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Equipment

Crown caps, swing tops and the tools that fit them

How do crown caps work and what capper do I need?

In short

A crown cap is a shallow steel cap with a fluted skirt and a liner inside. A capper crimps the skirt inwards under the ring at the bottle mouth, pressing the liner against the glass rim to make the seal.

It is used both as a finished closure and as the temporary closure during bottle conditioning of a traditional-method cider, before disgorging and corking.

Swing tops seal by clamping a gasketed stopper down onto the mouth with a wire lever. They are reusable and need no tool, and the gasket is a consumable that people forget is a consumable.

Caps, liners and sizes

The standard crown for beer and most cider is the 26 mm size, which fits the standard crown finish. A larger 29 mm crown is used on sparkling-wine bottles, whose mouth is wider, and the two are not interchangeable — a 26 mm cap will not seal a champagne bottle and attempting it produces a closure that looks fitted and is not. This is a common and consequential confusion.

The liner is where the sealing happens. Modern liners are moulded polymer rather than the cork discs of the original design, and they are available in ordinary and oxygen-scavenging types. An oxygen-scavenging liner contains a component that reacts with oxygen in the headspace and through the closure, and it is a genuine and cheap improvement in shelf life for a cider that will be kept.

Caps come lacquered inside for corrosion resistance, because cider is acidic and bare steel in contact with an acidic liquid rusts. A cap that has rusted through has both a leak and a metal contamination problem, and rust on the outside of caps in a damp store is a sign the batch needs looking at.

Cappers

Swing tops

The swing top — the Bügelverschluss — is a ceramic or plastic stopper carrying a rubber or silicone gasket, held against the bottle mouth by a hinged wire linkage that goes over-centre and locks. It seals well when everything is in good order and needs no tool at all, which makes it attractive for small batches and for cider given away rather than sold.

The gasket is the whole closure. Rubber gaskets harden, crack and take a compression set over a few years, and a gasket that has gone hard will leak slowly rather than fail obviously — the cider goes flat and oxidises over months while looking sealed. Gaskets are replaceable and cheap, and replacing them across a set of bottles every few years is the maintenance that makes the system work.

Swing tops also cannot be crimped to a defined force, so their sealing depends on the geometry of the bottle mouth and the wire being undamaged. A bottle whose wire has been bent out of shape does not seal reliably, and a bottle whose mouth is chipped will not seal at all.

Matching closure to pressure

The closure and the bottle must be rated for the same job. A crown cap on a proper sparkling-wine bottle will hold the pressures of a bottle-conditioned cider. A crown cap on a still-wine bottle will not make the bottle safe — the cap is not the weak point, the glass is. And a closure that fails under pressure releases suddenly: bottles are never pointed at anyone while being opened, and an over-carbonated bottle is chilled first so that less gas is available to expel the contents.

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