Most papers produced from the mid-19th century to the present become brittle in about 25 to 50 years. Surveys done in the 1980s showed that yellowing and brittleness is present in about 25 to 40 percent of research library collections.
Early paper made from materials such as linen, cotton, and hemp rags was relatively stable and durable. However, there was a chronic shortage of these materials, which became steadily worse as the demand for paper grew. In the early 19th century, the invention of the cotton gin (which separated the cotton fiber from the seed), provided additional relatively good raw material for papermaking. However, by the middle of the 19th century, papermakers began to look elsewhere for raw material.
In the 1840s, a process was developed to use groundwood pulp for paper. Groundwood papers contain more lignin (a naturally occurring organic acid) than is found in fibers such as flax or hemp. Lignin is not only acidic, causing the paper to become weak and brittle, but upon exposure to light it oxidizes, causing the paper to darken.
The desire to make paper more efficiently, quickly, and cheaply also contributed to the decline in paper quality over the course of several hundred years.
After about 1650, alum was added to the tub of gelatin size to harden it and keep it from spoiling (sizing makes the paper's surface suitable for writing, and can be done either by coating the paper by hand or dipping it into a tub of sizing solution). Alum is extremely acidic and leads to acid hydrolysis of the paper. In about 1680, the Hollander beater was invented, which was a mechanical device for chopping the rags to be made into paper. The Hollander, and later the Jordan (a rotary chopping mill) resulted in shorter paper fibers and weaker paper, and left residual metal particles in the paper. Chlorine bleaching began in the late 18th century, resulting in the formation of hydrochloric acid within bleached papers.
In the early 19th century, alum rosin sizing was introduced to speed up the papermaking process. The paper was sized in the vat (before the paper sheet was formed) by adding rosin to the vat, followed by alum. Alum rosin sizing creates sulphuric acid as a byproduct of the reaction between the alum and the rosin, and in-the-vat sizing meant that acids were distributed throughout the paper, rather than just on the surface. Cumulatively, this resulted in more acidic, weaker, and more brittle paper. Alum rosin sizing was widely used by the 1840s, and in the 1870s a cheaper (and even more acidic) form of the sizing was developed
It is possible to make good quality machine-made papers from wood pulp by using a chemically purified wood pulp (which separates the cellulose fiber from the other elements of the wood), but this is a more recent development.
A standard for permanent paper was developed during the 1980s because of demands by interested parties concerned about the brittle book problem. Permanent paper is defined by the National Information Standards Organization (NISO) in ANSI/NISO Z39.48-1992 (R1997), Permanence of Paper for Publications and Documents in Libraries and Archives. The standard defines permanent paper as having a 2% minimum alkaline reserve (to make it more chemically stable), less than 1% lignin, good tear resistance (a measure of durability), and a pH of 7.5 to 10.0.