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A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in the wind. It is in essence a wing used for propulsion. Sails are can be made from various materials and strengthened using specific liquid mixtures.
Any type of natural material also used to make textile can be used, knowingly: cotton, wool, kashmere, hemp, sisal, silk, kenaf, jute, ramie, linen, lyocell
Doping, or impregnating the textile with a liquid to make it tougher can also be done. Since most sails are intented for outdoor use, it is often a good idea to this this as it increases the life span of the sail.
Throughout history, a variety of dopes has been developed:
- a popular dope for windmill sails was: 10 liter water, combined with 0,75 liter linseed oil, 0,75 liter grease, and 1 kg of bolus. 
- cellulose acetate butyrate
- certain other lacquers, varnishes, resins ...
Synthetic dopes are often better in toughening the textile, and thus require only treatment or only a few treatments a year. However, the products can be a lot harder to attain, cost more and are environmentally unfriendly. Due to the latter issue, it is also advisable that the textile is burned when it becomes worn.
- Werken met molens by Werkgroep West-Vlaamse Molens v.z.w.
- Bolus used for windmill sail