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Bringing Oak to Life: Steaming, Toasting & Cauterising

In part one of this account of the work at leading Spanish cooperage Murua, I gave some details on how oak staves are weathered, cleaned and shaped, prior to their assembly into a barrel. With the staves in place, held together at one end by an iron hoop, the other ends must now by drawn in to create the barrel shape that is so familiar to us all. Here I continue my explanation of how this is achieved.

The barrel is brought into its final shape thanks to the increased plasticity of oak when exposed to heat, which can be dry heat (when barrels are shaped around an open fire), moist heat (steam or even a water bath) or a combination of the two. At Murua the process begins with a hot water bath, as shown in the image on the left. Hidden behind the steam in this picture is a floor-level vat full of scaldingly hot water (that should add a little spice to the annual health and safety inspection, I am sure) which is sufficiently deep and broad to take four barrels; in the image shown there are just three, one having just been removed. The ends of the staves drawn together by the first hoop are at the bottom, and poking out of the water at the top are the opposite ends, splayed wide; these are the ends that must be brought together. Once the staves are fully warmed through the embryonic barrel is lifted from the vat and then shaped, which today is generally achieved by applying a cable or metal strap around the open end of the barrel, and then closing the shape by applying force, tightening the strap or cable until the staves come together. If carried out when the wood was cold the staves may well snap, but thanks to the applied heat they will bend into shape; the cooper than knocks on another iron ring at this end to hold the staves together. At Murua the barrels are then placed over an open fire (as shown in the image below) which helps to dry the wood, but can also be used to toast the inner surface of the barrel, caramelising surface sugars in the wood and changing dramatically the flavours that may be imparted to the wine by the barrel. As with selection and weathering, the degree of toasting will also be to the client's demands.

Fitting the Heads & Finishing

Before the process goes any further it is necessary to fit ends to the barrel. For this a groove must be cut to take the end-piece (or 'head'), which itself is manufactured from five or six staves. These are planed into good condition, and then drilled twice on each edge in order to take two short pieces of doweling. During the assembly a single dried reed, gathered from a nearby riverside plot, is placed between each stave. Unlike the barrel itself, which is made watertight (or should that be winetight?) by the huge pressure exerted on the staves by the metal hoops, the heads need a little help achieving a good seal. This is the purpose of the reeds, which plug any potential leak between the staves. Watching a Murua employee place the reeds into position, before tapping the next stave down onto the dowels, did much to remind me of the artisanal origins of this manufacturing process, even if so much of it is now mechanised.

Once an adequate quantity of wood has been assembled, the circular head is cut from it and then introduced into the freshly cut groove. This is achieved by temporarily releasing the end hoop to allow the staves to open a little (other hoops, as shown in the image to the right, keep the barrel more or less in shape) and then pulling the head up into position using a special grabbing tool. Once in place the end hoop is replaced, and the process repeated for the other end of the barrel. At this point the barrel shape is complete, and what remains are largely finishing touches.

First the barrel is relieved of its iron hoops, as these are temporary manufacturing hoops which will be reused, and after a plane to the exterior to ensure a clean and splinter-free finish the nearly-completed product is fitted up with new, shiny hoops, usually of stainless steel, and which are forced into place using a hydraulic machine as shown on the right. At this point it is likely that the barrel will be tested, by drilling in and filling the barrel with water under pressure. A leak is a disappointment rather than a disaster; given the projected value of the finished product, which is considerable, a leaky stave does not condemn the barrel to destruction, but rather to replacement of the miscreant stave or staves. After retesting, the barrel may see some further finishing. The bung hole is drilled out and then cauterised using heat, as shown in the image below.

And finally the barrel is marked up, sometimes with the name of the cooperage, but Murua will also label barrels with the name or logo of the client; on the day I visited, a number of barrels destined for Marqués de Murrieta were being finished. Of all the processes I witnessed on the day, this was perhaps the most surprising. Naively I expected to see a heated brand applied to the wood, but the procedure turned out to involve a greater level of technology than this, as the required logo is seared onto the head of the barrel using a laser. The laser scans upwards rather like a laser-printer might, searing the image into the wood of the barrel head, slowing building up the images or words line by line. This process can be seen in the image below, where the laser has almost reached the tops of the letters. Wisps of smoke, from the smouldering wood, danced across the scene as I photographed it.

Once done the barrel is finished. It is first shrink-wrapped in a protective plastic coating and is then moved to a protected holding area, ready for shipping.

• Back to part one.