Crown Cork

Test only with cold, low-carbonation bottles, wear eye protection, and never apply heat. Glass under internal pressure fails without warning.

Painter specifies a fluted skirt. Twenty-one teeth became the standard and remains so — a number chosen because it works, not because it is round.

Record the outside diameter of the bead and the height from the top face to its underside. The cap is designed to the bottle, not the reverse.

Cut a blank a few millimetres wider than the finished cap. Thin sheet, because the skirt has to fold cold without tearing.

Press the disc into a shallow cup so it has a flat top and a straight skirt. The top must stay flat — it carries the liner and takes the pressure.

Notch the rim into evenly spaced teeth. The flutes exist so the skirt can shrink in circumference — a plain skirt has nowhere for the metal to go and simply buckles.

Mark them out rather than eyeballing. Uneven teeth crimp unevenly and leave one segment loose, which is exactly where it will leak.

Cut cork or rubber to sit inside the cap top. Painter used natural cork; the metal never touches the glass and never seals anything by itself.

The liner seals against the flat top of the bottle lip. The metal only supplies clamping force — confusing the two is the commonest misreading of the design.

Turn a die with a tapered bore. Pressing the cap in forces every tooth inward together, under the bottle bead.

Apply steady pressure once. Repeated part-strokes work-harden the teeth and they crack instead of folding.

Look round the whole skirt. One tooth sitting proud is a leak path and a cap that will blow off under pressure.

Seal a carbonated bottle and leave it cold overnight. Any hiss or loss of fizz means the liner is not seating flat.

Internal pressure pushes the liner up against the lip. The harder it is tested the tighter it seals — a self-energising seal, and the reason the design has outlived every rival.

The patent. US 468,258, "Bottle-Sealing Device", granted 2 February 1892 to William Painter of Baltimore. Painter held some eighty-odd patents and had already tried several bottle stoppers. The crown cork's competition was the wired cork, the Hutter swing-top with its rubber gasket, and the Codd-neck bottle with a glass marble held up by the gas. Every one of those was reusable and had to be cleaned, returned and inspected; the crown cork was cheap enough to throw away, and that changed bottling from a craft into a production line.

The flutes are a sheet-metal necessity. To grip under a bottle bead the skirt must reduce in circumference. Sheet metal does not compress along its length, so a plain cylinder can only buckle unpredictably. Cutting the skirt into teeth gives the material somewhere to go — each tooth folds inward independently and the gaps close up. Twenty-one is the number that distributes the grip evenly without leaving segments wide enough to spring back, and it has been standard for over a century.

Self-energising sealing. The metal never touches the glass sealing face; a soft liner does. Crimping supplies an initial clamping load, and internal pressure then adds to it by pushing the liner against the lip from beneath. Higher pressure means a tighter seal — the opposite of a cork forced outward by the same pressure. The same principle appears in lip seals, O-ring glands and pressure-actuated valve seats, and it is the reason a crown cork will hold several atmospheres on a two-cent piece of tinplate.

The link to a much more famous patent. Painter's business advice to one of his salesmen was to invent something that is used once, thrown away, and bought again. That salesman was King Camp Gillette, who spent six years failing to make a disposable razor blade before succeeding. The liner material changed — natural cork gave way to plastic in the 1960s — and the pull-tab and screw cap took much of the market, but the crown cork is still made in the tens of billions annually, essentially to Painter's 1892 drawing.