How wine corks are made

A cork tree can be stripped about every nine years, and the bark grows back each time.

stations: 8
things upstream: 21
raw sources: 11
steps from the ground: 5

The short version

Cork planks are stripped by hand from the bark of living oak trees, rested outdoors for months, and boiled for an hour. They are trimmed, cut into strips, and stoppers are punched out as cylinders. The corks are polished, scanned, washed, branded and coated with paraffin or silicone so they slide in and out of the bottle.

Rest the planks outdoors

Planks stripped from the forest are stacked outside for months to even out their moisture. Good processors keep them off bare earth to avoid contamination.

at least six months

Goes in
Cork oak bark
Boil the planks

The planks go into stainless steel tanks of hot water for an hour. That cleans them and softens them. They come out flatter and about a fifth bigger.

1 hour, 20% expansion
Trim and grade

Edges are trimmed. Each plank is graded by thickness, porosity and appearance. The best go for natural stoppers, and the rest for other cork products.
Punch out the cylinders

The planks are cut into strips, and a tube punch cuts a cylinder from each strip. Top-grade corks are punched by skilled workers by hand. What remains can be ground into granules for glued technical corks.
Polish to size

The cylinders are ground to exact dimensions and a smooth, even surface.
Scan and sort

Cameras sort each stopper by its look, and X-rays check for hidden flaws and density. Blasts of air push each cork into the right grade bin.
Wash and brand

Corks are washed to clean and whiten them, then branded with heat, ink or laser.
Coat for easy sliding

A thin coat of paraffin or silicone helps the stopper go into the bottle and come out again.

Goes in
Paraffin wax Silicone

End of the lineWine cork Next it goes into champagne.

Worth repeating

A cork oak is stripped for the first time after about 25 years, but that first bark is not good enough for wine stoppers.
A cork oak can live about 200 years, giving up its bark again and again.
The harvest year is painted as a number on the trunk, so each tree is not stripped before its time.