U of S | Mailing List Archive | alt-photo-process-l | Re: digital negative possibilities for gum

Re: digital negative possibilities for gum

On Oct 16, 2006, at 8:07 PM, Katharine Thayer wrote:

Chris, what kind of bitmaps were you using, a halftone screen, diffusion dither, or some other?
The reason I'm asking is that I'm not sure what it is you're comparing when you're comparing a negative made by printing the file as a bitmap file to a negative made by printing it as a regular file.

The assertion I've heard (Mark is the last person I remember making this assertion here) has to do with halftone separations rather than bitmaps per se, and goes something like this: gum printers "have found" that halftone separations give better clearer colors because the color is laid down next to each other rather than on top of each other. I said that might be true of opaque pigments, but certainly not of transparent pigments, which can be printed directly on top of each other without muddying the color.

With halftone separations, the screen angle for each of the separations is set so the color is laid down in a rosette pattern, each full dot being made of a rosette containing each of the three colors, like three different-colored petals making up a flower. In that case, the color really is laid down next to each other rather than on top of each other, and would give you the comparison you seem to be after. But if you were using halftone separations of this kind, it seems like you would have said so, rather than characterizing the negative type as a "bitmap."

So, some clarification would help me understand what it is I'm looking at here, thanks.


On Oct 16, 2006, at 6:57 PM, Christina Z. Anderson wrote:

Good evening all,

Over the last week I have been testing four kinds of gum negs, with tricolor
gum and gum over cyanotype (cyanogum). My goal was to see if bitmapped negs produced clearer, more brilliant colors as I read somewhere, or even worked with gum, and then to find an acceptable, cheap, low tech beginner mode of gum printing.

I made sure to actually attach the negs side by side so all other variables
were exactly alike--coating, dry time, development, etc.

So here's the skinny:
1. Trigum printed with a negative on cheapy Photo Warehouse OHP
transparency with all inks, no curves
2. Trigum printed with a negative on cheapy Photo Warehouse OHP
transparency with all inks, no curves, and bitmapped 360 ppi input and
3. Cyanogum printed with a negative printed on expensive Pictorico, all
inks, no curves
4. Cyanogum printed with a negative printed on expensive Pictorico, curved
correctly for cyanotype, magenta and yellow separately, colorized neg

These are my observations (NOT declarations or assertions); YRMV:
1. Bitmapping surprised me--it actually produced a pretty darn good image!
It was softer, a bit less contrasty, but heck, with what little ink
bitmapping uses and with the fact you can use cheapy transparency, it is
definitely a keeper,especially for teaching beginners low tech gum.
2. "All inks" was a bit smudgy and required drying with the PWOHP/ Epson
2400, not with Pictorico. Funny, my cyano layer printed with minute round
spots of lighter tone--not the dreaded speckles--and when I louped the
negative I saw that the printer lays down minute round spots of different
color inks that in turn expose cyano differently, like little mini filters.
Very interesting. With gum this is no problem--the spottiness, of course.
3. Cyanotype absolutely requires a curve--by the time the highlights are
printed in, the shadows are totally overexposed unless your image is short
scale to fit the 4 or 5 stops of that process's range. My next test is to
curve just the cyano and use the two bitmap magenta and yellow negs to print
gum over. And then next I will probably curve the individual negs and then bitmap.
4. If not printing with a cyano underlayer, you can get an acceptable
print with no curves, neg just inverted and printed as is, and adjust the
layer with exposure, development, pigment load, brushing. But all of you
already knew that, I'm sure I'll be told. I prefer the all inks unbitmapped
to the bitmapped--I think.
5. In my eye the better print was produced by a properly curved neg,
but how will your viewer know there is a "better" rendition unless all the
images are side by side, you know?
6. Bitmapping didn't produce clearer, better colors because of "individual
dots laid down side by side and not on top of one another".
7. All methods can be capable of producing fine prints, once the gum printer can meld his/her method to whatever workflow is chosen.
8. Bitmapping has....possibilities...I'm not sure what yet, but it really did surprise me.

If you want to see the visual, copy and paste this URL into your browser:


and scroll down to the very bottom of the images; it'll be there.