Charring occurs when the producers prepare casks to contain spirits by burning the inside of the vessel. Charring and Charred barrels are a big part of Bourbon production and the reason it tastes the way it does. As a result of its use there, charring is part of many other aged spirits that re-use American oak casks once they’ve been used for the Bourbon industry.
The use of fire isn’t just to shape the oak stave into shape, it does a number of things for distillers. It helps to break down the structure of the oak allowing deeper permeation into the oak by the spirit. The cracks in the wood produced by charring also increase the surface area available to the spirit. These two factors help to increase the speed in which the desired interaction and maturation occurs.
Practicalities of aging aside, charring barrels is fundamentally also about flavour - without first heat-treating casks (even just a light toasting), whisky would mostly taste quite sappy. This is due to the structure of wood itself which, amongst other things, consists of cellulose, hemicellulose and lignin. When heated, they break down and create new and very tasty compounds which are soluble in ethanol. Because of this, it’s fair to say that charring increases the levels of vanillin in a spirit and spiciness and the heavier the char is, the lower the levels of tannin there are seeping into the spirit. This trade-off is why the level of charring is so carefully considered by Whisky makers.
To achieve the right level of char, casks are made in the standard fashion with toasted staves. Once the cask has been constructed the whole of the interior is then given a controlled burn. A No.1 char will see the cask be fired for around 15-20 seconds; No.2 for 30-40 seconds; and No.3 for around 45 seconds, while the No.4 Char sees the cask flame just under a minute.