Blonde Girl With A Glass Of Champagne Strips A Pink Negligee On The Antique Chair

Corking a Champagne Bottle: 1855 engraving of the manual method
Champagne corks are built from several sections and are referred to as aglomerated corks. The mushroom shape that occurs in the transition is a result of the bottom section, which is in contact with the wine, being composed of two stacked discs of pristine cork, cemented to the upper portion which is a conglomerate of ground cork and glue. Prior to insertion, a sparkling wine cork is almost 50% larger than the opening of the bottle. Originally they start as a cylinder and are compressed prior to insertion into the bottle. Over time their compressed shape becomes more permanent and the distinctive "mushroom" shape becomes more apparent.
The aging of the Champagne post disgorgement can to some degree be told by the cork, as the longer it has been in the bottle the less it returns to its original cylinder shape.