Cask char levels

What charring does to the wood

To char a cask, the cooper sets fire to the inside of the assembled barrel and lets it burn for a measured time before putting it out. The flame turns the inner surface into a black, cracked layer of charcoal. Beneath it the heat soaks a few millimetres into the wood without burning it. That deeper zone is often called the red layer.

Both layers matter. The char acts a little like activated carbon, absorbing some sulphur compounds and other harsh notes from new make. The red layer is where heat has broken down the wood's hemicellulose into sugars with caramel and toffee notes and turned part of the lignin into vanillin and related compounds. Our guide to vanillin and oak lactones explains the chemistry.

The four common char levels

American cooperages usually describe char by number. The times are approximate and vary by cooperage, but the commonly quoted guide is:

• Char 1: about 15 seconds. A light char with a thin black layer.
• Char 2: about 30 seconds.
• Char 3: about 35 seconds. A medium to heavy char used widely in bourbon.
• Char 4: about 55 seconds. Often called alligator char because the surface cracks into scales like alligator skin.

Each step makes a deeper char layer and a deeper red layer. The difference between 35 and 55 seconds sounds trivial. In the warehouse it is large.

What each level tends to give

Light chars leave more of the raw oak character available: more wood spice and tannin, less colour, less caramel. Heavier chars give more colour, more sweetness and more toasty, smoky, caramel notes. They also filter more harshness from young spirit.

There is no best level. A distillery making a heavy, oily new make may want a deeper char to tame it. One making a light, fruity spirit may prefer a lighter char so the oak does not bury the fruit. Many cooperages also toast the staves before charring, which adds another layer of control. Our guide to toasted versus charred casks covers that step.

Char and the law

Bourbon must be aged in new charred oak containers. The rules do not specify a char level, so the choice belongs to the distiller. Because the container must be new, each bourbon barrel holds bourbon only once. It then goes on to other whisky producers around the world.

Scotch rules require oak casks of no more than 700 litres but say nothing about charring. Most ex-bourbon casks arrive in Scotland already charred from their first life. When a cask is worn out, it can be scraped back and recharred, which our guide to rejuvenated casks explains.

Char in a hot Indian warehouse

Heat speeds up everything a cask does, including the extraction of colour and flavour from the char and red layer. A heavily charred new cask in an Indian warehouse can push a young spirit to dark colour and heavy wood within a few years. That can be exactly what you want or far too much.

If you are buying new casks for an Indian climate, it is worth trialling more than one char level. Fill matched casks with the same new make, sample them every three to six months and compare colour, tannin and sweetness. The numbers on the cooper's order form only become useful once you know what they do in your own warehouse.

Ordering char from a cooperage

When you order new casks, specify the oak species, stave seasoning time, any toast, the char level and the cask size. Ask how the cooperage controls char time and whether it is measured by clock or by eye. Ask too for a sample cask or a stave so your team can see and smell the char before committing to a large order.

Keep the cooperage's char records with your own cask records. If a batch of casks turns out lighter or darker than expected, the first question is whether the char matched the order. Without the paperwork you cannot answer it.

Choosing cask specifications well is one of the bigger long-term decisions a distillery makes. It is set years before anyone tastes the result, which is exactly why it rewards proper study rather than copying what someone else orders.

Where to begin

If you already have casks, find out the char level of each type from your supplier's records and add it to your cask database. Then pull samples of a similar age from two char levels and taste them side by side with your team. That comparison will tell you more than any brochure.

Common questions

What is alligator char?

Char level 4, a heavy char of roughly 55 seconds that cracks the inside of the barrel into scales like alligator skin.

Does bourbon require a specific char level?

No. Bourbon must age in new charred oak containers, but the char level is the distiller's choice.

What does the char layer do to whisky?

It absorbs some sulphur compounds and harsh notes. The heated wood beneath it adds colour, sweetness, caramel and vanilla.