how watercolor papers are made
The technology for making paper from hemp fibers began to standardize in China around the first century BCE. The first true sheets of paper are said to have been the experimental papers made with a variety of materials bark, hemp, rag and fishnet pulp by the Chinese court eunuch Ts'ai Lun in 105 CE.
Chinese papermaking techniques migrated along the Asian silk routes in the 8th century CE, reaching Samarkind in 751 and Arab northern Africa shortly after. The Arabs innovated the wire mold and standard paper sizes before papermaking entered Moorish Spain around 1036. From Spain, paper mills were established in Fabriano, Italy around 1270, in Germany by 1320, and in England in 1495.
the paper furnish
The materials and chemical additives in a paper constitute the furnish or ingredients of the paper. All paper manufacture uses the fibrous plant material cellulose, which plants use to build the cell walls in stems and leaves. Cellulose is extracted for European and American papers from cotton, flax (linen) or (from around 1860) from wood pulp, and for traditional Asian papers from jute, kozo, salago or mitsumata. A sheet of paper is basically a thin mat of tangled cellulose fibers.
Types of Cellulose. Chemically, cellulose is a polymeric carbohydrate (C6H10O5), differing from starch or glucose only in the way the atoms of carbon, hydrogen and oxygen are linked together. The grade or quality of cellulose is determined by how much of the plant material dissolves in different chemical solutions. For example, the portion of plant material that does not dissolve in a moderate (18%) solution of sodium hydroxide at room temperature is termed alpha cellulose; the dissolved portion that solidifies (precipitates) when an acid is added to the solution is known as beta cellulose. Alpha cellulose is the most stable and permanent part of extracted plant material.
Cotton cellulose fibers are the long seed hairs removed from cotton seeds through ginning, a process of mechanical shredding and combing. These fibers are called cotton rag, as are any remaindered (used) threads, fabrics or clothing made from them. Cotton rag fibers are flexible and strong, and because they are naturally long, they produce papers that resist tearing. They are also almost 100% alpha cellulose, naturally white and already separated, which means little or no bleaching or chemical treatments are necessary.
The shorter fibers and lint left attached to the seeds after ginning are culled by crushing and boiling the pulp in an alkali solution; the separated fibers are then washed and formed into soft, blotterlike sheets called cotton linters. Linters are commonly used in the production of cellulose derived chemicals, but if washed they are also used in the manufacture of paper. Linters can replace up to one third of the rag content of paper without loss of strength, and they can improve the consistency, bulk, dimensional stability and whiteness of the sheet.
Linen cellulose fibers (from flax) are longer and stronger than cotton, which makes linen papers harder and more translucent. As with cotton, either the unspun flax fibers or discarded linen threads or fabrics can be used in paper manufacture.
By far the most common source of cellulose in machinemade papers is wood pulp. Wood from domestic hardwoods (including eucalyptus) provides short fibers with good bulk; wood from coniferous softwoods (pine or cedar) provides longer fibers for paper strength. Cellulose is extracted from wood pulp using either mechanical or chemical methods. The mechanical processes involve successive cutting, grinding, soaking and screening of wood chips or sawdust, which is then bleached in a sulfite or peroxide solution. This results in a coarse, brownish paper commonly used in wrapping papers, packaging, newsprint and paperboard.
Mechanical methods can leave a considerable residue of lignin, a kind of glue that binds together the cellulose fibers of a living plant and represents up to 30% of the bulk of wood pulp. Lignin repels water, causes clumping in the paper pulp during manufacture, and becomes acidic and turns a yellow or brown color with age, so it is necessary to remove it completely in quality paper production. There are various chemical methods of cellulose extraction, but most involve cooking the wood chips in an acid (sulfite) or alkaline (sulfate) bath, which dissolves the lignin so that it can be washed away.
Quality Designations of Paper. Several different designations are used to describe the quality of cellulose in paper, and these can be somewhat misleading.
At the very top of the quality scale, the term 100% cotton affirms that the paper is made entirely of cotton (usually from both linters and rag) and includes no linen or wood cellulose.
The term rag paper only means that some rag content is included in the paper, often mixed with linters or wood cellulose. (The label can also refer to papers made with a cotton and linen mix.) In fact, the amount of cotton cellulose in rag paper may be anywhere from 100% to as little as 20%! Gresham's law ensures that "rag paper" is therefore almost always of lower cotton content than 100% cotton, though this quality difference may not be noticeable or important for artistic uses.
Finally, wood pulp treated chemically is called woodfree paper or wood sulfite paper. The highest grade of wood pulp paper (which may contain as much as 93% cellulose) is labeled high alpha cellulose. Confusingly, this is often abbreviated to alpha cellulose in trade publications, where it still denotes cellulose derived from wood even though (as described above) alpha cellulose actually refers to "pure" cellulose of any origin.
Cotton cellulose is up to 10 times stronger than wood cellulose and naturally lignin free and acid free. Some residual lignin and chemicals remain in chemically extracted wood cellulose, which cause embrittlement and acidification over time. For this reason, wood pulp ("alpha cellulose") papers should generally be avoided for archival or museum quality artwork. Papers made from 100% cotton, 100% linen, or pure cotton/linen rag are all suitable for artistic use.
Many seasoned artists and art dealers swear the quality of most watercolor papers has declined noticeably since the 1960's, and some attribute it to a reduction in the linen content. However, the shift to cotton has also followed painters' preferences: linen creates a harder and less absorbent paper than cotton, which makes smooth washes trickier to execute; and because of the much longer fiber length, 100% linen papers tend to cockle or warp excessively when wet.
Acidity of Paper. The ideal paper should last for centuries under normal storage conditions, and papers that can meet this standard are referred to as archival quality.
Acids are the most pervasive and destructive hazard to paper permenancy. Acidic papers such as newsprint can embrittle and discolor in a very short time, can degrade acid sensitive pigments such as ultramarine blue, and can release acidic vapors that will degrade papers or mats placed next to them.
The pH scale, used in chemistry to measure acidity or alkalinity, is neutral at around 7.5; pure water has a pH value of 7, and a substance is considered acidic at a pH value of 5 or lower and alkalinic at values of 8 or more.
All newsprint and most magazine papers are made from wood that has been mechanically pulverized to the desired consistency, a process called mechanical pulping. However fine book and art papers are made from shredded wood that has been digested or broken down through wet mixture under heat and pressure with enzymes or pulping chemicals, which is called chemical pulping. The main choice is between the alkalescent kraft or sulphate process, or the sulphite process (which can vary from acid to alkaline). When carefully controlled, these processes break down the lignin content of the wood without significantly affecting the cellulose fibers. Enzymes are also used to increase the effectiveness of the bleaching agents (preferably chlorine dioxide or hydrogen peroxide) used to whiten the lignen residues, and to dissolve the ink residues that contaminate recycled paper stocks.
Thoroughly rinsing away all the chemicals used to pulp and bleach wood cellulose requires a significant amount of water, so pulp mills reduce water demand by chemically neutralizing the pulp during the final steps of manufacture. This is done by adding a sufficient amount of a base (such as calcium carbonate) to the paper furnish, creating papers that are usually designated pH neutral.
In contrast, the designation acid free usually means the paper furnish was made using only cotton linter, cotton rag or linen rag; the pulp was not chemically bleached during manufacture; and the paper contains no rosin or aluminum sulfate (alum) as sizing.
Neutralizing an acidic pulp is separate from the practice at paper mills of adding the neutralizing chemical to create an alkali reserve that counteracts any acidic pollutants the papers may be exposed to in the future from the painting process, cheap mat boards, or the atmosphere. In fact, an alkali reserve added to acid free paper is a good thing and is mandated by many archival standards. Paradoxically, if they are not adjusted at all, additive free papers can have a slightly lower (more acidic) pH than chemically pulped "pH neutral" papers, so "pure cellulose" by itself is no guarantee of archival quality. In addition, it's now believed that the more buffering there is in the paper the better, and an adequate calcium carbonate content can even protect against lignin degredation. So pH neutral papers are less desirable than pH alkalinic papers (up to pH 10).
As far as I can discover, chemically neutralized papers are believed to be less stable than papers that were never exposed to harsh chemicals in the first place. The most prudent course for watercolor painters is to prefer the explicit designation of acid free, buffered, 100% cotton papers to any other kind, and to prefer pH neutral cotton or linen papers to any "pH neutral, high alpha cellulose" or wood pulp papers. The label "100% cotton" by itself is no assurance either way, as recycled fabrics may be harshly bleached before use in paper pulp.
Chemical tests for pH are somewhat inconvenient but not difficult to do by the artist at home, and it is not unheard of for libraries to discover through their own pH testing that papers have a higher acidity than was guaranteed by the manufacturer. In addition, burning a small piece of your watercolor paper can be revealing. Ash from pure cellulose paper is white, fragile and wispy. Ash that is stiff, black or brittle indicates the presence of residual lignin, noncellulose (synthetic) fibers picked up from recycled consumer clothing, and/or large amounts of chemicals added as sizing or buffering.
The global population of large apes manufactures and consumes roughly 300 million tons of paper annually, under rising economic and resource constraints and demand that increases at about 3% per year. This dictates continuous innovation in manufacturing processes, recycling and productivity, which will only increase the diversity and complexity of modern paper stocks.
It's worth noting that handmade papers were traditionally bleached without chemicals by exposing the sheets to sunlight, just as linens are bleached by hanging them to dry on a clothesline; buffering was introduced by pulping and rinsing with "hard" spring water from limestone aquafers and by coating the papers with milk. Optical Brightening Agents or OBAs. It is not usually possible to assess the furnish of a paper from its color alone. The color of a sheet is influenced by the quality and amount of internal sizing, the purity of water used in manufacture, and the temperature and amount of time that the pulp was cooked.
The wood cellulose fibers used in paper have a natural yellow tint that is partially bleached out during manufacturing, giving the paper a warm but slightly dull finished color. To counteract this, many paper brands add a "bluing" agent to the paper, which nowadays are ultraviolet dyes that absorb UV light and fluoresce in the visible (violet and blue) wavelengths. The presence of OBAs can usually be confirmed by examining the sheet under an ultraviolet ("black") light: it will appear much brighter than an ordinary sheet of typing paper.
The consensus is that that OBAs impair paper permanency: they break down over time and can cause patchy yellowing or increased acidity in the paper.
In the USA, pressure from librarians resulted in standards for paper permanency, described in the ANSI/NISO Z39.48-1992 Permanent Paper Standard. Under this rule, fine art or archival quality papers should have a neutral to alkaline pH (7.5 or greater), be made entirely of high alpha cellulose (containing less than 1% lignin) or preferably 100% "rag" fibers, contain a reserve of calcium carbonate or equivalent buffer of 2.0% or more of total pulp mass, and contain no optical brighteners. (Other specifications include the paper's resistance to tearing.) Based on tests conducted by the ASTM, these papers have an estimated life expectancy of 500 years or more.
how paper is made
Whatever source of plant cellulose is used, the raw pulp is rinsed, screened, sometimes bleached, and mixed with water and pH adjusting chemicals to form a mulchy goop. Each batch of pulp and other ingredients, and all the papers that are made from it, represent a single making.
Manufacturing Methods. The pulp is thoroughly pounded to macerate and separate the individual cellulose fibers. Traditionally this was done by pounding the pulp in a stone trough with a large wooden pile or hammer; modern shops use a Hollander beater. This pounding bruises, cuts, folds and tangles the fibers in ways that alter the finished paper's character.
the tangle of cellulose fibers in a sheet of paper
As the pulp is beaten longer the finished pulp becomes denser and less porous: as a result the paper's "rattle" becomes brighter and more metallic, the paper translucency increases, the paper absorbs more water and cockles more extensively; the stretch resistance or tensile strength of the paper increases but paradoxically it also becomes easier to tear. Different types of fibers or raw pulp may be mixed in a making to adjust the qualities of the paper being made.
After the pulp has been thoroughly macerated, it is diluted with roughly nine times its bulk in water. Various chemicals, fillers and binders are added at this stage and the soup is energetically stirred in a vat, which may be as small as a bathtub or as large as a swimming pool. This milky liquid is the pulp actually drawn off in papermaking.
Most commercial art papers are manufactured by machine, though the term machinemade applies specifically to papers made with a Fourdrinier machine, which injects the pulp onto a running wire mesh, or between two meshes (depending on the machine type). The water is drained from the mesh, then the wet sheet is wicked between two felts and finally dried against heated cylinders. The finished sheet is extruded in a continuous roll or web. The web is torn or cut into sheets which are mechanically dried. A Fourdrinier machine can produce upwards of 23,000 sheets of paper in an hour.
Moldmade papers are manufactured with a cylinder mold machine. The pulp is poured over wire mesh cylinders operating at very low speed. The pulp adheres to the rotating cylinders in a continuous thin sheet. This web is pressed into a belt of wet felt, which lifts the web of paper away from the screen. A second felt is placed on top of the sheet and this sandwich is passed through a series of rollers that extrude the water and dry the sheet with forced air. Finally the bare sheet is run through a stack of heavy iron rollers (called a calender) that refines the surface texture. A cylinder mold machine can make about 200 sheets in an hour.
The variety and novelty of handmade papers has increased, part of a resurgence of interest in the papermaking craft. In America, this was due largely to the influence of artist, papermaker and historian Dard Hunter (1883-1966), and to the preservationist efforts of small, exclusively handmaking mills such as Dieu Donné (New York), Larroque-Duchêne (France), Twinrocker (Illinois), and St. Armand (Montréal).
Handmade papers are also made with a mold, though not the cylindrical kind. Usually the processes of pounding and mixing the pulp are also done by hand. The mold is an open rectangular frame slightly larger than the dimensions of the finished sheet, with a woven or laid wire screen stretched across the top. A second frame, the deckle, is placed over the screen to define the sheet's horizontal dimensions and (by the depth of the deckle) the paper's thickness. The vatman dips the entire frame into the pulp vat and by "throwing the weave" sloshes any excess pulp off the mold, then spreads the pulp into an even sheet by gentle horizontal shaking. This hand spreading of the pulp imparts many tiny variations to the paper surface and a slightly irregular thickness to the sheet, especially along the edges where excess pulp is cast away.
The sheet drains in place, and when it is firm enough the deckle is removed and the couchman inverts the frame on a thin sheet of moist felt, dislodging or couching (pronounced "cooching") the paper onto the felt. About 80 sheets of handmade paper can be shaken and couched in an hour; drying can take days or weeks.
Drying the Sheets. Methods of drying vary, and the different methods affect both the paper finish or surface texture, and its dimensional stability (resistance to cockling) when wet.
As explained by Sylvie Turner, a freshly couched sheet of paper contains water trapped between and absorbed inside the cellulose fibers. The water between the fibers can be mechanically pressed or blotted out, or evaporates early in the paper drying. As the remaining water inside the cellulose fibers evaporates, the fibers shrink along their length and form hydrogen bonds where they contact other fibers. This causes the sheet to cockle or distort as it dries, which can be minimized by slowing the drying process or restraining the sheet as it dries.
Traditionally, the highest quality handmade or moldmade sheets were hung individually over poles or horsehair ropes in specially built lofts that encouraged a gentle flow of clean air (loft drying); this is still done today for the highest quality handmade or moldmade papers. Because they dry unrestrained, these sheets can have a beautifully textured and unique surface. More often, a half dozen or so sheets of paper are gathered in spurs that are hung vertically or laid horizontally on racks or stretched canvas tarps (called "sails"), which tends to produce a slightly smoother finish. After two or more weeks of drying, the papers are stacked under weighted boards and allowed to stretch out flat.
Because these sheets have been allowed to dry slowly, the gradual shrinkage of the cellulose fibers can balance out across the entire sheet, which produces a puckering around naturally occurring irregularities in the pulp that results in a unique, minutely pebbled finish. These sheets tend to regain their original cockles when rewetted.
The more common and economical industrial method is restraint drying the sheet: removing water from the cellulose fibers while the sheet is held flat. The ancient methods were to let the sheet dry in the mold, or to spread the sheet against a wall or on a board that was left to dry in the sun (the traditional method in Japan). Various other methods exist in modern industrial production of book papers and less expensive grades of art papers. These include using a mechanical press to express water from a post or very large stack of paper sheets alternating with thin felts, or calendering the paper web between heated metal rollers.
The Deckle. Handmade papers will usually have a deckle edge on all four sides, a feathery thinning of the sheet caused when small amounts of pulp flow between the mold and deckle. These (and the mealy or pulpy variations in the sheet thickness and texture) create a unique material artifact, a presence that is very attractive in paintings that emphasize the object's surface rather than a representational illusion. However, this is a matter of taste: throughout the 19th and much of the 20th century, painters considered the deckle a manufacturing defect and always trimmed it away, usually when cutting the sheet from the board on which the paper had been stretched for painting. As machine manufacture became dominant in the 19th century, the deckle acquired a crafts cachet. Today many watercolor painters retain it as the sign of a handmade product.
Well, not quite. Deckle edges are sometimes simulated in moldmade papers by cutting or trimming a larger sheet with a dull blade or jet of pressurized water, which produces a frayed, irregular edge. Moldmade papers often have two genuine deckles on the long sides of the web, and smaller imitation deckle edges on the short sides where the sheet has been cut lengthwise from the web.
A large amount of water is required to manufacture paper. The quality of the paper is heavily dependent on the purity and alkalinity of the water. The best mills are always located near copious fresh water supplies, and frequently refresh the water in use.
The best way to learn about paper is to make some yourself! You can buy a papermaking kit with all the supplies and equipment included, or by reading books such as Arnold Grummer's Complete Guide to Easy Papermaking. You can also explore online at the Robert C. Williams American Museum of Papermaking, located in Atlanta, Georgia (USA); or through many online resources such as the site by Hand Papermaking, Inc. in Washington, D.C. (USA).
There are two types of paper texture, created by the weave of the wire screen in the mold.
The more primitive laid texture results when the screen is formed of closely spaced parallel wires that are held in place by more widely spaced transverse wires (or crosswires) woven between them (image right); this gives the sheet a characteristic ribbed or corduroy appearance. The wove texture, invented in England around 1755, results from wires woven evenly in both directions (like a window screen, image below). This gives the sheet a uniform texture in all dimensions, whether it is rough or smooth.
The felt side of the sheet faced away from the mold and only received texture from the felts used in rollers or drying. The wire side of the sheet settled onto the wire screen of the mold, and received texture both from the mold and any felts used in rollers or drying.
Textures often differ between the two sides; the quality and amount of difference depends on the type of papermaking machine, the quality of the pulp, and the drying process. The wire side usually has a more assertive and complex texture (both the screen and felt textures are visible), is more consistently flat (because the pulp settles against the taut screen), and also reveals any inclusions (impurities or bits of decorative fiber) that are heavier than the pulp and sink toward the wire during draining.
The Watermark. The watermark is a visible mark in the sheet, usually the name or an identifying symbol of the paper manufacturer, most easily seen by holding the sheet up to the light. Watermarks were introduced by the Fabriano Mill (Italy) in around 1282, and rapidly became a way for papermakers to assert the authenticity and quality of manufacture of their product, and to signal specific paper sizes, furnishes (100% cotton), customs declarations, and so on.
In handmade papers the watermark is usually created by a small copper wire image fixed to the wire screen of the mold (image at right); this creates a slight thinness in the pulp directly over the pattern, which appears as a more transparent area in the finished sheet. In machine moldmade papers the watermark is typically embossed on the still wet sheet with a rubber roller.
Some manufacturers emboss the sheet with a logo or trademark, instead of or in addition to using a watermark. This chopmark is usually small and placed unobtrusively along the margin of the sheet, as watercolor paints will not cover it.
Painters are usually taught that the watermark (and chopmark, if present) in handmade sheets is historically oriented to "read right" (is legible rather than mirror reversed) from the felt side of the sheet, considered the better side to paint on because the finish is often more consistent and inclusions (pulp impurities) show less readily (they usually sink to the wire side). But some manufacturers especially in England place the watermark or chop to read correctly from the wire side, which makes the manufacturer lettering less obtrusive to the eye. They did this because 19th century papermakers would sometimes shave away lint or clumps of pulp that protruded on the felt side of the sheet, and these cuts damaged the finish and surface sizing and would show up under washes. As a result the wire side became the "right" side to use.
This concern is no longer relevant, as paper sheets are no longer trimmed in that way. In nearly all modern papers, the quality, sizing and handling of the felt and wire sides are essentially the same; in fact, the texture of the felt side may be more desirable in some sheets.
The watermark orientation varies from manufacturer to manufacturer, sometimes even within the same manufacturer. For example, I have before me two sheets of Arches CP 640GSM watercolor paper: in the full sheet (22" x30") the watermark "reads right" from the felt side (opposite the watermark embossing), while in the double elephant sheet (30" x 40") the watermark "reads right" from the wire side! To make matters worse, the rubber roller used to emboss the watermark on moldmade sheets may be placed on the felt side (above the web), not on the wire side, so you cannot use the watermark indentation as a reliable guide.
However, on one side of the sheet the watermark will appear as a shallow embossing or indentation in the paper surface: that is usually the wire side. As a rule, you should put the watermark indentation on the back of the sheet, because watercolor paint will often make it more visible.
The most reliable method is to examine the watermark from both sides of the sheet, then evaluate the finish on both sides for surface texture and visible impurities. Then use the side you prefer. There is incidentally no reason at all to discard a sheet if you've botched a painting on the wire side, because both sides are usable: just flip the paper over, whistle a happy tune, and get cracking.
The Finish. Watercolor papers are supplied in three types of finish or surface texture: rough, cold pressed, and hot pressed. The finish is affected both by the mold screen and the texture of the felts or rollers used (or not used) in drying the paper.
Rough watercolor papers are dried as they are couched from the mold. Handmade papers are dried without pressing; the pebbly rough texture results from the shrinking of the paper around the natural irregularities in the pulp. (For this reason, the texture becomes rougher in thicker, heavier weight sheets.) Moldmade papers are pressed between rough felts on the paper machine as the first step in drying, which embosses the roughness of the felt fabric into the wet sheet. Because they have not been compressed, rough sheets often expand and cockle more than other types of finishes when wet.
Rough sheets are usually the most absorbent, and therefore produce the smoothest washes even with granulating pigments. However, if a juicy wash solution is applied thickly and the sheet is "rocked" or swirled slightly, the pigment will settle into the hollows of the paper, creating one of the most characteristic, expressive watercolor pigment textures. And texturing can also be produced with a brush charged with dry or thick paint lightly or quickly stroked over the surface: paint will only color the "peaks" of the paper, leaving sparkling white pinholes in the "valleys."
Cold pressed paper (in the UK called "NOT" paper, meaning "not hot pressed") is made by hanging the sheets to loft dry in spurs (clusters) or by pressing the posts under a mechanical press (for handmade papers), or by calendering the sheet with light pressure through felt covered metal rollers (moldmade papers). This cold pressing gives the sheet a subdued texture that is relatively easy to use, can tolerate a certain amount of corrections or lifting, and is congenial to almost any style of painting. The sheets are more dimensionally stable when wet, and (depending on manufacturer) are usually less absorbent than rough sheets.
Hot pressed sheets (both moldmade and handmade) are calendered at high pressure between heated glazing rollers or highly polished cold metal rollers, which creates a smooth, almost polished finish. These sheets show a high degree of brush detail and tend to show pigment color more brightly: the surface sizing and fiber density resist paint absorption so more of the paint stays on the surface; the drawback is that the lack of absorption tends to produce uneven and blotchy washes (especially in unstretched sheets that can cockle or buckle), and to amplify even minor variations in pigment granularity or flocculation. Hot pressed papers are often the most congenial to lifting dried paint by moistening and blotting, though the smooth surface reveals any abrasions more clearly. These papers are especially suited to painting styles that want to accent the watery irregularities of the paint, or styles (such as botanical illustration) where precise pen and ink outlines or drawings, or fine brush textures, are essential to the desired effect.
Sizing. In addition to these mechanical variations in surface texture, watercolor paper is also chemically treated to alter its absorbency. Internal sizing is added to the paper pulp before the sheet is formed and chemically bonds to the paper fibers. External sizing is applied to the surface of the finished sheet of paper after it has dried, sometimes by dipping the entire sheet into a tub of sizing solution (known as tub sizing). (Waterleaf papers are unsized.)
In the 19th century, a pine rosin sizing was added to the paper pulp to reduce the capillary action of the fibers and prevent inks or paints from blotting when applied; this also kept watercolors on the surface of the paper, making them appear as bright as possible. However, acidic chemicals had to be added to the pulp to bind the rosin to the cellulose, and all rosins yellow with age.
Animal gelatin sizing was (and still is) the preferred external sizing: it is naturally transparent, slightly water soluble, gives a hard surface that can be scraped or sponged away without damaging the paper itself, and imparts a warm tint to the sheet. However, gelatin is not the most common internal sizing for watercolor papers for this there are modern substitutes often used instead of animal gelatin sizing, such as carboxymethylcellulose (a common food thickener derived from cellulose, trade name Aquaplast) or alkyl-ketene dimers (AKDs or "dimers", trade name Aquapel). These add qualities of external sizing to a sheet but gelatin sized sheets are preferred by many artists and gelatin usually mentioned by name whenever it is used.
a laid screen (with part of the watermark visible near the edge)
a wove screen
The paper finish varies widely from one manufacturer to the next; papers labeled "rough" in one brand line may be equivalent to a "cold pressed" in another line. In general heavier weight papers have a more assertive finish than lighter papers, and handmade papers have a more assertive, irregular finish than moldmade papers. (Fabriano Uno papers also come in a soft pressed surface, with a finish midway between hot and cold pressed.)
Some manufacturing methods will align the paper fibers in the same direction, and these biases form the grain of the paper. In machinemade and to a lesser degree in some moldmade papers, the grain runs the length of the web (parallel to the deckle and perpendicular to the path of molds or rollers). There is no grain in handmade or roughshake papers (the alignment of the fibers is random). Papers are easier to tear or fold in the direction parallel to rather than across the grain. Cellulose fibers expand in width when wet, which causes machinemade papers to return to the curl of the mold cylinder that made them, or to produce cockles that lie in parallel ribs; in most moldmade and all handmade papers, the pattern of cockling is random.
Because variations in paper manufacture can produce variations in the dimensions of finished sheets, the British Imperial system of paper dimensions and weights standardized existing paper formats in 1836. Since then artists have referred to paper sheets by the Imperial standard names:
To assist your visualization of the various sizes and proportions, the dimensions of these sheets are shown below.
common watercolor paper formats in the
Larger sheets, up to 30' wide and 4' high, can be cut from watercolor paper rolls, and smaller sheets can be torn from any convenient sized larger sheet. Some artists prefer to paint on these smaller torn formats, as the tear makes an interesting contrast to the deckle.
The ratio of long to short dimensions in watercolor sheets varies widely. The super royal and USA "double elephant" sheets are close to 1 to 1.41; when a sheet in this ratio is torn in half across its width, the proportion of the half sheets is again 1 to 1.41. (Only the USA "single elephant" sheet is close to the "golden section" dimensions of 1 to 1.618.)
In general, the sheets with a higher ratio of length to width (the antiquarian at 1.71, the double elephant at 1.52 and the half sheet at 1.47) are especially pleasing in landscape format for landscape paintings and reclining nudes or figure portraits, or in portrait format for standing nudes or figure portraits. The sheets with lower ratios (the royal at 1.25 and the demi at 1.29) are more effective in portrait orientation for bust or head portraits and in landscape orientation for still lifes. The beautifully proportioned imperial sheet or full sheet (22" x 30", a ratio of 1.36) strikes a balance between those two extremes, and it has been the standard large format watercolor for almost two centuries.
Smaller sheets are often created by tearing the full sheet across its width to produce the half sheet (15" x 22"), and if needed torn again to produce the quarter sheet (11" x 15"). Note that the imperial sheet is itself roughly a quarter sheet of the huge emperor sheet (called "double elephant" in some watercolor books I have seen).
In the manufacture of machinemade papers and mass printing trades, a new metric system is now standard, based on the A0 sheet (84 x 119cm, or 33" x 47", a ratio of 1 to 1.41) and successive half sheets from it (designated A1, A2, A3, etc.), or the B0 sheet (100 x 141cm, or 40" x 56", also a ratio of 1 to 1.41) and successive half sheets (designated B1, B2, etc.).
In the manufacture of artist papers, particularly handmade papers, the traditional Imperial system remains common, especially for the half sheet, full sheet and USA double elephant. Sylvie Turner comments that one reason for the enduring popularity of the Imperial sizes is that they seem ideally suited for handmade production methods in papermaking, painting and printing rather than machine production. Unfortunately, even reliable authors give different dimensions for the different Imperial sizes (the "elephant" series is especially muddled), perhaps because variations in manufacturing methods or an increasing variety of paper sizes have outstripped the standards. The artists' confusion is likely to get worse before it gets better.
All the art suppliers I know of list watercolor sheets by actual dimensions rather than by the Imperial standard names. And many artists I have spoken with respond to terms like "double elephant" with an amused or uncomprehending look. The prevailing workshop and retail practice is simply to give the sheet dimensions, in inches or centimeters.
weights of paper
More confusion arises in measures of the basis weight or thickness of the paper. The lightest art papers are translucent, while the heaviest papers are as stiff as finely milled sheets of wood the rattle sounds as bright as sheet metal! The problem is finding a way to describe these differences.
The traditional specification (from the British Imperial system) has been the weight in pounds of a ream (500 sheets) of the paper. The imperial or full sheet (22" x 30") was taken as the standard size to define the weight of smaller sheets, as smaller sheets were typically torn from it. (The imperial equivalent weight is still used to describe the paper used in watercolor blocks, for example.) Unfortunately larger sheets can make this measure confusing if not useless, because these are also measured by the ream in their actual dimensions rather than scaled to approximate the imperial size. So a sheet of 1114 lb. 40" x 60" paper is exactly the same basis weight as a sheet of 300 lb. 22" x 30" paper!
The new and improved metric method is to measure the weight (in grams) of a single sheet of paper calculated to be exactly one meter square (grams per square meter or GSM). The following weights are most common:
Watercolor papers are becoming more diverse in size and weight, not less so. This means the traditional "by the ream" measures will become even more confusing to use they will be useful to distinguish between sheets from a single manufacturer, not to compare sheets between different manufactuerers. For that purpose, learn to think in terms of the metric weights, and insist on conversion to that standard when the antiquated Imperial system is offered in its place.
how paper is sold
Handmade watercolor papers are sold by the individual sheet. Catalog art dealers normally require a minimum number of sheets per shipment, and typically give discounts on quantities of 25 sheets or more. The going price for a handmade 300 GSM full sheet is anywhere from US$5 to US$20 or more. (When pricing papers, keep in mind that handmade sheets often come in unusual sizes and weights.)
Machinemade or moldmade papers can be purchased as individual sheets or in packs, rolls, or watercolor blocks.
Watercolor rolls are typically about 10 yards long and, depending on manufacturer, come in widths from about 40" up to about 60". The wire side usually faces toward the spool, which means the edges should curl downwards if you want to paint on the felt side. The artist can get about twelve 29" x 40" sheets, or six 40" x 60" sheets from one 43" wide roll. Dollars per inch, rolls are usually the most economical form of paper packaging, which makes them good for classroom use (a single roll can be cut or torn into smaller sheets). They are also the obvious choice for paintings in unusual sizes, or for creating very large works several feet long. A 43" wide 10 yard roll of 300 GSM paper costs anywhere from US$30 to US$100, depending on the manufacturer.
Individual Sheets are commonly available from all retailers in the widest range of sizes and weights and in any quantity. These are usually wrapped for shipping in brown paper inside a carton. Slit one side of the wrapping to pull out individual sheets, but leave the rest in the wrapping to protect the papers against moisture and dirt. Most retailers provide quantity discounts, and shipping charges are tacked onto every delivery, so it's most economical to order the largest numbers of sheets you can afford.
Plastic Packs are normally provided for the larger sizes of watercolor papers full sheet and above in counts of 5, 10 or 25 sheets (depending on manufacturer). This is usually less economical than ordering a larger number of individual sheets. The price of a 10 pack of moldmade, 300 GSM full sheet watercolor papers is usually around US$30.
Watercolor blocks are for paper in sizes smaller than the full sheet from 18" x 24" down to postcard size. These are made of a stack 20 sheets (usually only in 185 GSM or 300 GSM weights) that are bound together to a very heavy paper board backing by a thick adhesive applied around all four sides. A heavy sheet of cover paper, printed with the manufacturer's usual marketing gaudiness, is folded over the front to protect the exposed top sheet.
Paintings are done with the paper fastened to the block, which eliminates stretching and fastening a single sheet on a board support. (Block papers are not stretched, but fairly well resist cockling or warping when wet, at least in the 300 GSM weight.) When the painting is finished, the artist inserts a blade or palette knife under the top sheet at an edge area where the sheets are left ungummed, then runs the blade around all four sides to cut the sheet away, exposing a fresh sheet underneath.
Blocks transport well and are really ideal for field painting. The main drawback is that the cold pressed or rough sheets lack the character of individual sheets: the textures are smoother, less irregular, more flattened, in order to make the blocking uniform. Blocked sheets also do not have deckle edges, though these can be simulated by tearing after the painting is finished.
You pay approximately a 30% premium for this convenient and durable packaging. A typical 20 sheet block of 10" x 14" 300 GSM paper costs about US$20. A pack of 5 full sheets of the same paper from the same manufacturer (which will tear into 20 11" x 15" quarter sheets) would cost around US$15. In general the largest blocks are the most economically priced, pound for pound.
Sketchbooks come in a variety of sizes and formats. A perennial favorite is the "Basic" brand with the word "sketchbook" printed in rows of blue letters across the softcover binding; it contains 144 sheets of white heavy wove paper. Another artstore familiar is the black hardcover sketchbook with unruled white pages in a range of formats. Punjab makes a romantic if somewhat impractical watercolor sketchbook of handmade watercolor papers and a sewn binding under brown handmade hardcovers. Strathmore makes a range of spiral bound watercolor sketch pads. And so on.
I enthusiastically pass on David Dewey's recommendation of the NY Central Art Supply custom sketchbooks. These are a little pricey ($45 for a 48 page 8"x12") but well worth it: bound in durable, natural (beige) linen covers with dark green endpapers, the Arches CP 300 GSM text laid paper is a receptive, resilient ivory surface for pencil, pen or watercolor sketching. They are listed in index to the fine papers catalog under "Arches text laid sketchbooks," or ask the staff for the product by name.
Holbein makes an elegant if somewhat fussy small (16cm x 23cm) "Clester" sketchbook of CP 300 GSM watercolor paper, perfect bound (that is, with glue) under stiff green covers with a ribbon clasp. I bought one as a novelty item and never used it.
Sketchbooks or sketch pads are one of the most valuable tools you will buy. They can assume the importance and personal value of a diary or an experimental journal. It's worth your time to find a really good art store with a large selection: go there and browse until you find something that genuinely appeals to you.
The last trick is to use the sketchbook. Make it your own, fit it into the habits of each day, and you'll find it will yield you unexpected insights and accelerate the progress of your art.