactured they can be made entirely homogeneous as they will not have any of the variations seen in natural cork products. The categorization of agglomerated cork closures depends on the cork granule size and the surface treatment used.
Capsulated Cork Closures (Bar Top Cork Stoppers)
Capsulated corks are made using natural (or colmated) cork stoppers with a wooden, PVC, porcelain, metal, glass or other material attached to the top. This stopper is generally used in the bottling of spirits, liquors and fortified wines which are ready for immediate consumption and allow the bottle to be easily opened and closed.