Fibers Used in Papermaking

(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)

IN THE WEST, paper is generally made from wood or cotton or a combination of both. The way the fibers are chemically treated during the manufacturing process affects the appearance and the durability of the paper. The most suitable papers for permanent artwork are those made from cotton. Though some people believe that a paper is safe if it is at least half cotton fiber, only paper made of 100 percent cotton fiber is recommended for artwork that is expected to last for decades. Papers derived primarily from wood pulp, even pulp that is treated or buffered, are best used only for printing, graphics, or student purposes.

In Oriental papers, three types of fiber are commonly used–kozo, mitsumata, and gampi. These three fibers, used alone or in combination with each other, are the basis for the largest variety of papers in the world today. When used without fillers, these fibers produce durable, permanent papers, often of great beauty.

GROUND WOOD

Wood that is ground into a pulp without any additional purification is called ground wood. All the impurities-lignin, resin, and hemicellulose-remain. Paper made from ground-wood pulp is the least durable and the least permanent, and has the poorest appearance of all papers. However, the paper is more opaque than most and absorbs printing inks well. Since paper of this type is also the least expensive, it is popular primarily among printers and schools. Newsprint, construction papers, oak tag, newsboard, chipboard, railroad board, and oatmeal paper are among the many ground-wood papers. It is my opinion that this type of paper should never be used as a primary ground for permanent art work.

Chemically Treated Wood Pulp. In an attempt to improve the appearance, durability, and permanence of papers derived from wood, a variety of chemical treatments have evolved to make lignin soluble for easy removal and to separate the wood pulp into individual fibers. In 1851, a treatment using caustic soda (sodium hydroxide) was developed, which resulted in a pulp referred to as “soda pulp.” This treatment increased bulk, but weakened the fibers. During the 1870s, a treatment that reduced the weakening of the fibers involved the use of sulfur dioxide and bisulfate, forming a “sulfite pulp.” This method significantly reduced the lignin content to between 2 to 5 percent, which could be further reduced through bleaching and thorough washing. This pulp was given the name “alpha-pulp.” This type of treatment is used today to produce the better-quality wood-pulp artists’ paper.

The sulfate process developed by Carl Dahl in 1884 is the process used today for the production of most paper products in general use. It is more efficient than the sulfite process, which did not remove or break down wood chips, so they had to be removed before processing. This is not necessary in the sulfate treatment; however, the resultant pulp is much higher in impurities that resist bleaching. The sulfate process produces a much stronger fiber and is often referred to as the Kraft process-the term “Kraft” is derived from the German word for strong. Although this process produces a stronger fiber, it is also much rougher and less suitable for use in the production of artists’ papers.

Although sulfite, or alpha-pulp, papers are compared to cotton papers, they have neither the quality nor the permanence that only cotton papers have. The sulfite papers are a tremendous improvement, but, in most cases, the lignins are replaced with a lesser evil-the natural resins and chemical residues left over from processing. These are rarely dealt with, and must be before this pulp can be considered an equivalent to cotton. Ground-wood pulp papers have a lifespan of only a few years before embrittlement and yellowing take their toll. Sulfite papers are a vast improvement since it usually takes between one and three decades before the residual chemicals and the remaining natural resins produce significant yellowing and embrittlement.

A distinct advantage to sulfite is its moderate cost compared to cotton. A wide selection of papers are made from this pulp, including drawing, tracing, layout, bond, colored, decorative, and watercolor papers, and boards for illustration, mounting, and matting. This type of paper is fine for artwork that will be reproduced and the original may be sacrificed, but it should not be used for original artwork that will be considered an investment.

Chemically Treated and Buffered Wood Pulp is simply chemically treated wood pulp with the additional enhancement of a chemical buffer. The buffer keeps the paper made from this pulp looking better longer because yellowing will not occur until the buffers are exhausted. Buffering will also help to maintain the color quality of pH-sensitive inks, dyes, and certain watercolor pigments. Drawing and watercolor papers made from this type of pulp are being introduced by only a few companies, including Seth Cole, Rising, and Pentalic, but are nevertheless widely distributed.

Chemically Treated, Lignin-free, Buffered Wood Pulp is the result of a new process in which, it is claimed, all the undesirable natural residues and chemicals used in processing have been removed so that a pure cellulose pulp remains. This pulp is then buffered for extra safety. This is a costly process and if this pulp were used only to make a cotton paper substitute there would be little recognizable saving. In the case of museum boards, however, which are expensive partly because of the large amount of cotton fiber used, the savings may be as high as 25 percent when substituting this type of pulp for cotton. Currently, this pulp is being used primarily to produce a less expensive board for use in archival storage and framing.

COTTON

Cotton is virtually pure cellulose fiber, which has a natural resistance to deterioration and to many forms of chemical attack. Cotton fibers range in length from 5/8 to 1 1/2 inches. The longer fibers are separated by the cotton gin for use in the manufacture of textiles. The shorter fibers left in the gin, which are called cotton linters, are collected separately and used to make cotton papers and boards.

Papers made from 100 percent cotton are among the most permanent, most durable, and strongest. They possess all of the most desirable characteristics and all the best drawing, watercolor, charcoal, and tracing papers and the finest museum and illustration boards are made from 100 percent cotton fibers. Papers and boards made of 100 percent cotton fiber are among the most expensive. Keep in mind that many 100 percent cotton tracing and visualizing papers are impregnated with oil or chemicals, which can dramatically reduce their lifespan.

Rag and cotton are terms that today are virtually interchangeable. At one time, rag meant cotton taken exclusively from cotton textile remnants. Very few cotton papers are still made from rags, either entirely or partially. Fabriano’s Roma paper, which is said to have been used by Michelangelo, is still, however, made of 100 percent rag. Strathmore’s Artist Bristol is also still made of rag.

The difference between true rag papers and cotton papers made from linters is that the rags have the longer cotton fibers and the weaving seems to add strength. The symbol of quality is still a paper that is made from 100 percent cotton rags. That is probably why people prefer to call both cotton linter and cotton rag papers “rag.”

COTTON AND WOOD

The larger the ratio of cotton to wood, the better the paper will handle and age. Most artists and conservators do not consider a paper with less than 50 percent cotton safe for permanent artwork. Owners of some paper mills of long standing, as well as some government guidelines, claim that paper made with at least 50 percent or more cotton fiber is safe. In Europe, most fine art papers have some refined pulp but the majority of the 100 percent cotton papers produced there are sent to the more extravagant Americans. I have yet to find an art restorer in this country, however, who will vouch for a paper that contains less than 100 percent cotton fiber.

KOZO, MITSUMATA, AND GAMPI

These are the most common fibers used in making Oriental papers. Only the inner bark, or bast fiber, of Broussonetia kazinoki for kozo, Edgeworthia ehrysantha for mitsumata, and Diplomorpha shikokiana for gampi, is used to make paper. When these fibers are used without wood pulp or other fillers, they are as permanent as cotton and, in the case of gampi, even more permanent.

Kozo, known in North America as mulberry, makes the strongest and most durable of the Oriental papers. It has the longest fibers, will not shrink or expand when wet, and produces a paper with an uneven surface. This fiber is used alone to make paper and is added to other fibers to give them additional strength and durability.

Mitsumata is traditionally described by the Japanese in what they feel are female terms. They say it is the most beautiful, softest, most absorbent, and the weakest of the three fibers. It is often used to balance kozo fibers, which are described in male terms, to increase the absorbency, even the surface, and add beauty to a paper.

Gampi is described by the Japanese as having both male and female characteristics. Its fibers are long, thin, somewhat shiny, and very tough. The fibers are so durable that paper made of gampi is referred to as “paper cloth.” Gampi paper is smooth, lustrous, and has its own natural chemical resistance to papereating insects. It is nonabsorbent, damp-resistant, and may well be the most permanent paper in the world. The best paper is made from uncultivated plants, but the plant is rare because it was overused to the point of near extinction. Most available gampi papers are made from a species of the plant found in the Philippine Islands and processed in Taiwan.

Many Oriental papers available in the West are made from one of these three fibers. They are called Japanese papers, or Japanese-style papers, even if they are made in another country.

Tan-hi is the Chinese version of kozo fiber and is the primary ingredient in such traditional Chinese papers as gasen. Today, tan-hi is more commonly referred to as tampi.