Back in the days, sails were tanned to make them more durable. They dipped the sails in a heated big bucket with water and crunched bark from an oak tree. The sails got a brown tan, this procedure we call tanning.
Normally, a cotton sail would last for about one year and a tanned sail would last 20 years or more. The first dungaree was made from tanned sail or tent canvas. This is our connection with the past. Amsterdenim was the first denim brand with the tanned denim.