The use of oak in winemaking dates back more than 2,000 years. Originally, oak was used as a flavor addition, along with herbs, to improve the taste of crudely made wines of the time.
The barrel's purpose, aside from being a vessel, is to age wine and add a subtle oak flavor. Most barrels used in California are French. There are many aspects of the manufacturing process that influence wine flavor, all of which the winemaker must consider when ordering the precise barrel needed.
Most French oak forests used for cooperage are owned by the government. The wood is inspected by the cooper (barrel builder and/or barrel manufacturer) or wood broker in the forest and bought at end-of-the-year auctions when the tree's sap level is lowest. For this reason, it is illegal to harvest wood in the summer.
A winemaker can choose from a number of forests of oak. Many of the popular forests or wood types, from loose (open) grain to tight (closed) grain, are Limousin, Nevers, Bourgogne, Trancais, Allier, "Center of France" and Vosges. Wines aged in looser grain barrels age and acquire oak flavors faster. Oak types also offer subtle differences in flavor. Some impart more buttery characteristics, while others offer a stronger vanilla flavor.
The wood is cut to rough stave size and split mechanically or manually with axes, following the grain of the wood. This is important, as it reduces porous leaks often associated with "quarter sawn" barrels. The oversize staves are stacked and open-air dried for a minimum of two years. This aging in wind, rain and sun serves to reduce the wood's harsh, sappy flavors.
The aged staves are sorted and the damaged ones culled. Then, edges are jointed in order to form a circle when bent and assembled. Each stave is a different width, which necessitates a different angle for each when fitted together.
The staves are jointed and temporarily hooped at one end to form an open cone shape. This is called "raising the barrel" or "shaping into rose." The shell is heated over an open fire, causing water in the wood to steam, which allows the staves to be bent without cracking. This takes about 20 minutes. Afterward, a cable and winch are used to close the other end, which is then hooped. This is known as the first firing.
The second firing, also about 20 minutes, conditions the barrel and relieves stress. The interior becomes drier and hard where it is closest to the fire, while the outside softens and stretches as it warms.
Aside from wood choice, firing is the most important process to the winemaker. One can choose from light, medium or heavy toast levels, each of which has a significant flavor profile ranging from slightly toasty to caramelly to smoky. A heavily toasted barrel can even be brought to the slightly burnt point.
After the second firing, the barrel is fitted for its heads. Like the barrel, a head consists of many pieces fitted together. Each joint has a slight gap near its center, which allows for expansion when soaked in water or wine. The head pieces are assembled with double-ended dowels with needs in-between to act as a gasket.
The inside edges of the barrel end are cut with a croze into which the head fits. Each head is then fit into place. The outside is surfaced and sanded. Permanent hoops are sized and put into place. The barrel is pressure-tested. Any seeping, porous staves or heads are removed, replaced and retested.
A winemaker has many considerations when choosing barrels. Each wine style requires a different oak influence. Barrels fashioned from the same tree, fired to the same toast level, made by two different coopers may have totally different flavors. So winemakers have the opportunity to choose the cooper, wood type, forest, size of barrel and toast level that best suits their wine. Some barrel makers have a "house toast," which is their own particular style. (You get it their way.)
There are also choices of barrel shape. A Bordeaux barrel -- used for aging Bordeaux varietals such as cabernet sauvignon, merlot, cabernet franc, petite verdot and malbec -- is taller and narrower than the Burgundian (chardonnay and pinot noir) barrel, which is short and fat (happy). There is also a choice in stave thickness: "Chateau" barrels, with their thinner staves -- which age wine faster -- and the thicker staved "export" barrels.
To select barrels, some winemakers order as many as 15 different barrels from top cooperages, fill each with the same wine, then taste for different nuances to decide which copper offers the flavors that fit their style.