A barrel, cask, or tun is a hollow cylindrical container, traditionally made of wooden staves bound by wooden or metal hoops. Traditionally, the barrel was a standard size of measure referring to a set capacity or weight of a given commodity. For example, in the UK a barrel of beer refers to a quantity of 36 imperial gallons (160 L; 43 US gal). Wine was shipped in barrels of 119 litres (31 US gal; 26 imp gal). A small barrel is called a keg.
Modern wooden barrels for wine-making are either made of French common oak (Quercus robur) and white oak (Quercus petraea) or from American white oak (Quercus alba) and have typically these standard sizes: "Bordeaux type" 225 litres (59 US gal; 49 imp gal), "Burgundy type" 228 litres (60 US gal; 50 imp gal) and "Cognac type" 300 litres (79 US gal; 66 imp gal). Modern barrels and casks can also be made of aluminum, stainless steel, and different types of plastic, such as HDPE.
Someone who makes barrels is called a "barrel maker" or cooper. Barrels are only one type of cooperage. Other types include, but are not limited to: buckets, tubs, butter churns, hogsheads, firkins, kegs, kilderkins, tierces, rundlets, puncheons, pipes, tuns, butts, pins, and breakers.
Barrels have a variety of uses, including storage of liquids such as water and oil, fermenting wine, arrack and sake, and maturing beverages such as wine, cognac, armagnac, sherry, port, whiskey and beer.
Though Herodotus mentions palm-wood casks used in shipping Armenian wine to Babylon in Mesopotamia, the barrel as we know it today was most likely developed by the Celts. Around 350 BC they were already using watertight, barrel-shaped wooden containers that were able to withstand stress and could be rolled and stacked.
The technique of bending wood into shape through heating they most likely borrowed from boat building, where it was used to bend planks for ship's hulls as early as the ancient Egyptians and Phoenicians. The barrel, apart from serving as a receptacle for wine and other liquids such as water, vinegar and oil, was also used for the storage and transport of precious metals, nails, powders, ocher and sulfur, as well as for the preservation of fish, olives, mustard and other sweet, salted or pickled foods.
In Asia and Europe, liquids like oil and wine were mostly carried in earthenware vessels. The Greeks and Romans kept their wines and other liquids in large vases called cadus or amphorae, sealed with pine resin.
Archaeological research has found coopers' tools and wooden seals dating from as early as 100 BC. The Romans began to use barrels in the 3rd century AD, as a result of their commercial and military contacts with the Gauls.
Toward the end of the era of Augustus, the Allobroges of the Rhone valley had selected a vine species that was acclimatized to their region. This "Allobrogic" variety bore frost-resistant grapes, thus pushing back the northern limits of vine cultivation by several hundred kilometers. At the same time, on the plains of Aquitaine, in the Gaillac region, another vine variety—"biturica"—made the Bordeaux prosper.
The wines from the Gallic province were highly appreciated in Rome, and the progression of the vine worried the Romans to such an extent that, in 92 AD, Emperor Domitien decided to destroy the Gallic vineyards. Around the same time, Pliny the Elder noted—in the regions near the Alps—the appearance of an exceptional container specially conceived to hold and mature wine: the wooden barrel, which became inseparably associated with wine, from its fermentation to its transport.
For nearly 2,000 years, barrels were the most convenient form of shipping or storage container, for those who could afford them. All kinds of bulk goods, from nails to gold coins, were stored in them. Bags and most crates were cheaper, but they were not as sturdy and they were more difficult to manhandle for the same weight.
The use of barrels for the transportation of bulk goods slowly lost its importance in the 20th century with the introduction of pallet-based logistics and containerization. However, they are still of great importance in the aging of wines and spirits.
Starting in the late 19th century, barrels were largely superseded by corrugated fiberboard boxes for the storage and transport of dry goods; and in the mid-20th century, steel drums began to be used for the storage and transport of fluids such as water, oils, and hazardous waste.
An "aging barrel" is used to age wine; distilled spirits such as whiskey, brandy, or rum; beer; tabasco sauce; or (in smaller sizes) traditional balsamic vinegar. When a wine or spirit ages in a barrel, small amounts of oxygen are introduced as the barrel lets some air in (compare to microoxygenation where oxygen is deliberately added). Oxygen enters a barrel when water or alcohol is lost due to evaporation, a portion known as the "angels' share". In an environment with 100% relative humidity, very little water evaporates and so most of the loss is alcohol, a useful trick if one has a wine with very high proof. Most beverages are topped up from other barrels to prevent significant oxidation, although others such as vin jaune are not.
Beverages aged in wooden barrels take on some of the compounds in the barrel, such as vanillin and wood tannins. The presence of these compounds depends on many factors, including the place of origin, how the staves were cut and dried, and the degree of "toast" applied during manufacture. After roughly three years, most of a barrel's flavor compounds have been leached out and it is well on its way to becoming "neutral". Barrels used for aging are typically made of French or American oak, but chestnut and redwood are also used. Some Asian beverages (e.g., Japanese sake) use Japanese cedar, which imparts an unusual, minty-piney flavor. In Peru and Chile, a grape distillate named "pisco" is either aged in oak or in earthenware, in which case the minerals from the fired clay leach into the liquor, giving it a unique flavor.
Some wines are fermented "on barrel", as opposed to in a neutral container such as a steel or wine-grade HDPE (high density polyethylene). Wine can also be fermented in large wooden tanks, which—when open to the atmosphere—are called "open-tops". Other wooden cooperage for storing wine or spirits range from smaller barriques to huge casks, with either elliptical or round heads.
The tastes yielded by French and American species of oak are slightly different, with French oak being subtler, while American oak gives stronger aromas. To retain the desired measure of oak influence, a winery will replace a certain percentage of its barrels every year, although this can vary from 5 to 100%. Some winemakers use "200% new oak", where the wine is put into new oak barrels twice during the aging process. Bulk wines are sometimes flavored by soaking oak chips in them instead of being aged in a barrel.
NOTE: This section includes both the U.S. spelling, whiskey, and the U.K. – Canadian spelling, whisky.
Laws in several jurisdictions require that whiskey be aged in wooden barrels. The law in the United States requires that "straight whiskey" (with the exception of corn whiskey) must be stored for at least two years in new, charred oak containers. Other forms of whiskey aged in used barrels cannot be called "straight".
Since the U.S. law requires the use of new barrels, which is not typically considered necessary elsewhere, whisky made elsewhere is usually aged in used barrels that previously contained U.S. whiskey (usually bourbon whiskey). The typical U.S. bourbon barrel is 53 US gallons (200 L) in size, which is thus the de facto standard whisky barrel size worldwide. Some distillers will transfer their whiskey into different barrels to "finish" or add qualities to the final product. These finishing barrels frequently aged a different spirit (like rum) or wine.
Sherry is stored in 600-litre casks that are made of North American oak, which is slightly more porous than French or Spanish oak. The casks, or butts, are filled five-sixths full, leaving "the space of two fists" empty at the top to allow flor to develop on top of the wine.
Maturing is very important for a good brandy, which is typically aged in oak casks. The wood used for those barrels is selected because of its ability to transfer certain aromas to the spirit. Cognac is aged only in oak casks made from wood from the Tronçais and more often from the Limousin forests.
Traditional balsamic vinegar is aged in a series of wooden barrels.
Since its invention in 1868, the pepper mash used to make tabasco sauce is aged for three years in previously-used oak whiskey barrels.
Beers are sometimes aged in barrels which were previously used for maturing wines or spirits. Cask ale is aged in the barrel (usually steel) for a short time before serving. Extensive barrel aging is required of many sour beers.
"Angels' share" is a term for the portion (share) of a wine or distilled spirit's volume that is lost to evaporation during aging in oak barrels. In low humidity conditions, the loss to evaporation may be primarily water. However, in higher humidities, more alcohol than water will evaporate, therefore reducing the alcoholic strength of the product. In humid climates, this loss of ethanol is associated with the growth of a darkly colored fungus, the angels' share fungus, Baudoinia compniacensis, on the exterior surfaces of buildings, trees and other vegetation, and anything else that happens to be nearby. 
Water barrels are often used to collect the rainwater from dwellings (so that it may be used for irrigation or other purposes). This usage, known as rainwater harvesting, requires (besides a large rainwater barrel or water butt) adequate (waterproof) roof-covering and an adequate rain pipe.
The standard barrel of crude oil or other petroleum product (abbreviated bbl) is 42 US gallons (34.972315754 imp gal; 158.987294928 L). This measurement originated in the early Pennsylvania oil fields, and permitted both British and American merchants to refer to the same unit, based on the old English wine measure, the tierce.
Earlier, another size of whiskey barrel was the most common size; this was the 40 US gallons (33.3 imp gal; 151.4 L) barrel for proof spirits, which was of the same volume as 5 US bushels. However, by 1866 the oil barrel was standardized at 42 US gallons.
Oil has not been shipped in barrels since the introduction of oil tankers, but the 42-US-gallon size is still used as a unit for measurement, pricing, and in tax and regulatory codes. Each barrel is refined into about 19.74 US gallons (16.44 imp gal; 74.7 L) of gasoline, the rest becoming other products such as jet fuel and heating oil, using fractional distillation.
Barrels often have a convex shape, bulging at the middle. This bulge facilitates rolling a well-built wooden barrel on its side and change directions with little friction, compared to a cylinder. It also helps to distribute stress evenly in the material by making the container more curved. Barrels have reinforced edges to enable safe displacement by rolling them at an angle (in addition of rolling on their side as described).
The rings holding a wooden barrel together are called hoops and are generally made of galvanized iron, though historically were made from flexible bits of wood called withies. While wooden hoops could require barrels to be "fully hooped," with hoops stacked tightly together along the entire top and bottom third of a barrel, iron-hooped barrels only require a few hoops on each end.
The "head hoop" or "chime hoop" is the hoop nearest the extremes of a barrel, the chime being the beveled edge and the head being the flat circular top or bottom of the barrel. The "bilge hoops" are those nearest the bilge, or bulging center, while the "quarter hoop" is located between the chime and bilge hoops.
The stopper used to seal the hole in a barrel is called the bung and mostly made of silicone.
A barrel is one of several units of volume, with dry barrels, fluid barrels (UK beer barrel, US beer barrel), oil barrel, etc. The volume of some barrel units is double others, with various volumes in the range of about 100–200 litres (22–44 imp gal; 26–53 US gal).
- English wine casks
|gallon||rundlet||barrel||tierce||hogshead||puncheon, tertian||pipe, butt||tun|
|1||1 1⁄2||3||puncheons, tertians|
|1||1 1⁄3||2||2 2⁄3||4||8||barrels|
|1||1 3⁄4||2 1⁄3||3 1⁄2||4 2⁄3||7||14||rundlets|
|1||18||31 1⁄2||42||63||84||126||252||gallons (wine)|
|1||15||26 1⁄4||35||52 1⁄2||70||105||210||gallons (imperial)|
Pre-1824 definitions continued to be used in the US, the wine gallon of 231 cubic inches being the standard gallon for liquids (the corn gallon of 268.8 cubic inches for solids). In Britain the wine gallon was replaced by the imperial gallon. The tierce later became the petrol barrel. The tun was originally 256 gallons, which explains where the quarter, 8 bushels or 64 (wine) gallons, comes from.
- Brewery casks
|= 4.621 l||= 36.97 l||= 73.94 l||= 147.9 l||= 221.8 l|
|= 4.621 l||= 41.59 l||= 83.18 l||= 166.4 l||= 249.5 l|
|1||8 1⁄2||17||34||51||ale gallons||1688|
|= 4.621 l||= 39.28 l||= 78.56 l||= 157.1 l||= 235.7 l|
|= 4.621 l||= 41.59 l||= 83.18 l||= 166.4 l||= 249.5 l|
|= 4.546 l||= 40.91 l||= 81.83 l||= 163.7 l||= 245.5 l|
Although it is common to refer to draught beer containers of any size as barrels, in the UK this is strictly correct only if the container holds 36 imperial gallons. The terms "keg" and "cask" refer to containers of any size, the distinction being that kegs are used for beers intended to be served using external gas cylinders. Cask ales undergo part of their fermentation process in their containers, called casks.
The modern US beer barrel is 31 US gallons (117.34777 L), half a gallon less than the traditional wine barrel. (26 U.S.C. §5051)
- Dry goods
Barrels are also used as a unit of measurement for dry goods (dry groceries), such as flour or produce. Traditionally, a barrel is 196 pounds (89 kg) of flour (wheat or rye), with other substances such as pork subject to more local variation. In modern times, produce barrels for all dry goods, excepting cranberries, contain 7,056 cubic inches, about 115.627 L.
- ^ Oak Barrels: French vs. American
- ^ a b "27 C.F.R. sec 5.22(b)(1)(i)". Ecfr.gpoaccess.gov. Retrieved 2010-08-05.
- ^ "ASIL Insight: WTO Protections for Food Geographic Indications". Archived from the original on 14 August 2007. Retrieved 2007-08-25.
- ^ "Canadian Food and Drug Regulations (C.R.C., c. 870) – Canadian Whisky, Canadian Rye Whisky or Rye Whisky (B.02.020)". (Access date December 15, 2010.)
- ^ Why's a bourbon barrel 53 gallons?
- ^ ISC Barrels
- ^ Dixon, B. (March 2009). "The mystery of the warehouse stains (Animacules)" (PDF). Microbe 4 (3): 104–105.
- ^ Byland, Hannah (September 4, 2012). "Whiskey Aging Warehouses and the Effects to Surrounding Residential Neighborhoods in Louisville, Ky." (PDF). Louisville, Kentucky: Louisville, Kentucky government. Retrieved July 30, 2014.
- ^ Slate
- ^ 
- ^ What's In A Barrel of Oil?
- ^ http://www.sizes.com/units/barrel_wine.htm
- ^ http://www.sizes.com/units/barrel_alebeer.htm
- ^ 27 CFR 25.11 – Meaning of terms
- ^ Dry goods barrel: size defined by U.S. Congress in 1915