A sheet material made up of a network of natural cellulosic fibers. Paper is a renewable and recyclable material that has been in use for almost 2000 years. Since its invention in AD 105 (Ts'ai Lun, China), paper has become a highly engineered material that is used for a great variety of applications. Paper grades are commonly categorized as graphic, packaging paper and paperboard, hygienic, and specialty grades. A steady increase in worldwide paper production and consumption leads to regular development of new grades.