Paper is made of cellulose fibres from plant materials. Over the years, there have been many sources of cellulose fibres such as cotton, wheat straw, sugar cane waste, flax, bamboo, wood, linen, rag, and hemp. Lignin is the natural glue that holds the cellulose fibres together in plants. When making paper, lignin is broken down either through a mechanical or chemical process and the cellulose fibres are converted into a watery pulp containing tiny individual fibres. The watery pulp is then bleached, spread into sheets in a mould (or rolling wire screen mat), pressed, and dried. During drying, the tiny individual fibres interlock and bond with the aid of the natural sugar in the pulp to form sheets of paper.
By and large, paper and paperboards used in the fine art industry are either from cotton, rag, or groundwood. The terms “rag” and “cotton” although often used interchangeably have different origins. Rag refers to papers made with cotton textile remnants. Cotton refers to paper made of cotton linters, which is a byproduct of cotton processing. Cotton linters are pure cellulose fibres compared to rag; however, rag has longer fibres and makes stronger papers that are good for printing and drawing. At one time, it was common for papers to consist of 100% rag; however, that changed as the textile industry slowly shifted to synthetic fibres and caused a reduction in the raw material available for the paper industry. Today, most fine art papers are made from 100% cotton linters or a combination of rag and cotton linters.
Groundwood pulp has significant lignin content and requires additional chemical refinement during processing. On the other hand, rag and cotton linters have negligible lignin content. The presence of lignin and chemicals such as sulfites that are used in processing (but not completely washed out) impact the pH level of the paper and cause it to turn acidic during ageing. Acidity breaks down the cellulose fibres in the paper and causes embrittlement. Historically, groundwood pulp paper was not suitable for artworks due to the lignin content. These days there are processes in place to refine and remove most, if not all, lignin content from the groundwood pulp. These types of paper are described to be 'lignin-free or made of 'lignin-free cellulose/pulp'.
The longevity of lignin-free groundwood pulp is yet to be tested because there is insufficient data to determine the effects of the chemical used in the process to make lignin-free wood pulp. Theoretically, it's meant to be as durable as cotton or rag paper. It may be wise to use paper made of 100% cotton/rag as support for artworks until such time as we have further data on the longevity of lignin-free groundwood pulp.