Whisky words: charring
Charring is a heat treatment whereby the wood of the cask is ignited. It helps to break down the structure of the oak, allowing for an easier and deeper penetration by the spirit and a more intense interaction with the flavours produced through lignin degradation.
The charcoal layer also acts as a filtration system, helping to remove unwanted flavours, such as sulphur, from the new spirit.
There are generally four grades of charring:
No.1 = 15 seconds
No.2 = 30 seconds
No.3 = 35 seconds
No.4 = 55 seconds
This heavy char creates layers of charcoal on the inner surface of the staves, hence its nickname, the ‘alligator char’.
‘Exhausted’ and ‘tired’ casks can be rejuvenated by going through a process called ‘de-char and re-char’. Essentially, a few millimetres from the inside of the barrel is skimmed off and removed, thus removing the old char. The cask can then be charred again (or toasted - see toasting), thus regenerating the char and also producing a new ‘red layer’ further into the wood. More lignin is thermally degraded, generating more compounds that are water and alcohol soluble. This action grants the ‘exhausted’ a new lease of life in terms of its use for further maturation.