Printing techniques

A brief explanation of the three printing techniques I use. Of course, they can also be combined.

Screen printing

In screen printing a stencil acts as a mask. It is supported by a fine material mesh that is stretched over a metal or wooden frame. The print is made by pressing colour through open areas of the stencil.

Preparing the screen: A positive of the desired picture that is opaque to UV light is required, e.g., a high quality photocopy. The material mesh of the screen is coated with a light-sensitive layer. Positive and screen are placed on the glass surface of a special vacuum table, positive under the screen ready for illumination by a short intense burst of UV light. This follows once the cover has been closed and the pressure reduced. After the photoreaction, the screen is extensively washed with water. Regions of the photosensitive layer that were exposed to the UV light remain, others wash away, i.e., the stencil, a negative of the desired picture, if formed.

Objects that will not damage the fine mesh of the screen can also be used as 'positives', e.g., leaves, flowers, pieces of string etc.

It is also possible to paint a negative of the desired picture directly on the screen using a liquid filler. A stencil can also be modified in this way.

Screen printing is an 'open' or 'shut',  'colour' or 'no colour' technique, intermediate tones are not possible. These can be achieved by using a raster of dots when preparing the 'positives', cf. newspaper pictures.

A screen has to be made for each colour used. A third colour will be obtained if regions of two screens overlap and the second colour printed is partially transparent.

Printing: The frame is positioned slightly above the paper. Paint is poured onto a closed region at one end of the mesh and dragged across the stencil using a squeegee, which is at the same time pressed down onto the paper. The method is simple but requires practice. The angle of the squeegee and the speed with which it is moved are both important.

Further information can be found in Wikipedia under Screen printing.


Woodcut printing

Like linocut, woodcut printing is a relief printing technique. The areas of the wood block that should not print are cut away using various gourges and knives. A mirror image of the desired picture is cut.

There are two approaches:

1. The multi-block method: A different wood block is cut for every colour of a multicolour woodcut print. The different colours do not have to be printed on top of one another. Regions where the different blocks overlap have a 'mixed colour' depending on the transparancy of the inks used cf: watercolour techniques.  The artist is able to test various colour combinations and print different series.

2. The single-block method: The same block is used for all colours. Once the first colour has been printed the block is cut further and used to print the second colour etc. One colour is always printed on top of the other i.e., all colours except the first are mixtures depending on the transparancy of the inks. The artist has to decide on a colour scheme before starting to print. There is no 'going back'.

Printing: The wood block is inked up evenly using a roller; only the raised areas are coated.  The type of press used determines exactly how the print is made i.e., whether the block is placed face down on the paper before pressure is applied or face up on the press and the paper rolled over it. The use of too much ink or too much pressure will fill up the finer lines and result in loss of detail.

Further information can be found in Wikipedia under Woodcut and Woodblock printing.


Lithography on limestone slabs

Lithography exploits the fact that oil and water do not mix. It is a planographic printing technique i.e., the picture and the blank 'undrawn' area of the lithographic stone are on the same level.

Preparing the stone: The previous drawing has to be removed and the stone has to be given the required grain. This is done manually by grinding the surfaces of two stones against one another. The lower stone is coated with a sludge of coarse silicon carbide (carborundum) and the upper stone is moved on top of it in a regular motion over the whole surface. It is essential that the surface of the lower stone remains flat. A less course carborundum is used in the later stages, i.e., once the picture has been removed, to give the stone the required grain.

Drawing the picture: The mirror image of the desired picture is drawn using special lithographic ink or crayons. These contain grease or soap (fatty acids). The upper surface of the stone should not be touched by hand as a finger print could show up on the final print.

Processing the stone: When the picture is finished the whole stone is coated with a thin film of gum arabic acidified with nitric acid (2%) and left for at least 8 hours. A hydrophillic layer of calcium nitrate and gum arabic is formed on all blank areas of the stone. The stone is then wiped with a cotton rag soaked in terpentine. This removes most, but not all, of the greasy drawing materials; a thin hydrophobic film remains tightly bound to the limestone surface.

Printing with a hand press: The stone is transferred to the steel bed of the press and dampened with water. Oily printing ink is then rolled out evenly over the whole surface of the wet stone. The blank hydrophilic areas repel the ink,  hydrophobic regions where the drawing was accept it. A sheet of paper is placed over the stone and covered by several sheets of soft backing paper, and finally by a sheet of perspex with a lubricated upper surface. The teflon scraper of the press is lowered onto the perspex and applies an even pressure while the stone and its covering paper and perspex layers are manually drawn through the press with an even motion.

The first few proofs vary, becoming darker as more ink is retained by the stone. Optimum printing conditions are then attained and retained until the stone suddenly starts to accept too much ink.

A new stone has to be prepared for each colour of a lithograph.

Further information can be found in Wikipedia under Lithography and Lithographic_limestone.



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