"hanji" is made out of inner bark of mulberry, a tree native to korea. the inner bark of the mulberry is so strong and durable that it does not decay in water up to a year. korean paper’s durability comes from this tree. the life span of korean paper is 1,000 years. for example, "mugujeonggwang daedaranigyeong," the world's oldest woodblock print was created in 751. also, thanks to its durability, koreans even used it as body armor in war. 1 year old trees are used to make the high-quality "hanji", and older trees are used for cheaper modern papers.