Design Context: Offset Lithography

For high volume commercial printing it is usually most cost effective to use rotary printing machines because they are fast and can produce really high volumes of print. Rotary Printing machines use rollers to print and there are a few different types of rotary printing. One of these types is Offset Lithography.

Lithography Is a process of using stone or metal plates to transfer ink to a flat surface.

Traditionally Lithograpy was based around the fact that water repels oil which sounds quote complicated and I don' really fully understand, but you can sort of see how it might work by separating the plate into sections and then the ink would be repelled by some and stick to certain areas so it would make a design when printed.

Below is an example of a stone lithography plate and the score that it has printed when used.

In modern day lithographic printing, metal plates are used with a slightly roughened texture and are covered with photosensitive emulsion. A negative of the image is then placed on the plate and it is exposed to ultraviolet light in a similar way to contemporary screen printing.

This makes the emulsion react to the light and the plate then shows a reverse image of the negative which make it positive again.

This is how the process works for Offset lithography in which the plate is used to transfer the ink on to a separate rubber blanket and then again on to the final stock.

The plates are really thin and are wrapped around cylinders in a rotary printing press as in the diagram above. This process is much more efficient for mass print jobs than digital print which would take a long time and cost considerably more. Once the machine is set up it can produces probably hundreds of thousands of reproductions without the need of human interference.
Some of the more intricate details are probably a lot more complex than the way I have described it but I think that this is basically right.