Lithography (Offset)

The first process we'll be talking about is Lithography, particularly offset. It is based on the principal that oil and water don't mix. A plastic or aluminum plate is coated with a photo polymer that's exposed through a photographic mask (this contains the image to be created on the plate). The plate then goes to a chemical process which hardens or "fixes" the exposed area and then another that washes away the unexposed area. Now the plate contains an ink receptive area and a non ink receptive area. When you print a page, the plate needs to be dampened. During this process, the water sticks to the unexposed areas of the plate. The ink, applied via rollers, will stick to the exposed (image) area. If the plate is dampened correctly, the oily inks do not stick anywhere but the image area.

In summary or things you might see on a quiz:

• Discovered by Alois Senefelder in 1798.
• Based on the principle that oil and water don't mix.
• Plates are created much like an old time photograph (the image is etched).

referenced: Brief Definition @ PC Magazine