Screen stencils were originally produced through the use of a paper stencil or a "glue - tousche" method which paved the way for today's hand cut and photographic stencils. The development of the photographic stencil enabled screen printers to create and print designs with much more detail and even allowed for the printing of photographs in both black and white and color.
As the name implies, the photographic stencil process uses light sensitive materials to create the stencil and is usually produced under subdued light or darkroom conditions. Most photostencils are exposed by light in the blue spectrum of light, like tungsten, carbon arc, and flourescent light. A normal incandescent light bulb ( like in your house lamps) can be used as a safelight for most photostencil materials.
The theory behind a photographic stencil is that the light that strikes the surface of the stencil material causes a chemical reaction. The chemical reaction causes the surfaces struck by the light to harden. Any area that is not struck by light remain water soluble, and can be washed away. The areas that wash away become open in the screen mesh. Those are the areas that print.
There are three basic types of photostencils in use today. There are different variations and different manufacturers of these three, resulting in many products on the market. You may want to experiment with several products, until you find the one that works best for you. The three types are listed below:
Direct stencils - A light sensitive liquid emulsion is coated or spread onto the surface of the screen. It is then allowed to dry. The entire screen is then placed into the exposure unit and exposed. The screen is washed out, producing the stencil.
Indirect stencils - A light sensitive film is used. It is cut and placed into the exposure unit. After exposure, it is washed out, creating the image. However, it still needs to be "adhered" to the fabric. This is done while the film is still wet. The film is adhered to the screen fabric, and then allowed to dry. A protective plastic backing sheet is removed, and the stencil is complete.
Capillary Direct Film (Direct/Indirect Film) - This type of stencil combines characteristics of both of the above. The stencil consists of a film. However, the film is cut and adhered to a wet screen in the darkroom. After the film is dry, the protective plastic backing sheet is peeled and removed. Now, the stencil can be exposed just like a direct photostencil. This method is used a lot for t-shirts. It allows for a consistent stencil coating on the screen and a very even ink deposit on the surface of the shirt when printed.
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