What is a printing plate?

The plate works on the mutual repulsion of oil and water. After imaging, the areas that carry the picture are hydrophobic — they repel water and accept greasy ink. The background areas are hydrophilic and hold a thin film of damping solution that keeps ink off. On press the plate is wetted and inked in that order, so ink lands only where the image is.

That single mechanism is what lithography is, and it has been the basis of commercial printing for over a century. What has changed is only how the image gets onto the plate.

Because there is a plate per colour, plates are a fixed cost per job. Print five hundred units or fifty thousand and the plates cost the same, which is the first half of why offset is expensive short and cheap long. The second half is make-ready.

Plates are consumable. They are good for a large number of impressions but not unlimited, and they are not normally kept between jobs — a reprint months later is usually a new set of plates from the retained file rather than the original plates coming off a shelf. That is worth knowing when comparing a reprint quote with the original.