Boiled leather, sometimes called cuir bouilli, was a historical construction material for armour. It consists of thick leather, boiled in water (some sources hold that oil and wax were used as well; others posit the use of ammonia from fermented animal urine). The boiling causes the leather to be harder but also more brittle. The boiled leather can be fashioned into lames or scales to make lamellar or scale armor. The leather remains flexible for a short time after boiling, allowing it to be molded into larger plates.
Cuir bouilli has also been employed to bind books.