Books, periodicals, documents and reports are the main means of recording and storing knowledge. Contemporary mass-produced paper, although possessing many qualities, frequently lacks that of endurance. One of the main reasons of paper deterioration is the cross linking of molecular and supermolecular fibre structures (so called 'hornification'). This, together with the acidic degradation of cellulose chains, leads to the brittling of fibers and in consequence of paper . The main reasons are low purity of cellulose, use of alum to fix the sizing (which gives rise to acidity in the paper) and the increasing amount of acid fumes in the environment.
A survey of books published between 1900 and 1939 suggested that nearly 40% could not survive 25 years, even in moderate use, and most would be unusable in the 21st century. A later survey showed books published between 1940 and 1949 to be even shorter lived than newsprint. In the case of books of the 19th century, the deterioration can be indicated by the percentage categorized as needing restoration. Only 5% from the first fifty years are in this class, rising to 10 for the period 1850-1869, and to 48 for the last decade, due to use of new techniques.