SILK SCREEN PRINTING. Screen stencil printing is the modern form of what is probably the oldest of all printing proc esses. It is the logical development of the principle used by the Chinese and Japanese, who long ago learned to make very intricate stencils by gluing the cut design to a web of hair, thus making it possible to use isolated elements which could not otherwise be held in place.
The screen stencil ordinarily used consists of bolting silk stretched over a wooden frame. On this silk is the design to be stencilled. This design may be produced by simply painting the areas which are to remain white with some substance, such as gum or shellac, which will make the silk impervious to the ink used. The stencil is placed in contact with the surface to be decorated and a puddle of ink is scraped from one end to the other by means of a rubber squeegee. This forces the ink, which resembles ordi nary house paint, through the open areas of the stencil and it ad heres to the surface beneath. The stencil is then lifted from the decorated surface and the ink allowed to dry, after which another colour may be applied from another stencil, and so on.
For special purposes, wire gauze or cotton fabrics may be sub stituted for the bolting silk, and a great number of ingenious methods have been devised for putting the design on the screen. Fine detail in the pattern generally makes it advisable to make the stencil photographically, by impregnating the screen with a light-sensitive material, such as dichromated glue or albumin, and exposing it to light under a tracing of the pattern. The image is developed by washing with water, which removes the coating from the open areas where it has not been light-hardened. Simple de
signs are often made by cementing patterns cut from paper, cellu loid or metal, to the screen, a common method being to cut the design from shellac-coated paper and fasten it to the cloth by passing a hot iron over the two in contact.
As compared with other printing processes, screen stencil print ing has the advantage of requiring relatively cheap equipment and little preliminary work. It is very economical for short runs of suitable subjects, and is widely used for show-cards, posters, charts, etc. It permits printing on almost any surface, so it is used for decorating paper, cloth, glass, wood, celluloid, rubber, metal, linoleum and oilcloth. The inks used are usually oil paints, but may be water colours, dyes, or lacquers, so that the process may be used to produce any effect from the most delicate of pas tel shades to glossy, weatherproof colours, so heavy that they stand out in visible relief. Areas of relatively great size are easily stencilled: frames so" x 6o" are frequently used for tex tile printing. Finally, as the object to be decorated need not be limited in thickness or even perfectly flat, the process is much used for printing on objects such as toys, novelties, and machin ery which could not be run through a printing press.
The limitations of the process lie in the fact that it is not suitable for reproducing subjects with fine detail and delicate gradations of tone, such as photographs or process colour work. It is unsuitable for reproducing very long runs because it is a relatively slow printing process and because the stencils are of limited durability.