Re: aguatint screens and photogravure screens

[Gary Beasley <beasleyglb@mindspring.com>]

At 01:19 PM 8/12/01 +0200, you wrote:
> > I work with photolithography on a daily basis
> > so you should check the details on that gravure screen to find out
> > exactly what it is.
>
>Gary, could you explain in a few sentences what photolithography is? I
>have seen a print recently and wondered ever since.

Photolithography is the most common process printers use in their printing 
methods. The name litho  comes from the fact the process was first done 
with a smooth stone as the image bearing surface (probably a limestone). A 
greasy image was drawn onto it then the stone was dampened and a greasy ink 
applied. The damp stone repelled the ink and the grease image attracted it. 
Then a clean sheet of paper was pressed onto the surface that picked up the 
ink resulting in a reverse of the stones image. If done right its pretty 
high quality.
Todays method involves treated metal plates with similar properties to the 
stone and the image was exposed onto it via a coating of a diazo type 
sensitizer which was developed up with a special plate developer or a 
photopolymer coating which left a water repellant image on the plate. All 
these create an image that is flush with the plate, like a painted sign. So 
sometimes these processes are referred to as planography.
The plates are wrapped onto a cylinder on a printing press where first it 
rotates under a dampener roller to wet it, then is inked by a set of 
rollers. After that the ink is transferred to a rubber mat wrapped on 
another cylinder and transferred to the paper. This last action re-reverses 
the image to read right.
There are also new processes out that do the same thing without the water 
but has a silicone surfaced plate that is hard for the ink to stick to. 
This ones a bit tricky to make work but the resulting quality and stability 
of the ink balance make it worth developing. Theres not many printers using 
this method yet.
Nowadays the new digital systems expose the plates directly with a laser, 
so the film is eliminated and the process from image file to plate 
drastically shortened.