preparing papers for painting
The paper or support you choose can be prepared for painting in different ways. This page primarily describes methods for stretching paper. (Methods for preparing the surface with resist or maskoid are described in the page on resists & edge control.)
evolution of the paper surface
Watercolorists come to know their papers as intimately and with as much passion as their paints (and their spouses). And they also come to realize that the paper surface changes in subtle ways as they work. Every paper has particular pathways of change that affect your working methods and the appearance of the finished work, and you can take the paper down different paths depending on how you treat it. Let's first look at these main pathways.
Wet on Dry. First is what might be called the "dry" evolution of the paper surface, and it will be familiar to painters who build up their painting with glazes or repeated layers of diluted paint.
Nearly all watercolor papers come from the manufacturer with a coating or sizing of gelatin, starch or rarely Aquapel (a synthetic glue normally used as internal sizing). This coating can be thin to moderately heavy. If you paint directly onto this dry surface, the paint vehicle primarily bonds to the sizing rind rather than the cellulose fibers of the paper, and this effect is greater in papers with heavier sizing.
If you make a mistake, you can scrape or dissolve the paint away (especially from a gelatin sized paper) without damaging the paper itself: the mistake is removed with the sizing. This lack of absorbency can also cause the paper to produce backruns more easily, as often happens on a hot pressed paper, and most painters control this problem by tilting the working surface and wicking off excess paint before backruns can form.
If the paper is lightly moistened first, the surface rind of sizing is loosened or dissolved, and the surface cellulose fibers absorb water and expand lengthwise; they contract again when the paper dries. This washes away any oils or dirt that may have strayed onto the paper surface, softens and breaks apart the sizing rind, and opens many small paper crevices underneath, creating a more complex bond with the paint. The surface cellulose fibers have also "fuzzed" slightly by this expansion and contraction (much as a wool sock comes out of the washing machine much fuzzier than it went in), and this delicate nap intertwines with the paint and produces a smoother application of diluted color. At the same time, all the paint remains on the surface of the paper, rather than sinking into the pulp, which yields the brightest possible color on that paper.
So paint, especially the diluted paint used in washes, will go down with fewer pinholes, blotches or irregularities: you especially won't have that dismal experience of running your wash brush into a heavy patch of sizing or oily smudge that repels the color and creates an unsightly white spot or a frantic episode of scrubbing the spot with the brush while the wash is still wet. However, corrections now may require some scraping away of the paper itself, as the paint is deeper in the cellulose mat and has bonded with the cellulose fibers directly, rather than the dissolvable sizing.
Many artists get the benefit of this preliminary moistening, and minimize the risk of scraping corrections, by blocking out the major shapes with a light background wash, letting the paper dry completely, then adding darker, more concentrated color to develop the established shapes further. For this layer of foundation color paints made with iron oxides (see earth pigments) are especially effective.
This preliminary wash moistens the paper surface, so that any trouble spots can be identified and remedied before heavier color applications are attempted; the wash color is light enough so that any irregularities won't be noticeable in the finished work; the wash visually establishes the shape and edges of the color area so that you can focus solely on expressive paint application (rather than correct shape and outline) when you add new layers of color. Finally, the pinholing or broken color effects possible by rapidly stroking a wicked brush across the paper become easier to produce and control, because the fuzzy nap on the paper produces a somewhat more irregular surface.
If each application of paint is allowed to dry completely before a new layer of paint is added, the new coatings of water and paint vehicle partially dissolve and mingle with the layers of sizing and paint vehicle below, but most of the paint remains on the surface of the paper. This creates an accumulation of gum and gelatin that essentially acts as a new, dense sizing layer, which holds the later applications of color on the surface. Painters usually notice that the paint is easier to apply, can be applied more accurately along the edges of existing color areas, is easier to manipulate and edit by rewetting and lifting, and is easier to remove if scraping is necessary. The main drawback is that the paper surface becomes increasingly less absorbent, so backruns are somewhat more likely to occur.
The difference in the paper surface texture between the virgin dry sheet and the sheet with many layers of color applied is very noticeable to the touch. The paper that has been repeatedly painted will have a very stiff, velourlike nap to the touch, while the virgin paper will feel relatively smooth and hard, even polished.
Wet in Wet. The "wet" evolution of the paper requires first thoroughly soaking the paper down to the pulp, then painting wet into wet with concentrated colors as it dries. This is usually done in several cycles of wetting and drying to complete a single painting.
The thorough wetting dissolves not only the surface sizing but the inner sizing as well, producing a sheet with a much more porous, receptive surface. (You can easily feel the difference by stroking your hand over this dried sheet and then over a fresh, unwetted sheet.) There is greater "fuzzing" of the paper surface because the cellulose fibers have been soaked more thoroughly and deeper into the paper core.
Much more of the paint is pulled into the paper pulp, dulling the color, but the painter can compensate by increasing the concentration of the paint and not editing or fussing with the paint after it is applied. (This would disturb the subtle effects created by the water anyway.) As a result, the amount of vehicle, pigment and sizing on the paper varies considerably across the surface of the paper, inch by inch, because more concentrated applications of paint have been applied to a deeper foundation of soaked fibers and have been allowed to diffuse outward and soak downward in a more random way. This makes later applications of wet in wet effects that depend on paper wetness harder to predict.
The main hazard with the wet in wet method comes when you try to brush on several applications of paint while the pulp is wet, which only drives paint deeper into the paper pulp and tufts up more cellulose fibers on the surface. This results in an unpleasantly lightened, dulled color when the sheet dries.
Stretching. The third way is to stretch the paper first. The normal procedures for stretching paper will soften or dissolve the surface sizing and remove surface impurities, which makes stretched papers more reliable and receptive to work on. Washes go down more evenly and easily, the brush seems to kiss the paper more intimately, and the incidental scrubbing or touching up movements that grow habitual through working on a raw paper surface disappear: brush movements become pure expressive gesture. Drybrush or pinholing effects become especially easy to control. At the same time, more of the paint in a juicy wash or thick application of paint can dive into the paper pulp.
Painters who stretch their papers are often partial to glazing their colors and are familiar with scraping their edits (because the paint goes deeper into the paper), but many wet in wet painters also stretch their papers to eliminate cockling.
If you do not stretch the paper, but still want a more receptive surface, lay the sheet flat on your table or kitchen counter, then either lightly swab the paper with a small cellulose sponge that is dripping wet with lukewarm water, or drench the paper with a hand sprayer. Let sit for a minute or two, then lift the sheet to vertical, one corner lower than the other, to drain off excess moisture, and lay flat again to dry. With either method, lay the water down evenly and wait a minute or two so that the surface sizing can partly dissolve. Never wipe the sheet firmly or repeatedly, as this will raise the fibers on the surface.
to cockle or not?
As you work with watercolor paper you get it wet. When cellulose fibers get wet, they expand, and the fibers expand more lengthwise than in girth. The expansion of each fiber is constrained by the sizing matrix around it and by random ionic bonds with other cellulose fibers. These competing forces of expansion and constraint cause the paper to warp or cockle, which is really an incomplete attempt by the sheet to return to its original arrangement of wet fibers as couched from the mould or rolled off the web. Machinemade sheets tend to cockle substantially and curl lengthwise, "remembering" the curvature of the rollers on which they were made and dried; handmade and air dried sheets tend to cockle less and in random "hills and valleys" evenly distributed over the whole sheet.
Cockling begins when the pulp is wetted, increases unpredictably as the fibers absorb water, then tends to relax and stabilize once all the fibers have completely soaked up moisture. At that point the paper pulp is very close to its original state, and the paper becomes unusually fragile and liable to tearing or sluffing.
The normal solution to the "problem" of cockling is to stretch the paper. Stretching imposes a tension on the sheet that counteracts any lengthening of the cellulose fibers through wetting.
Most books on watercoloring explain how to stretch the papers before painting, as if this were the universally accepted method. However, the questions arise: is cockling a bad thing, and if it is, should you stretch the paper to avoid it?
If you work with very small areas of paper and apply limited quantities of paint, then variations in the paper surface will be minor and transient. Your style of limited or controlled paint application also means that variations will not affect the behavior of the paint on the paper if the paper does warp the paint won't run into the valleys. Typically you also work seated at a desk, with the paper at a slant or angle, like a writing desk, as this is more comfortable for long periods. (Some "dry" artists, such as Joseph Raffael, vertically hang unstretched sheets of paper on a wall while they work.)
At the other extreme, the warping of the paper is a natural part of the painting process, and therefore in some styles of work it has expressive possibilities. Charles Demuth intentionally worked with thin sheets of unstretched paper because he found the resulting unevenness and banding in his washes interesting and exciting.
So cockling in itself isn't a bad thing, and there are other ways than stretching to avoid it.
Whatever your painting habits, it's instructive to try painting in your normal style on an unstretched sheet of paper. You will then know what you're trying to avoid by stretching the papers before working. You may find that it's not worth the effort to stretch paper in the first place, or that the resulting effect on washes and colors is attractive.
If you work with large areas of the paper and apply large quantities of washes, use repeated glazes or splashy brushwork, you typically must work with the paper flat (or close to flat) to control the flow of the water. This holds the excess moisture on the surface, where it can sink in and cockle the paper. This causes your wet, splashy style to run into the valleys when more paint is applied.
Painters who blend large color areas wet in wet, like Roland Roycraft or Nita Engle, have good reason to stretch their papers not so much to even out the flow and mixture of atmospheric background color, which is done when the paper is soaking anyway, but to prevent the resulting cockles from interfering with the work that must be done after the background has dried.
However the flow and pooling of the paint, and the uneven surface of the paper, create new visual elements in the painting variable paint densities, visible signs of water movement, blooms and blossoms in the drying paint, runs and drips. (See for example this painting by Eric Fischl.) You may find these effects are useful; with experimentation and practice you can learn to control them, rather than force them not to appear at all.
My message is that you are not painting a house or the fender on a car. Natural irregularities in your media can be pleasing and interesting to the eye. Explore the possibilities, and stretch paper only when it's necessary to meet your artistic goals and your work preferences.
how not to stretch papers
Many contemporary watercolorists work without stretching their papers. This has now become a popular approach among professional artists: it preserves the deckles on the paper and gives the sheet an individual, handmade feel. Some paper manufacturers, such as Twinrocker, even manufacture papers with enhanced deckle edges solely for decorative effect.
The common approach is to work on a heavy paper stock, usually 400 GSM or more. These papers are so heavy they cockle only slightly under heavy or repeated washes. For extreme wet in wet techniques, 640 GSM paper is thick enough to resist buckling or cockling under most circumstances. These sheets also have a robust, assertive presence that is attractive with the deckles left intact.
Heavier papers may still cockle along the grain when kept very wet. These deformations tend to cover a large area and are relatively gentle. If you work with large sheets on a painting table, door stops or the small vinyl covered exercise freeweights, placed around the area to be painted, will hold the surface flat.
Smaller papers can be clipped to the stretching board. Black metal bulldog clips (the kind that lawyers use) are very good and are available in a range of sizes at any stationery store. However, the grip on these clips is so strong that they can dent or scar a soft or wet paper, so you may want to wrap the edge in duct tape, or insert a tongue depressor or piece of cardboard under the grip, to prevent damage if the paper is extremely wet.
Art supply stores sell a nickel plated paddle clip that does not grip quite as strongly, is easier to loosen and move around, and has rounded corners to prevent scarring the paper. These work just as well to hold the sheet, but won't restrain it from buckling. The clips can be moved around as you work, even when doing very large washes.
Use a board that is only 1/2" larger than the paper on all sides, so that clips can reach the paper from all sides. Clip the paper along the top edge, at each corner, then at each lower corner from the side. This lets you rest the stretching board along the bottom edge if you need to tilt it up to do washes.
The other method, advocated by Edgar Whitney, is simply to hold the sheet to the board with water. The back of the sheet is soaked first with a spray bottle, then the board is wetted the same way and the sheet pressed against it. The water excludes air from circulating behind the sheet, so air pressure on the front of the sheet holds it fast to the board.
This method works best if the board is completely nonabsorbent and smooth: finished plywood sealed with several coats of varnish or urethane sealant, plexiglas, or (in a pinch) the smooth side of pressure treated masonite.
Depending on the thickness of the sheet and the ambient humidity, a sheet will usually remain fixed in this way for 5 or 10 minutes. Water will naturally evaporate away from the edges of the sheet, and if the board is tilted will flow to the lower edge, so the sheet may need to be rewetted before you have finished your preliminary washes or wet in wet work. No problem: just peel it up at each corner, spray the underside and board with a liberal dose of water, and flatten the corners back down again.
It's possible to work with a sheet mounted this way so that the exposed surface remains relatively dry you can lay a large sky wash before the sheet begins to buckle. However, you may find that the wash blotches because moisture creeps up from underneath the sheet. The more reliable approach is to soak the exposed surface of the sheet as well, and work with it wet in wet. This is consistent with the basic style of Whitney and his students.
Whitney's method has a fine logic to it: the worst inconvenience occurs when you try to lay an even wash or work wet in wet on cockled paper. By the time the sheet starts to dry out and cockle, you've finished the wet in wet work anyway. At that point, you can clip the edges of the sheet using the nickle art clamps. The sheet will remain cockled somewhat, but this will be less of an inconvenience because you have moved to more focused work using less moisture.
how to stretch papers
The other approach is to use any weight paper you want, but soak and stretch it when you need to: when you use large sheets or medium to light weight paper, or when you paint with heavy washes.
Some painters stretch their papers in advance and store them on the stretching boards, ready for use. In this form the papers are also very easy to transport, and you don't have to fuss with the sometimes inconvenient clips during work.
Others use watercolor blocks instead: these don't require any clips or stretching boards, because the block or mounting board lets you set the sheet in any position and still keep a firm painting surface. Blocked papers will still buckle under heavy or repeated wash applications, however, and their surface textures are not as inviting as single sheets. (I've also encountered occasional and annoying blotching in watercolor block sheets, usually due to irregularities in the surface sizing.)
Soaking the Paper. Soaking sheets is the easy part if you can find a place to do it. The two basic approaches are to immerse the sheet in water, or to soak it with a sponge or spray bottle.
If you have a tub or sink large enough, submerge the sheet in pure lukewarm water. You can try rolling the sheet to place it in a smaller tub, provided you are careful to get the rolled sheet evenly wet. I soak very large sheets on the floor of my shower, scrubbed to remove soap residue from the tiles, the drain covered with a piece of soggy cardboard.
If the bathroom won't serve, lay the sheet flat on a clean kitchen granite or formica countertop, or clean vinyl or stone floor, that has first been wetted evenly with a generous amount of water. Lay the sheet in the water, then wet the top surface as necessary with a mist bottle, sponge or large wash brush. Turn the sheet over after a few minutes to soak the other side; repeat until the sheet is saturated. If you have an area of clean pavement outside (a patio or oil free driveway), you can soak it there with a garden hose.
"Soaking" here means only that you keep the paper wet enough, by whatever method you use, so that the surface appears brightly wet all over. If the paper develops dull patches, it can absorb more water, or water has evaporated. Wet it some more.
For most 300 GSM papers, the optimal soaking time is about 5 minutes. You will need to adjust this time to suit the specific paper you are working with. The following factors will require you to increase the soaking time: a heavier weight (thickness) of the paper, a greater amount of sizing in the paper, a paper made with thoroughly macerated pulp (a paper with a brighter, more metallic rattle), colder than usual water temperature, and spraying the paper rather than immersing it in water. Lighter, unsized, soft papers, soaked by immersing in hot water, will require less soaking time.
You'll have to experiment: the longer the paper soaks and the warmer the water you use, the more the paper will expand by absorbing water, so the more it will contract as it dries. If the paper is not soaked long enough, it will continue to expand after you have fastened it to the stretching board, causing undulations or buckling in the surface when it has dried. If the paper is soaked too long, the resulting high tension will put unnecessary strain on the stretching fasteners, and may even tear the paper where it is fastened, rip up the fastening tape, or buckle the stretching board. The best tension is roughly in the middle of those two extremes.
Some authorities suggest testing the paper's wetness by gently bending a corner toward the center of the sheet. If it springs back to flat, the sheet is probably not wet enough. If it bends back to flat very slowly or remains where you flexed it, it probably has the right amount of moisture. If the corner remains folded or flops over under its own weight, then the sheet is probably too wet at that point, it may buckle, fold or tear when you try to lift it.
Prolonged soaking will dissolve the surface sizing and some of the internal sizing; the paper surface contains more small holes and crevices. Capillary action will pull the paint below the surface of the paper, dulling the finished color. Soaking also removes some of the surface alkali buffering, which can make the paper more vulnerable to acids or embrittlement over time. Moral: don't soak the paper any longer than necessary to get the results you require.
Stretching the Paper. I've discovered that artists have devoted unusual ingenuity to the problem of stretching the paper, and have come up with a number of solutions. However, all these stretching methods cover or badly damage the deckles of the sheet, which usually means you must trim them off once the painting is finished.
In the traditional method, the stretching board is about 1" larger on all sides than the dimensions of the sheet, including deckles. Hardwood plywood is best for the purpose if you use staples or tape as fasteners. Do not use masonite or the shelving particle board made with thin slivers of wood, because these may swell or dissolve under repeated or prolonged exposure to moisture. (The kind of particle board used in home construction, made with thin shaved chips of wood, is fine it's manufactured with waterproof glue.) Plexiglas is very good, provided the tape you use to fasten the sheet will stick to it.
After the sheet is soaked to your requirements, pick it up by two corners with the "good" side of the sheet facing away from you. Lay the sheet on the stretching board by aligning the top edge of the sheet about 1" from the edge of the board, then lowering it gradually toward you.
Gently smooth out any air pockets under the sheet, and any excess water under or on the sheet, by rubbing it with the side of your hand. (Remember, rubbing the wet sheet can scuff the surface, creating a roughness that will soak up paint and dull the finished color or appear as blotchy areas in washes.) Lightly blot the surface and edges with a paper towel.
Now you must work quickly. The main fastening methods are:
Paper tape. For this method you must precut 4 strips of 2" wide brown paper tape, about 2" longer than the length of each of the edges.
Lay each strip on a hard surface, glue up, and with a sponge gently moisten the glue backing of one strip at a time.
Lay the tape along one edge of the paper, covering about 1/2" of the paper edge, the rest of the tape on the board. Smooth the tape out quickly from the center to each end, then move on to the next piece of tape.
When all four pieces are in place, burnish the tape with a clean dry cloth to ensure the bond with the paper and board is secure.
Lay the board flat until the paper has thoroughly dried (usually several hours).
If the tape pulls free from the board, just moisten it and secure it again. You cannot easily remove the tape from the paper, so if the stretching fails because the tape pulls away on two sides, there is little you can do to fix it. Soak the sheet again, gently remove the remnants of tape after the glue has dissolved, and stretch the sheet once more.
When you are finished painting, cut the painting away from the board along the inner edge of the tape, using a razor blade or Xacto knife and a steel straight edge to guide the blade. (I prefer to use a large carpenter's square to get the corners at right angles.) Finally, soak the tape thoroughly with a sponge, and scrape it off the board with a putty knife.
Tacks or staples. Fasten the wet smoothed paper to the wooden board using tacks or staples from a staple gun. (Stationery staples from a desktop stapler will not be strong enough for the job.) Space the tacks or staples, depending on the size of the sheet and how long it has been soaked, about 2 to 4 inches apart along all four edges.
Some artists recommend push pins for this purpose, but you may find the rows of stubby heads an inconvenience as you paint.
I have never been able to get this method to work: the paper simply contracts away from the tacks, creating small tears all the way around the sheet. The trick is to get the paper soaking exactly right, so that the tension of the sheet doesn't reach the tearing point.
Folded edges. Several artists have written to me describing methods that secure the paper by simply folding it around the edge of the stretching board or an oil painter's canvas stretching frame. The logic is much the same as the circular clamping mechanism used to stretch needlepoint work.
Frank Sheldon wrote: "For day to day plein air painting, I use 140 pound paper stretched in one of 2 ways: (1) For quarter and half sheets I use 1/8" tempered Masonite with the paper overlapping by one inch on all sides. The wet paper is then folded over the edges of the board with gusseted corners, then secured with strips of mitered PVC strips which are friction fitted over the folded edges of the wet paper. (These PVC strips, are available from Home Depot in 8' lengths and are normally use to cap the edges of tile board in bathrooms.) (2) For half sheets and larger I use ordinary wooden canvas stretchers and staple the folded edges of the wet paper to the stretcher bars. These become tight as a drum and are a joy to paint on. You will appreciate the mystical aspects in that on hot summer days the paper becomes "soft" and on cold dry days it becomes "crisp" and snappy."
In the same vein, Ken Bromley has developed and patented The Perfect Paper Stretcher that consists of a specially designed stretching board and four edge rods. The board is grooved along its four sides; the paper is folded over the edges and the rods tamped gently into the grooves, gripping the paper.
The advantage of Sheldon's canvas stretcher method is that you can build stretcher frames to fit any size paper you use, even odd sized handmade papers. Bromley's mechanism only comes in the standard paper sizes.
Paste. Karen Guzak developed a method for pasting the sheet to a stretching board using arrowroot paste. This method required more time and care than the other stretching methods and some practice to perfect; I don't recommend it. There used to be a technical document on the method at the Daniel Smith web site, but it has been removed.
my fastening method
I've tried the usual methods for fastening wet paper brown paper tape, staples, tacks and they don't work for me. Paper tape is messy to apply and gums up the mounting board; worst of all, you must cut away the paper deckle to remove it. Staples tear holes in the paper if it shrinks too much, leave rust stains, and cause further damage when you try to pry them out with a screwdriver. Tacks have similar problems, and give a weaker grip.
Then I hit on a method that is extremely strong yet leaves minimal traces on the paper. This is to use a staple gun to drive 1/2 inch, heavy duty staples through wooden popsicle sticks placed along the edges of the sheet (right). Two sticks in each corner of the short sides, and one stick in the middle of each long side, are enough for a 22x30 sheet under moderate tension. For best results the mounting board should be a high density ("Swedish") plywood and not one side ply or particle board. The best weight seems to be 3/8" thick; thinner sizes may buckle under the strain. (Daniel Smith sells a hard and durable applewood plywood that is perfect for the job.)
The sticks distribute pressure over a larger area of the sheet and grip the paper with the texture of their grain, which eliminates tearing. Wood quickly wicks up moisture around the staples, eliminating rust stains. When it's time to pry the staples out with a screwdriver, the sticks take all the abuse caused by forcing the screwdriver under the staple. And the method requires many fewer staples, making it faster to do.
The only catch is that each stick covers a sizeable area of the sheet. As these areas are narrow and close to the edge, you will typically trim off this area of the sheet once the painting is finished. To minimize the problem, use only corner sticks along the short sides, which conserves the true deckles along the long sides of the sheet. (The "deckle" edge on the short sides of mouldmade full sheets is typically cut by a high pressure water jet or mechanically torn.) For paintings that do not require washes to extend to the edge or under the areas covered by the sticks, I've found that I can paint most of the sheet while it is firmly stapled, then pop the sticks off one at a time to finish painting the covered areas. This preserves the deckles in the finished work. If necessary, the sticks can be remounted after the paint layer has dried to hold the support to the stretching board.
how to cut or tear paper
You are not limited to the paper formats you buy in single sheet, block, or roll: you can and should prepare sheets with the best proportions for your subject.
The two ways to do this are by cutting and tearing. For either method you need a metal straight edge; cutting requires a single edged razor blade or strapping knife.
Metal straight edges come in a full range of sizes, from 1 foot to 4 feet long. Many come with a cork or rubber strip affixed to one side, which prevents the straight edge from slipping or sliding on the paper surface when pressure is applied to it. A 2 foot (24") ruler is the minimum adequate to tear full sheet papers (22" x 30") into halves or quarters. Do not buy plastic, or the wooden rules with an embedded metal strip: they aren't strong enough for the work.
To cut a sheet of paper, lay the paper face down on a completely clean table, stretching board, or other flat surface. Protect the surface from scarring with heavy cardboard, newspaper or a painting board.
Measure the cut you want to make from the edge of the paper; if the edge is deckled, measure from the base of the deckles (where the deckle frame leaves a faint edge). Mark the cut location with a pencil, near the opposite edges of the sheet, then lay the straight edge along these marks. Cut once firmly with the razor in a single stroke. (Heavy paper weights will require two or more cuts: make these while holding the straight edge in place.)
To tear a sheet of paper, mark the tear location as before and align the straight edge against these marks. Press down firmly on the straight edge with one hand, and with the other grasp the upper corner of the paper. Lift the paper up to cut the top edge of the paper with the straight edge, then pull firmly and steadily straight upwards against the straight edge. The paper should tear easily and cleanly; sharp tugging or yanking motions will provide a clean edge if the tear has a tendency to leave the edge. Shift the pressure hand down the straight edge as necessary.
This procedure works well for papers with a basis weight of 400 GSM or less. For heavier papers, follow the procedure just described but score the paper along the edge with a dull razor blade or knife. This takes practice to get right, since it depends on the weight of the paper and the keenness of the blade. Then hold the straight edge firmly along the cut as you tear up and slightly across, against the metal edge.
A third method is to first score the paper, then fold it back and forth along the score to break the fibers. Alternately, flex the sheet along the cut until it is weak enough to tear. (These tears tend to puff out somewhat and to absorb paint.)
A final method is to turn the sheet over so it is face up, and place the cut against the edge of a table, and tear by pulling down against the table edge. I find this makes it easier to hold the sheet in place, and this makes a coarser tear that can resemble a deckle edge.
where to position the wooden
You can also buy a special brand of metal straight edge, called the Art Deckle Tear Bar (right), with a irregular sharp edge along one side. This is claimed to tear a very raggedy deckle without first scoring the paper. Well, I can struggle through a deckle tear with a 300 GSM sheet, but cannot make the thing work with papers 400 GSM or heavier.
Although a straight edge or tear bar works well at first, paper is a very abrasive substance, and soon the edge isn't tearing the deckles as crisply as you're used to. As long as you are tearing lighter weights of paper, this does not seem to become much of a problem.
the Art Deckle Tear Bar