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View Full Version : Photogravure UV exposure unit - diffuse vs point light sources



banksy
11-29-2010, 09:59 AM
Is it possible to use an Edwards Engineering UV unit + a vacuum frame to expose photopolymer plates - or would a point light source such as a NuArc 26-1k be advisable?

I'd like next year (once I've built my etching press) to have a shot at photogravures. I own something similar to the Edwards unit, but a local printworks is selling off a NuArc quite cheaply, so if I'll need it for exposing the plates now might be an opportune moment to pick one up.

Thanks.

banksy
11-29-2010, 01:06 PM
This (http://www.silverclayart.com/ppplates-instr.htm) would seem to indicate that a bank of UV fluorescent tubes is fine. Shame, I quite fancied a NuArc!

clay
11-29-2010, 04:38 PM
I can tell you from personal experience that you will need a point source light. The NuArc should work fine. The site you linked to applies to art reproduction. Continuous tone photogravure is a whole 'nother can of worms and requires an aquatint screen exposure in addition to the image exposure. Check out the information here (http://intaglioeditions.com/procedures/polymer_photogravure.html) for some other corroboration about the dismal results you can expect with fluorescent tubes.

The issue is that the omni-directional fluorescent light will start bouncing around between the positive and the plate and give you light streaks. This is frustrating and expensive.

banksy
11-29-2010, 07:08 PM
Thanks Clay. I did see John Lybrook's site - I just assumed (for some reason - wishful thinking perhaps) that the problem was confined to blacklight bulbs in particular. Better clear a space for the NuArc...

clay
11-29-2010, 10:27 PM
A point source is definitely necessary. I found out the hard way that the fluorescent light boxes don't work. I tried to get my box to work on an 'on-the-road' workshop and we had a lot of failures. We ended up getting better results using the sun and ratio-ing the screen/image exposures the same as I use here at home with my exposure unit.

It is a beautiful process, but not the easiest thing to get dialed in. A lot more moving parts than pt/pd for instance.

iansand
11-30-2010, 02:51 AM
Just a thought, if using an aquatint screen with a positive requires a 'sandwich' when exposing with UV tubes then light might be diffused due to the thickness on the film. Perhaps it would be better to incorporate a grain directly on the film positive using photoshop.

clay
11-30-2010, 07:17 AM
It isn't really a sandwich, it is two different exposures, one with the screen and one with the positive. The screen provides the 'ground' for the etching process. What it does is provide microscopically tiny little exposed areas (roughly 20-30% of the total area) in a random pattern on the plate. This gives broad areas of tone the ability to retain ink when the plate is prepared for printing.

What you are proposing can be done, but actually is a process called photo-etching, where a single exposure is made and the density of the dots provide the gray tones. It is somewhat akin to what you would see in a black and white newspaper print where the relative density of the black dots provides the tone. The look is not nearly as smooth as the two step screen/image positive approach.

banksy
12-01-2010, 09:33 AM
Would it be possible to create your own aquatint screen on OHP - say by printing the dots via QTRip using only a single ink with a high UV blocking ability e.g. the matte black in my Espon 2200?

If not...I see quite a few aquatint screens for sale - are they all suitable for use with digital negatives of are some more useful than others (the one that Mark Nelson sells perhaps?)

Loris Medici
12-01-2010, 02:47 PM
banksy, you'll need a much higher resolution screen methinks. (I mean if you want to print - virtually - continuous tone images...)

clay
12-01-2010, 11:45 PM
The screens that most printers are using are from imagesetters. The dot size on those with a stochastic screen usually is 1800 dpi or better and is on film. I doubt you can get the same resolution from inkjet printers that have a native cell size of 360 dpi. It also needs to be a very optically dense dot.

But I haven't tried doing it and I could be totally wrong about it. I do know the imagesetter screens work well.