The Oxford Companion to Beer definition of
A firkin
A Firkin is a cask used by British brewers for delivery of cask-conditioned beer to the pub. A firkin holds a quarter of a barrel (9 Imperial gal; 10.8 US gal, 41 l). A firkin was originally coopered from wooden staves bound with iron hoops, but is now more commonly made of stainless steel or aluminum. Like all proper casks, it has a hole in one of the curved sides, to which a wooden or plastic bung ("shive") is fitted when the cask is filled. There is a smaller hole in one of the flat ends, or "heads," of the cask, which is also bunged. The bung is knocked out by the tap when the cask is broached. Although other sizes of cask are also used, the firkin is by far the most common.
The name "firkin" marks a milestone in the development of British brewing. The term originates from the Netherlands, as does "kilderkin" (18 Imperial gal; 21.6 US gal). Originally, ale was the common brew in Britain and was made without hops. See ale. Hopped "beer" was introduced to Britain from the Netherlands, probably sometime before the 15th century; many of the early beer brewers in England were Dutch in origin and used their own terms for the casks in which they sold their products. The beer barrel was established in 1420 at 36 Imperial gal (43 US gal) compared with the ale barrel at 30 Imperial gal (36 US gal). Because hops not only conferred flavor to the beer but also acted as a preservative, beer eventually displaced unhopped ale, and the 36-gal barrel became the standard in Britain.
Kilby, KThe village cooper. Haverfordwest, Wales: Shire Publications, 1998.