Article by Llewellyn Matthews
The Papermaking Process – It is all cellulose, but there are differences
Papermaking has come along way since it spread to Europe, but the basic idea is the same. It is typically made from wood chips but can be made from any cellulose containing plant such as cotton, straw, hemp, or bagasse. The cellulose fibers that becomes paper is separated either mechanically or through chemical processing. Cellulose is composed of innumerable fibers, which are finer than human hair that are held together by a complex substance called “lignin,” mixed with other carbohydrates necessary for the plant to grow. The exact ratio of these ingredients varies greatly among plants. Most wood used in papermaking is approximately 50% cellulose, 30% lignin, and the rest carbohydrates. Cotton on the other hand is 99.8% cellulose that can be easily cleaned of its few residuals of wax, dirt and minuscule amount of lignin. It has the added advantage of greater durability and strength than wood pulp. Cotton would seem to be ideal, but is simply not economical and available in sufficient quantities.1 A paper labeled as cotton rag may have as little as 25% cotton, so it is wise to check the fine print. Still, cotton is probably the most elite fiber giving us the world’s finest bond and writing papers, vellums and bristols.
A word should be said about alpha cellulose as you find this label used by some manufacturers of fine art papers to denote greater purity and hence greater longevity. Alpha cellulose fibers are a set of longer fibers that are left remaining after the cellulose is subjected to further chemical processing.2 It is represents the effort of papermakers to separate from wood fibers the sub-set with the most desirable qualities of cotton. A good alpha cellulose paper is likely equal in longevity to good cotton paper.
The challenge in making paper is removing everything that is not the desirable cellulose that ultimately becomes paper. These extraneous materials reduce the longevity of the paper.
From the point of view of an artist concerned about the longevity of paper it is helpful to know some basics about the different paper making processes.
There are two reasons why there may be extraneous material in paper that reduces longevity: it either originates with the plant or it is added later to the paper.
Longevity Issues Due to Inherent Qualities of Wood Pulp:
The lignin that forms the glue needed by the plant is a very complex chemical that is responsible for the beautiful tan color of some paper, but broken down in the papermaking process it becomes detrimental to longevity. It tends to become brown, ages poorly and becomes acidic over time. Groundwood papermaking process removes a relatively small amount of lignin, as it is simply mechanical grinding of wood chips assisted with by heat and sometimes a little chemical processing, followed by washing. It has the added advantage of yielding more paper for a given quantity of wood chip than other processes but is not particularly strong. This process is useful for producing papers that are intended for a short life such as newsprint paper. Who has not seen their newspaper yellow if left too long in the sun? This is lignin letting us know it is still present and deterioration has begun.
The Kraft and sulphite processes separate the cellulose from the lignin and other carbohydrates through a chemical cooking process followed by washing of the pulp before being made into paper.3 The Kraft process results in a beautiful tan color that is typical of groceries bags. Although this is a strong pulp, good for short-term uses, over time the residual lignin will cause the paper to decompose. Too many artists have used newsprint, paper bags or Kraft wrapping paper without concern about longevity and are later disappointed. Nevertheless, an artist should freely use newsprint or brown Kraft paper for practice and gesture drawing and eventually recycle these papers.
To make white paper from wood pulp, a bleaching process is needed to break down and remove the lignin. No matter how well washed, a goodly amount of lignin will remain4. Modern bleaching methods do not pose the environmental problems of the past but sensitivities around this issue remain. Regardless of sensitivity around bleach issues, the author is of the opinion that an artist concerned about the longevity of a work on paper is likely one that has devoted many hours to the piece or may be creating a legacy piece. The artist with this intention should feel free to buy and use the best possible quality bleached paper.
For many reasons, there is confusion in the market place about white paper and how it is labeled. All white paper that is made from wood pulp is bleached to remove the lignin. Once the lignin is removed, the paper does not need to be bleached again when it is recycled. A different process is used to remove ink, called surprisingly enough, “de-inking.” A recycle paper manufacturer may label the paper as “recycled, chlorine-free.” This simply means the recycled manufacturer did not bleach the paper; but it was bleached initially in the original manufacturing process to remove lignin. This label is for marketing; it is not useful information. Similarly, for at least the past fifteen years, there is no paper made in the U.S. (and not likely in the E.U) using the old style bleaching method, so the label of “elemental chlorine-free” as a reference to using the new method is also not so important.5
Longevity Issues Due to Materials Added After Manufacturing:
The second category of extraneous material in paper are those materials added after the paper was manufactured. Today the U.S. recycles approximately 60% of the paper used. This is extremely important for reasons too numerous for the scope of this article. In many communities all recycle material is put into common bins. This means that not only is the paper not sorted by grade (newsprint, office papers, mixed papers, etc.) it may become contaminated with all manner of materials. Today virtually all recycled paper has a mix of paper from differing manufacturing processes. We know from the papermaking processes described above that some papers are manufactured from processes that are not intended to produce paper with strength or longevity. Furthermore, there is the question of the condition of the fiber in the recycle mix. It is thought that an individual fiber can be recycled 5-8 times before disintegrating, but there is no way to track the number of times an individual fiber has been through the wash.
What is this means, in the opinion of the author, is that use of recycled paper is best for short-term uses as there are too many unknowns in terms of longevity. Paper manufacturers have all manner of tests and standards for strength and longevity, but there is no test for time except time itself. And ultimately time is the enemy of paper. Nevertheless, with the advances in understanding of paper chemistry, it is not unreasonable to expect that a work on good modern archival paper will have the same longevity as an oil painting.
Labels that are important for longevity purposes are “acid-free” and “archival.”
This means some additional steps, beyond washing, were taken to counteract the acidity inherent to the wood pulp. There are various labels and standards for these papers as well.6 Paper with a pH of 6 or higher (a pH of 7 is neutral) can be labeled “acid-free,” but clearly has some acidity. “Archival” papers are likely not only closer to a neutral pH but are typically buffered as well. Buffering chemicals may be added to counteract the acidity of subsequent chemicals used on the surface of the paper such as sizing (printing papers) or atmospheric exposure to air contaminants.
Among archival papers two additional categories exist:
Conservation-grade means acid –free, buffered paper made from wood pulp
Archival or museum grade means paper made from cotton
1The first mill in the Pacific Northwest was built in 1860 and used 100% cotton rag. It closed ten years later due to lack of sufficient raw material. A ditty of the times exemplified the shortage, “Kind friend, when thy old shirt is rent, Let it to the Paper Mill be sent.” Modern wood pulping was established shortly thereafter with the first commercial grade mill built in Quebec in 1866. The first successful paper mill in the PNW was built in the 1880s and still operates today.
2Etherington and Roberts (1982ed), “Bookbinding and the Conservation of Books” offers this definition: “That part of a cellulosic material that is insoluble in a 17.5% solution of sodium hydroxide at 20° C. under specified conditions. While alpha cellulose consists principally of cellulose, it does include other components that are insoluble under the test conditions. Because the permanence of paper depends to some extent on the absence of non-cellulosic impurities, the determination of true cellulose (alpha cellulose) gives an indication of the stability of the paper, and therefore its permanence.”
3The Sulphite process is largely phased out in the U.S. It has the advantage of producing a pulp that required less bleaching than Kraft, yielded more alpha cellulose, but typically produces a lower overall yield for a given quantity of wood chip. It is still in use in Europe for specialty papers, including art papers.
4Overbleaching reduces paper strength, so a manufacturing solution is to use optical brighteners or optical brightening agents to enhance the white appearance. You will see some paper advertised as OBA-free. This is in response to the concern that optical brighteners will fade over time and the paper will look less than pure white over time.
5Modern bleaching methods use chlorine dioxide as a substitute for the problematic elemental chlorine of the past. Newsprint is typically lightened with hydrogen peroxide but will not become as white as paper manufactured from a chemical process and bleached.
6From the time that wood pulp was widely substituted for cotton, it was a mere 70 years before concern was triggered for “brittle book” syndrome and standards were developed to establish criteria for paper to last from several hundred to a thousand years.
Image Credits: Andrei Kozlov