🚀 go-pugleaf

[In article <1991Apr20.163349.14280@gdt.bath.ac.uk>,
     ma9mgg@gdt.bath.ac.uk (M G Gidley) writes ... ]

> Also, does anyone know a decent book about printing methods (ie the different
> process - litho etc, screening photos). I have the constant problems of
> contacting printers and them blinding me with jargon so I'd like to find
> out more rather than claim ignorance !

I'm not sure what books might be available in the U.K., but here are
some definitions that you might find helpful. Feel free to ask for more;
it's a far more appropriate use of the net than the flame wars you see in
most newsgroups.

Lithography: Literally, ``stone printing.'' Traditional lithography (still
used by many artists) employs a stone surface onto which an image is drawn
using a greasy crayon. The stone surface then is dampened. The moisture
won't stick to the greasy image, but it coats the stone. An ink roller
then is applied to the moistened stone. The oily ink won't adhere to the
damp areas, but sticks nicely to the image area. You then press a piece of
paper over the stone, and the image is transferred to the paper. Modern
commercial lithography uses a flexible aluminum plate in place of the
stone, and a photosensitive coating in place of the greasy crayon. The
coating is exposed to light through a photographic negative. Unexposed
areas then are removed in a developing process. The resulting plate then
is mounted on a rotary press.

Offset: Actually, ``offset lithography.'' Instead of letting the paper
come into direct contact with the printing plate, the image is ``offset''
first onto a rubber roller or ``blanket'' and then transferred to the
paper. By avoiding direct contact between the rough surface of the paper
and the printing plate, abrasion is reduced. Also, since the rubber is
slightly flexible, it can deposit the ink onto the paper more evenly.
Most modern lithography is offset.

Letterpress: The traditional Gutenberg method of printing whereby a raised
surface is inked and pressed into the paper. Letterpress printing from
phototype (or laser type) requires that an engraving be made.

Gravure: This is sort of a mirror image of letterpress. The inked areas
actually are engraved into a smooth surface, usually copper. Ink flows
onto the surface, then is scraped off (except in the depressed channels).
The paper then is placed into contact with the plate. Gravure (called
rotogravure if it's a rotary press) is quite expensive, but yields high
quality over extended press runs.

Screening photos: By nature, photographs are ``continuous tone'' images
(at least at a macro level). They're a mixture of grays. Since printing
deals with only binary states -- there's either ink there, or there isn't
-- the grays have to be simulated by breaking the image into dots whose
sizes are in proportion to the density of the original image. A screened
photo is often referred to as a ``halftone.'' Traditionally, halftones
have been created using photo-optical techniques, but now you can produce
them digitally by scanning the image into a computer and using appropriate
software.

 ----
 Steve Yelvington, Marine on St. Croix, Minnesota, USA / steve@thelake.mn.org