PhotogravuKM73, NuArc 26-1KS - need help with initial exposure values!

[PaddyMac]

(oops, not sure what happened to my title, "Photogravure, KM73..."

hi all, I just joined the forum and hope someone is using the same setup as me to create photogravure. I've read everything I can for the past few days on exposure, have read books, but am having trouble deciding on the initial base exposure using a NuArc 26-1KS. Every book and blog seem to use either a homemade unit with light unit in "minutes" or a 5000-watt unit, even though I think the NuArc is common. I have a looming deadline, and could use some help that might save me a day or two of tests. (I've taken classes on this process using the same unit, but was using their clear transparency film but printing on a dye printer, and I'm also thinking the Pictorico OHP transparency will be quite different than clear. In the class, common light units were 16 to 22.)

Here is my setup:
Stouffer 21-step transparency for test
KM73 Printight Toyobo plates
1000W NuArc 26-1KS (pressure is low at 17, but I'm hoping it will be ok for now until I can look into it - otherwise seems to work fine)
Takach Aquatint screen

Takach Press says that "Evaluated and calibrated test strips of [KM73] photopolymer plate using a Stouffer 21 Step Scale to achieve a solid 14/15 with the actual positive transparency."
Dead Link Removed

But I have no idea what this means?!! Do I expose it twice - once with the Aquatint screen and then again with the Souffer film? Then look for a scale that goes from 0 - 15, and then 15-21 is the same?

I tried putting the Stouffer film on a test plate exposed with "18 units" of light (no idea how much time that is) with no aquatint screen, and the number from 7-21 are pretty much all the same. Next I'm going to expose it again, with the aquatint this time, both at 18 units.

Next step is to print a grayscale step wedge that I printed out on Pictorico OHP film on an Epson 2200, Black Only, gamma 2.2. Epson set to Photo Glossy Black.
I also have access to an Epson 3800, if that was significantly better I could switch to using that printer. I think slowing down the drying time as the head moves.

But I was really hoping that someone here uses the same equipment and film and can tell me what range of number of NuArc "units" I should be looking at to get started, with each pass (screen and image). 15? 20? 25? ?

Thank you in advance.

 

[clay]

Read this: Polymer Plates - Polymer Photogravure
Then this: Screen exposure calibration - Polymer Photogravure
Then this : Image exposure calibration - Polymer Photogravure

This will work

 

[PaddyMac]

Thanks. I had read that info a few times. I wanted to start with the Stouffer test I'd seen on this video on YouTube,
https://www.youtube.com/watch?v=X25aRMRnoQs

I had the same little 21-Step Stouffer scale. I think the logic used seemed to line up with the info on the Takach site: The KM73 plate should be hitting the 14/15 marks when exposed to the correct amount of light. However, the video didn't show using a screen first, but the screen seems to help.

I spent today making tests from 40 - 80 light units, and somewhere around 70 (3 mins or so) seems to be a reasonable spot to start the next round of testing with the image.

What was throwing me into a panic yesterday was that in the workshop I took, we used unit in the 16 - 22 range on the same NuArc machine. So I started testing with 14 - 24 and it was seriously off. The polymer remained green, the washout water was filthy, and the KIM wipes were sticking to the plates when I dried them...! I still don't understand how the same Nuarc system and aquatint screen could end up with such different numbers. The only difference that I can think of is that they are using an office inkjet printer to make the transparency?

My plates have also been stored for about five years, so I was afraid they were old. But they seem to be behaving alright now. Onward to image exposure testing.

 

[PaddyMac]

Clay (assuming you're Clay Harmon) - I forgot to thank you so much for your excellent resource, which I'm still devouring! Your images are amazing too.

BTW, I found an old link on the right sidebar to Keith Taylor's website.

 

[clay]

Remember that these plates are designed for relief printing, and the exposure recommendations are what are needed to expose a negative properly to make a relief plate, not an intaglio plate.

Also remember that these plates will age and require less and less light for exposure. It is likely that your earlier work was done with a batch of plates that respond differently.

 

[PaddyMac]

Thanks Clay. I read as much as I could about the Nuarc and from what I've read, one unit should be roughly one second. On my Nuarc, 60 units = @ 2.5 mins (150 seconds), so not sure what that is all about.

Going to try your 51 step chart now. Fingers crossed it makes some kind of sense!

 

[PaddyMac]

FYI, I finally found the info I needed for how to "read" the exposure tests I did yesterday with the Stouffer 21-step wedge. Page 95 of Dan Weldon's book "Printmaking in the Sun" talks about running this est to initially test your exposure unit and plate. You do not need an aquatint screen, and you don't even need to harden and print it. You just need to develop it and dry it. Then look at the transition between solid and washed away. (This seems to be what the guy in the YouTube video did too - he just "read" the numbers without printing it.)

So if my goal is 14/15 to get the full range of the plate, then I need to expose it so that numbers 1-13 are solid (=shiny clear), numbers 14/15 are partially washed away (somewhat grainy), and 16-21 are washed away (or solid grain since I did use the aquatint screen). For my unit, the correct number was 70 units. Any higher than that (I tried 75 and 80) gave the same result as 70, so that seems to be the maximum possible from this plate. 65 and 60 were noticeably lower.

 

[PaddyMac]

What is everyone using for a "washout brush"?

I found an unused brush in my art supplies that seems to work well for now - Martha Stewart 4 in Wavy Graining Brush! It's soft but not too soft. Polyester/nylon. It was a wavy set of bristles.
Martha Stewart Living 4 in. Wavy Graining Brush-MS-WGB at The Home Depot

Will have to research the PVA sponge. I've been using Kim wipes, but I like the idea of even less chance of a pattern forming.

 

[PaddyMac]

Now I'm back to square one. I think the lamp in this exposure unit is worn out, and I have a service technician willing to come out to service it, check the voltage, and replace the lamp. Over the phone, he's guessing that it is putting out mostly white light, and not so much UV. He can also check the vacuum pump as the pressure isn't as high as it should be (only 17).

I can print - barely - a gradient that might suffice for a looming deadline, but these units and exposure times (in minutes) seem way too high for this unit. Sigh.