Why do crown corks on beer bottles 鈥� at least the few hundred thousand that I have drunk from 鈥� always have 21 sharp bits?
鈥� We have three explanations for this one. We鈥檙e still waiting for a bottle-top aficionado (of which there seem to be many) to rule between them 鈥� Ed
鈥� The crown cork on any bottle is regulated by the internationally accepted German standard DIN 6099, ensuring all crown corks are the same. Along with specifying the diameter of the bottle neck, the form of the rim around which the cap is crimped and the materials the cap may be constructed from, this document specifies the form of the crimp. One requirement is that the closure be sufficiently circular to maintain a tight seal all around the circumference, which implies a high number of crimps (and thus points). It must also be robust, however, which implies reducing the number of crimps to give each crimp a larger bearing surface. Using 21 crimps is a good compromise between these requirements and is mandated in the standard. As to why it is 21 crimps rather than 20 or 22, the best answer is simply 鈥渂ecause it is鈥�.
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S. Humphreys, Oxted, Surrey, UK
鈥� Through trial and error, William Painter, the inventor of the crown cork, discovered that the optimum number of teeth on a mould made of steel for securing carbonated drinks was 24. He registered a patent for his design and for many years the 24-tooth capping mould was standard. However, around 1930 the steel mould came under threat from a cheaper version made of tinplate. This newer mould could not win a patent if it also had 24 teeth, so was changed to 21 to avoid infringing the original design. The new figure is the smallest number of teeth needed to prevent leaks and is now used across the world.
Chitran Duraisamy, Seven Hills, NSW, Australia
鈥� The crown cap was patented by Painter on 2 February 1892 (US patent 468,258). It originally had 24 teeth and a cork seal with a paper backing to stop drink and metal touching. The current version has 21 teeth.
The 24-tooth caps were originally fitted to bottles one by one using a foot-operated press. When automatic machines were adopted, the crown caps were loaded into tubular feed tubes and the 24-tooth caps frequently jammed. With an uneven number of teeth, this doesn鈥檛 happen, and because the sealing quality of 23 teeth was no better than 21, the smaller number was adopted.
The height of the crown cap was also reduced and specified in the German standard DIN 6099 in the 1960s. This also defined the 鈥渢wist-off鈥� crown cap widely used in the US.
Barry Painter, Niedernhausen, Germany