Gum Calibration 3 (Dark Tricolor)

[Loris Medici <mail@loris.medici.name>]

I mixed stok paint/gum solutions as following:

1. 15ml Schmincke Aureolin Modern (208) PY151 + 65ml gum = 90ml total
2. 15ml Schmincke Ruby Red (351) PV19 Rose + 165ml gum = 180ml total
3. 15ml Schmincke Helio Cerulean (479) PB15:3 + 165ml gum = 180ml total

As you can see above, tricolor mix will consist of 2Y + 1M + 1C for
neutral grays. (3Y + 1M + 1C is remains on the warm side with my
paints...)

I will dilute the paint/gum stok solution 1+1 with gum at time of use, and
I will add equal amount of 10% ammonium dichromate solution to the working
strength paint/gum solution; I like the consistency I get with 1 part  gum
solution (which is 1+2 gum powder+water w/w) + 1 part dichromate (v/v)
solution...

I will fully calibrate each color individually (using Luminosity channel
at Lab mode), but since the paints look quite strong even at working
strength, I will probably(!?) end up with a tricolor print that looks too
dark when the colors are printed over each other (as noted by others
before).

If I end up with a print that is too dark, then I will do a final
calibration printing all colors together (hopefully giving a balanced
grayscale gradient), and make a curve that is going to be used before
splitting the channels of the original(*), again using the Luminosity
channel of the source image.

(*) This curve will be used to lighten the image (applied to the
Luminosity channel - in order to keep the colors intact), so that it
prints right.

I hope to get a print that is both colorful (no muted colors because of
excessive paint dilution pass the optimum chroma) and luminous (thanks to
the lightening curve)...

David, I think this is close to what Denise Ross is doing by increasing
saturation - but in a more controlled manner methinks...

Short list of calibration phases:
1. Calibrate Y, M and C individually (= curves for individual R, G and B
channels, uni-dimensional approach)
2. Print tricolor grayscale step tablet (three-dimensional approach)
3. Calibrate tricolor print (= curve for composite RGB image)

Short list of negative making phases:
1. Apply the "composite" curve to the original RGB image
2. Split channels
3. Apply the respective "channel" curves to each channel (+ colorize - if
using colorized negatives) and print the negatives

I'm starting tonight, with Cyan. Will post whenever I progress. Any
thoughts? Please warn me if I'm making an obvious mistake, since all this
will take precious time and effort -> I want to be sure I'm not fighting
against windmills.

Regards,
Loris.