Screen process printing
A screenprint, or serigraph, is a print made by pushing ink through a screen. It is basically a stencil technique. The number of prints in an edition is limited, as is true of lithographs, etchings, woodcuts and other printmaking techniques. The number of each print in the edition is signified by a number such as 18/20 which means that this particular copy is the 18th of an edition limited to twenty prints.
The use of a screen with silk bolting cloth as a stencil carrier was developed in Europe in the early 20th Century. The method most widely used today is essentially the same though more refined. The screen is a frame of wood or metal with silk cloth or other material stretched drum-tight over it. Nylon and polyester monofilament material are at this time preferred over silk by most screenprinters. These synthetic fibers are tougher, longer lasting, easier to clean, do not absorb ink resins and fats, can be reclaimed after using with photo emulsion, and they print more sharply than silk. Organdy, Dacron and stainless steel mesh can also be used.
The stencil is either attached to or formed on the screen. The screen is then hinged to a printing support, usually a table, which has register tabs to position each sheet of paper correctly. Then ink is pushed through from the top side with a special squeegee onto the printing paper underneath.
Although screenprinting is in principle a simple stencil process, the kinds of images which can be produced cover a wide spectrum. Depending on the stencil used, the artist can produce a range of effects - from broad, simple areas to fine, detailed, even photographic images. This wide range is broadened further by the use of various combinations of transparent and opaque colors, and by printing on various kinds of paper.
The stencil carried on the screen is the heart of the process since it determines the character of the images printed onto the paper. There are several methods of preparing a stencil, five of which are mentioned here. The stencil must be made of a material that will not be damaged, distorted or dissolved by the ink and must be durable enough to withstand abrasion and flexing during printing.
The paper stencil is probably the simplest. It is nothing more than a thin mask of paper or other thin material such as mylar or acetate which is attached to the underside of the screen; ink squeegeed across the screen will pass through the open areas of the mask and be stopped or blocked from the not-to-be-printed areas by the paper mask. Color areas printed through the paper stencil are generally flat and bold. Edge quality of these areas can be manipulated by the manner in which the stencil is made. The stencil can be cut with a sharp blade to produce crisp edges and sharp detail, or the edges can be torn, burnt or abraded. The stencil can also be punctured to allow ink to pass through the openings or it can be worn thin with sandpaper or a razor blade to allow ink to seep through as a light film. (Of course, these operations are performed on the paper stencil before it is attached to the screen.)
A second kind of stencil is profilm, or hand-cut film. Profilm is a very thin but tough material layered onto a transparent and flexible plastic sheet. Profilm is available in a wide range of colors, all transparent, to fit various uses and may be either water soluble or of the lacquer type, but only the latter may be used with water-based inks. It is used as follows: the profilm is taped over the master drawing so that it lies flat and doesn't shift. A sharp stencil- cutting knife is used to cut through the film (but not the backing) and the areas which are outlined with the knife, i.e., those through which ink is to pass, are lifted away or "stripped." In principle it is similar to the cut paper stencil, but it can produce images of much greater detail and precision. After the areas which are to print have been cut and stripped, the rest of the stencil remains attached to the plastic backing sheet. It is then placed under the screen and adhered to the fabric with the proper solvent. When dry, the backing support is peeled away, leaving the stencil firmly attached to the screen.
The third type of stencil, the negative liquid stencil, is the most direct. The stencil consists of a thin layer of a liquid blockout applied directly to the screen; when dry, the stencil is ready to print. There are no intermediate steps such as cutting or washing out. The blockout can be brushed onto the screen crisply or loosely, with full brush or "dry" brush. It can be sprayed, dabbed, or dripped on or it can be applied with paper, cloth or any other "tool." Alternately, the entire surface of the screen can be covered with a layer of blockout and while it is still wet, materials such as cloth or crumpled paper pressed against it to remove the blockout in some places; after the blockout dries, ink can be pushed through the opened areas in a rich texture. This method is called negative because the blockout is applied directly and purposively to those areas of the screen which block the ink from reaching the printing surface. Stencils made from a glue such as LePage's are used with solvent-based inks; for water-based inks, a screen filler which is insoluble in water is used.
The end result of the fourth method, the positive liquid blockout stencil, is much the same as the previous technique but the process leading to that result is more involved. The areas which are to print are filled in on the screen with a special wax which is applied directly to the fabric. The wax most used by screenprinters is tusche (pronounced toosh), the same material used by lithographers. It is available as a liquid and in solid form (pencil and crayon) and both forms are used in screenprinting. With tusche, many effects related to drawing and painting and to texture rubbings can be achieved. After the tusche has been applied to the screen and dried, a thin and even coat of liquid blockout is spread over the surface of the screen completely covering the tusche. The blockout is repelled by the oily tusche but it adheres to the mesh in areas where tusche was not applied. When the blockout is dry, the tusche is removed. If solvent-based inks are to be used and the blockout is glue, the tusche is removed with turpentine or mineral spirits which does not affect the glue; if water-based inks are to be used, the blockout is a screen filler which when dry is not affected by water, so the tusche is removed with water. The areas which were originally covered by the tusche - the positive areas - are now the open parts of the stencil through which ink will pass. The blockout, of course, remains as the stencil which holds back the ink.