Keystone (cask)
A replaceable fitting that seals the cask and accepts the tap.
A keystone is a small, thick disk made of wood or plastic that fits into a cask's bunghole, sealing it completely until the cask is tapped. The bunghole is located on one of the cask's circular faces, near the rim; when the cask is in use, it lies on its side with the bunghole at the lowest point. The center of the keystone, called the tut, is deliberately weakened—in plastic versions it is much thinner, while wooden ones are partially pre-cut—so that a tap can be hammered through it with a mallet. The tap's tapered shaft seals against the undisturbed outer part of the keystone as it is driven in. A small amount of beer may leak during this process, but with skill and a firm mallet blow, no spillage is necessary.
Although wooden casks are now largely obsolete, with most being aluminum or stainless steel and some sturdy plastic ones appearing, the standard tap is still designed to be hammered into a yielding material. All modern casks, including the few wooden ones still in use, employ a replaceable keystone to accept the tap. When a cask is empty, it is customary to plug the keystone's hole with a small cork bung to prevent old beer from spilling on the drayman or in their vehicle; similarly, the shive hole may be sealed with a spile. Wooden keystones often develop a layer of mold on the outside between filling and tapping, which does not affect the beer, but the keystone must be thoroughly cleaned before tapping because part of it is driven into the cask and floats on the beer. This cleaning is worthwhile even with plastic keystones.
Leaks can occasionally occur around the tap due to the rustic nature of the connection. If beer leaks around the tap, hammering it further in usually fixes the problem; otherwise, the keystone may be split or the tap faulty, and the offending part must be changed. Because ale conditioning makes upending the cask to bring the keystone above the liquid level ruinous for at least a day, the keystone must be changed in situ. This can be done with minimal beer loss—half a pint is reasonable, and half that is possible. If possible, soak a new wooden keystone in water for some hours to reduce brittleness (plastic keystones do not need this). Ensure the keystone is the correct size for the cask, as most fit most casks but exceptions exist.
- Materials
- wood or plastic
- Location
- bunghole of the cask
- Weakened center name
- tut
- Typical beer loss when changing keystone
- half a pint or so is reasonable; half that again is eminently possible
Lore & Background
The keystone is inserted into the bunghole, which is on one of the circular faces of the cask near the rim. When the cask is in service, it is laid on its side with the bunghole at the lowest point on the circumference. The keystone's center is deliberately weakened; in plastic varieties it is much thinner than the surrounding material, while wooden ones are partially pre-cut. To tap the cask, the end of the tap is placed against this weakened section and driven through with a mallet. The tap's tapered shaft seals against the undisturbed outer part of the keystone. A small amount of beer may leak during this process, but with sufficient skill and firm action, no spillage at all can occur.
Wooden keystones commonly acquire a layer of mould on the outside between filling and tapping, which does not indicate a problem with the beer. However, because part of the keystone is driven into the cask (where it floats on top of the beer), it must be thoroughly cleaned before tapping. This cleaning is worthwhile even when plastic keystones are used. When the cask is empty, it is a courtesy to the drayman to stop up the hole in the keystone with a small cork bung to prevent old beer from spilling.
Reader's Guide
The keystone is notable as a replaceable component that allows modern casks—mostly aluminium or stainless steel, with a few sturdy plastic ones—to accept a standard tap designed to be hammered into yielding material. Even the few wooden casks still in use employ a replaceable keystone. Changing a keystone in situ is necessary because upending the cask would render the beer undrinkable for at least a day. The process can be achieved without serious loss of beer: half a pint or so is reasonable, and half that again is eminently possible. If the keystone is wooden, it may be soaked in water for some hours to render it less brittle, though plastic keystones do not require this. The procedure involves hammering a hard spile into the shive hole to prevent air entry, dislodging the old keystone, swiftly inserting the new one, and then hammering the tap through it. If the tap itself is faulty—more likely at beer festivals where taps are infrequently used and maintained—one may replace the keystone and insert a new tap, or remove only the tap and quickly insert a new one, with some authorities recommending a small cork bung between removal and insertion.
Did You Know?
- The weakened center of the keystone is called the tut.
- When changing a keystone, half a pint of beer loss is reasonable, and half that again is eminently possible.
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