Characteristics of Paper

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

There are several terms that are often misused in describing paper. Durability, for example, is often confused with permanence, formation with grain, and finish with surface. Understanding the applicable terms will greatly improve your chances of getting what you want when buying paper.

Acid-free is a term used to indicate that a paper or board, when it leaves the factory, has a pH of 6.5 or higher, which indicates that there is little or no acid that can accelerate aging or deterioration. However, “acid-free” does not mean that the paper or board is guaranteed to remain in this condition or that it is free of any other undesirable chemicals. It also does not mean that it is safe to use the paper or board for archival storage or framing. “Acid-free” is a label sometimes used in misleading ways by a few manufacturers to imply permanency when it does not exist.

Conservation is the repair, restoration, and preservation of documents, objects, and artwork with the intention of preserving them permanently.

Deckle refers to a wooden frame used in the papermaking process and to the irregular edge on the paper produced by the use of that frame. In papermaking, a screen or mould is passed through a suspension of pulp to form sheets of paper. A deckle is a separate wooden frame fitted over the papermaking mould to prevent excess pulp from spilling over the mould as it is lifted from the vat. A deckle edge is formed when a small amount of pulp seeps under this frame, producing an irregular edge on the paper. This deckle edge is often left untrimmed. Handmade papers have four deckle edges because each sheet is made individually and the water is allowed to drain from the mould in both directions. Mouldmade papers have two deckle edges and two torn edges because one continuous sheet is formed and the water is allowed to drain in one direction. The one long sheet is later torn into smaller sheets, which accounts for the two torn edges.

Durability is the ability of a paper to retain its original qualities under use. Most print papers, for example, do not have a durable surface and will be ruined by erasure, but bristol paper is very durable because it will maintain its surface qualities under repeated erasures. Durability does not necessarily indicate permanency. The American Society for Testing and Materials defines durability in their publication ANSI/ASTM D-3290 as “the ability of a paper to resist the effects of wear and tear in performance situations. For example, paper currency should be made durable, but permanence is not a problem.”

Fillers include such materials as kaolin (clay), calcium carbonate, and titanium dioxide, which are used to fill the pores on the surface of papers and boards to make a paper whiter and more opaque, as well as to give bulk. The more filler that is used, the less fiber there will be, and itis the fiber that maintains the strength of a paper or board. Today, it is becoming increasingly common for manufacturers to load up ground-wood boards with calcium carbonate to buffer them against the significant acidity that develops as the relatively unrefined pulp ages. Too much of this chalk will result in a great sensitivity to changes in humidity, which can cause severe warping. If such a board gets wet, the chalk dissolves and the board can easily fall apart.

Finish is the condition of the surface of a paper. The terms used to describe the finish of drawing papers and illustration boards are similar to those used to describe watercolor papers and boards, but refer to different finishes.

In the case of drawing papers and illustration boards, the terms “hot press,” “plate,” or “smooth” refer to a surface that is as slick as glass with virtually no tooth. This type of finish is produced by pressing the paper through hot rollers. The terms “cold press,” “kid,” “vellum,” “regular,” and “medium,” refer to a surface that ranges from a barely detectable tooth to the feel of a medium-grade sandpaper. “Rough” refers either to an irregular bumpy texture of the surface, or to a laid surface that is characterized by lines caused by the way the screen was sewn to the mould, like an impression of Venetian blinds.

In watercolor papers, “hot press” or “smooth” refers to a surface like that of a “vellum” or “cold press” drawing paper. Hot-press watercolor paper is not formed by pressing with hot rollers, but by the using of a fine wire screen to collect the fibers. A cold-press watercolor paper is formed by pressing the still wet paper with a textured surface like a felt mat, giving the paper an irregular surface. This watercolor finish would be equivalent to a “rough” finish in a drawing paper. “Rough” in watercolor paper is an exaggeration of “cold press.” A “laid” surface finish is determined by the pressure and the grain of a wire grid against the surface of the paper.

Formation is determined by the manner in which the fibers collect upon the screen during manufacture. For example, when “wove” paper is held up to the light, the formation of the fibers appears uniform. When the same is done to an inexpensive bond paper, the formation will appear mottled.

Grain is evident in machine-made and mouldmade paper. The fibers tend to align themselves in the one direction that the wire screen used in the machine process pulls them from the water. This gives the machine-made paper more strength in one direction than another. In handmade paper, the fibers are pulled more slowly and they are lifted up and out, without directional preference. Handmade papers, therefore, have no grain.

GRS/m2 is a unit of measure for the weight of paper in the metric system. It is the weight of one square meter expressed in grams. A 140 lb. 22″ x30″ watercolor paper is equivalent to 3000RS/m2.

Handmade Papers are usually made of cotton or Oriental fibers. Strong chemicals are not used because these fibers do not have to be treated as wood does. The hand process of lifting the fibers from the water allows the fibers to align in all directions and interlock among themselves. This gives handmade papers great strength. When this type of paper gets wet, it will expand evenly in both directions with minimal buckling and warping.

Mouldmade Paper is halfway between handmade and machine-made paper. When making mouldmade papers, the individual screen used in handmade paper to pull the fibers from the water is replaced by a rotating screen. It slowly pulls the fibers from the water in a long continuous sheet. Individual sheets are made by passing a stream of water or air at intervals across the continuous sheet while it is still wet. This creates a weakness where each sheet can be gently torn free after drying.

Permanence refers to the length of time materials will maintain their original integrity. Materials that will not maintain their original integrity for a minimum of twenty-five years are not considered permanent. Materials that will maintain themselves for fifty to seventy-five years are considered relatively permanent if handled with care. A life expectancy from seventy-five to one hundred or more years is considered permanent. Materials that have shown little or no deterioration within seventy-five years will probably continue to remain in good condition for some time longer.

The permanency of paper can be reduced by the presence of wood fibers, alum or rosin sizing, residual chemicals from bleaching, traces of iron and copper from the water used in processing, exposure to sulfur dioxide or nitric oxide, and a pH of 5.5 or less.

Ply refers to one sheet of paper that is bonded to another of the same kind. For example, two-ply bristol is made of two one-ply sheets laminated together.

Sizing is the use of a glutinous material to fill up the pores in a paper’s surface. Paper is composed primarily of cellulose. Cellulose possesses a great many “free radicals,” a chemical term for atoms containing unpaired electrons. Free radicals love to attach themselves to water molecules. Therefore, when the cellulose of paper comes into contact with water, the water forms a temporary chemical bond causing the paper to swell to accommodate the increased volume. The purpose . of a size is chemically to tie up a certain percentage of these free radicals and make them unavailable for water molecules. The more size, the less absorbent the paper will be, resulting in less buckling of the surface and less bleeding of color.

Animal gelatin or glues were early sizing agents. In the mid-seventeenth century, alum (potassium aluminum sulfate) was introduced to harden the gelatin sizing. Hardening the soft gelatin increases the surface tension and results in the color staying more on, and spreading less over, the surface. This allows the painter to work more slowly and achieve greater detail. By the nineteenth century it became clear that the animal gelatin aged poorly, and that the alum acidified the paper. These deleterious effects led to the unpopularity of animal-source sizes. Rosin (a residue collected from the processing of pine trees) was then introduced as a possible substitute, but it proved to be even worse for the longevity of art paper. In the recent past, vegetable gelatins replaced the animal gelatin because of their better aging characteristics. Efforts to deal with the acidic alum-sized papers through the use of buffers proved unsuccessful; alum requires a slightly acidic pH to attach itself to paper.

Today, a synthetic size, Aquapel, manufactured by Hercules, Inc., is replacing the traditional sizing agents because it possesses all the ideal characteristics of a hardening size without the disadvantages. Chemically, Aquapel is an alkene ketene dimer. Dimers are chemicals with two chemically active parts and Aquapel has one area that is attracted to water and one that is attracted to oil. The part that is attracted to water attaches to the free radicals of the cellulose in paper and the other part repels water. The most important characteristic of this size is that it works without an acidic pH.

Most art papers are sized in one of two ways-tub or surface. Tub sizing is commonly used in handmade and mouldmade papers, where the whole sheet is dipped into a vat containing a size. This results in sizing equal on both sides. Surface sizing is done on machine-made papers, where the size is applied by roller to one side only; only the sized side is meant to be used.

Surface refers to the front and back, or top and bottom, of a piece of paper. The top of the sheet of paper is determined by the manufacturing. A piece of artwork is usually done on the top surface, that is, the side in which the most care has been taken, the surface to which the sizing has been applied. Some illustration boards and bristol paper are specially manufactured to have two identical working surfaces.

Tooth is a term that describes the arrangement of small peaks and valleys that the fibers form on the surface of the paper. The degree of difference between the peaks and valleys determines how the pencil or pastel will be caught and how the graphite or color will settle on the surface.

Weight describes one aspect of a paper’s quality. The “basis weight” of a paper is based on the weight of 480 to 500 sheets (500 sheets equal one ream) of a paper in its standard size at a temperature of 75°F and a humidity of 50 percent. For example, 140 lb. watercolor paper-a typical artists’ paper-indicates that approximately 500 sheets of this paper in its standard size, 22″x30″, would weight 140 pounds. The standard size used for most print paper basis weight is 24″x36″. It is particularly difficult to compare weights of art papers to commercial print papers because of the difference in standard sizes.