gravure plates under vacuum
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- Subject: gravure plates under vacuum
- From: SusanV <email@example.com>
- Date: Sun, 04 Mar 2007 09:54:12 -0500
- Comments: "alt-photo-process mailing list"
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Jon, I did the experiment you suggested with a piece of virgin km73
and pictorico ohp under vacuum. I watched as the vacuum pulldown
advanced, right along with the splotchy look of the contact between
plate and film. dark areas where the film contacted (maybe too
tightly?), and lighter elsewhere. I came to realize that the darker
areas seemed related to the plate and film being drawn up against the
glass of my exposure unit very tightly. (Jon, this would go along
with what you've said about too much vacuum being a problem)
I wondered if the lighter areas were indeed close enough for a sharp
exposure. I couldn't stop the darker splotches from happening even by
using as little as 5"mercury pressure... the moment the sandwich
raised up and touched the glass, the splotches appeared.
At this point I should maybe describe my vacuum/exposure bed because
I'm not sure if they all work this way... The actual pebbled
surface bed of the frame is rather thin and flexible, and during
vacuum drawdown it rises up, lifting the plate and film up to come in
firm contact with the glass, which rests on a raised rubber gasket
around the perimeter of the bed..
So... how to draw a vacuum for close contact, but not caused the
sandwich to be pressed together physically by contacting the glass.
I placed a piece of masonite, maybe 11x14" on the vacuum bed. Then I
cut a 5x5" window in a similar sized piece of foamcore, and placed
that on top of the masonite. (To allow air to escape I used a
burnisher to draw some deep indentations in the underside of the
foamcore, radiating out from the window to the outside edges.) I then
put the plate and ohp positive in the window (no powder), and turned
on the vacuum. The vacuum drew down to 22" pressure, and the window
and masonite under it didn't allow the plate to rise up to the
glass!!! The film maintained a totally measle free, even, light
tone... even after an extended drawdown time (like 3 minutes or so).
I exposed the plate, developed, etc... and when I printed it, there
were no measles. What's more... there is very fine detail in this
image, and it was ALL there, crisp and sharp as can be! There was
definitely close enough contact with the film without it being
physically pressed together. The vacuum was enough.
So for me, I think this is going to be my solution to the measles.
I'll post this test print to my blog later, including some closeups of
the sharpness of the detail.
gravure blog at www.susanvossgravures.blogspot.com