From: Dennis Ritchie <email@example.com>
Subject: Mimeo, ditto
Date: 13 Oct 1998
Surprising that no one has clarified the technologies
for these. I'm weaker on the trademarks and common names.
Mimeograph uses a penetrable-fiber waxy paper. Typing
on it pushes away the wax at the key impressions.
When the masters are attached
to a drum with liquid but thick ink inside, some thousands of
copies can be produced from each master: the ink "flows"
through the master where the wax is pushed aside. The
wearout mechanism is just wear on the master. Some
solvent smell from the ink. Fair amount of set-up.
Ditto or spirit process used masters in two parts:
top sheet a carefully-formulated paper, bottom a solid ink
surface with a coating so it didn't rub off easily.
Typing on the top physically transferred the ink
from the bottom sheet to the front. Fairly
readily done in more than one color by replacing
the bottom ink-sheet and typing (or drawing) some more.
Reproduction was done by taking the reverse-inked top
and putting in on a similar drum.
Good only for a finite (~100) number of copies, because
the solvent used during printing softened the ink
sticking to the master and transferred it to the copies; when
the ink was gone, it was gone. This process definitely
generated organic fumes, but the setup was cheaper
than mimeograph. The closest analog today is making
overhead slides with marker pens.
There was also a process (whose name I forget and never
actually saw) with the same idea, except that it was
manual by sheet: it made a master like the Ditto, transferred
the ink to a tray with a gelatin-like surface, and you
pressed pieces of paper onto the gelatin. Yuk. But you
didn't need to buy a machine with a drum.