WHAT IS IT

Lithography: A printing method based on the principle that oil and water do not mix, but remain separate when added together. It was invented in 1798. The 'litho-' part of the word means stone. The '-graph' part means writing. So lithograph means stone writing.

When it was first invented, a greasy crayon made from oil and black pigment was used to draw a picture onto a smooth flat stone. Water was then applied to the stone drawing. The black greasy drawing repelled the water, pushing it away from all the parts of the stone where the crayon was used.

Does this sound like doing a drawing the hard way? It would be, if the only purpose was to make a single drawing. Here's where the next step of the invention took place. With the stone still wet, any wet surface areas on the stone repelled the oil-based ink. When applied to the stone, this ink remained only on the parts of the stone with the grease crayon drawing.

A blank sheet of paper was then pressed down onto the stone. When the paper was removed, a copy of the drawing was copied onto the paper. When additional ink and water were applied to the flat stone, the water stuck only where there already was water. The ink stuck only to where there already was ink, So by adding more water and more oily ink to the stone, another paper copy was made. This process was repeated so that many paper copies were made from the single stone drawing. The copies dried and became permanent.

This was a "eureka moment" in the history of printing. Did the copy match the original drawing? Not exactly. Why not? It was because the process created a mirror image of the original drawing. With current lithographic systems, the final paper print does match the printing master. Why? See Offset-Lithography below.

How is Lithography used today?

Instead of drawing the original picture, photography is used in lithography.

How is the oil and water principle used with photos?

A printing master* can be made by using photography. This printing master would take the place of the stone. The photographic process can be chemical or electrostatic*. The photo process creates a printing surface where the image is divided up into areas that attract water and repel oil -or- areas that attract oil and repel water. First water, then an oil based ink is applied to the surface of the printing master. The inked image is then transferred from one surface to another mechanically by bringing the printing master in physical contact with the paper.

*A "printing master" is similar to a printing plate except it is made of a thin flexible sheet of material. A finished printing master is flat in appearance. It is flexible so it can be wrapped around a cylinder or drum that rotates. See the diagram below.
*Electrostatic: This is a photocopy process used by laser printers and copy machines. The same method has been adapted to make lithographic printing masters. Computers can also be used to assist in the making of printing masters electro-statically.
 
Here is a diagram of a typical offset system

With this system, the inked image is offset. This means that the inked image from the printing master is transferred first to a rubber covered drum which then comes in contact with blank paper. This go-between step saves wear and tear on the printing master which never directly touches the paper. The image is reversed like a mirror on the rubber drum, but reverses back to normal on the printed paper. The transferred ink portions will produce the printed image. Presses may be designed to handle paper in single sheets or continuous rolls. Paper printed from continuous rolls is cut into sheets after printing.

Since there can be only clear paper areas or solid ink areas in a printed copy, how are different shades and colors made in a lithographic picture?

Since the oil and water principle allows for only two possibilities, areas with ink, and areas without ink, there needs to be a way to print different shades of light and dark. One answer is to break the picture into very tiny dots.

Printing different shades or tones

So a way to break up the inked parts of a picture into tiny dots of different sizes was invented. Each dot will print ink as dark as any other, but the size of the dots and the spaces between the dots will make a difference. It will make parts of the picture look lighter or darker. If these dots are made small enough, we see different shades instead of the individual dots. If you examine a printed photo closely or use a magnifying glass, you can see the dots. A picture that is broken up into small dots is called a halftone image.
 
When greatly enlarged, different tones or shades can be seen as separate black dots on white - or - white dots on black. At a normal viewing size and distance, halftone images appear to be a wide range of tones.
 
CREATING A RANGE OF TONES FROM A PHOTO

What about color?

How are dots used to make different colors?
Color lithography applies the same half-tone technique to color. In the case of color, dots comprised of four basic colors are used to print most full-color images. However, the ink for each of these four colors is printed one at a time on top of one another. In this case, the lithographic printing machine would have four sets of printing cylinders, four sets of offset blankets, and use four master printing images, one for each color.

Due to the way color mixing works, the human eye can resolve these four basic color dots into an entire range of different colors and shades. When magnified, you can better see the individual dots of color. From normal viewing distances, the color dots blend visually and we usually do not notice the individual dots.

The colored shapes shown on the left appear as if viewed at a normal viewing distance. The dots are very small and our eyes tend to blend them together. The shapes at the center and right appear as they would if greatly magnified. The dots are "screened" at different angles so that they do not create distracting moiré or wavy patterns.   The four basic colors used in offset printing often go by the names "process red", "process yellow", "process blue" and "black".
(Process red is somewhat brighter and pinker than what we might call red. It is like the magenta ink used in inkjet printers. Process yellow is a bright lemon color. Process blue is a bright blue that is like the cyan ink also used in inkjet printers.)