Starting curve for gum bichromate?

The Curve Exchange
1

Does anyone have a starting PiezoDN curve for gum bichromate process on Rives BFK paper (actually I’ll take any paper). If so could you also let me know your parameters (e.g. paper, pigment/gum amount, pigment/gum to dichromate ratio, dichromate strength and type)?

thank you!
Michael

2

Start with the cyanotype curve.

best,
Walker

3

Walker,

Thank you!

I’ve looked in the Piezography/Curves/4880-7880-9880-PiezoDN-Pro folder and there is no cyanotype curve (there is only the x8-PiezoDNPro-Master-beta.quad and x8xx-PiezoDN-Master-goldmastr.quad). Can you point me to the url with the current list of starting curves?

Michael

4

Sorry. The PRO ink curves have been in beta for a long time. Please ping me on Monday (when I’m not crazy teaching) and I’ll build you out a starter gum curve from the Gold Master curve I posted on this forum a few months back.

best,
Walker

5

Monday ping, :-). No hurry as I am hunting down an interesting “problem” (or as they say maybe it’s a feature).

thanks!
Michael

6

Hi Michael,

We forgot to discuss this when I saw you last week! Ack!

I think you may find this to be a bit like nailing jello to the wall. Gum is generally high contrast like traditional cyanotype, but each pigment or combination of pigments (since I know you are mixing them) will ideally need it’s own curve. Add to that variations in contrast you can achieve with the different dichromates (ammonium, potassium, sodium) in different concentrations, and there may be as many combinations and permutations as there are with the Pro inks! And then those curves can be targeted to particular segments of the tonal scale. Yikes!

But I don’t think you need to wait for Walker to make a starting curve for you. Take whatever your favorite pigment mixture is and mix with dichromate and gum as you are already doing. Do the Stouffer 21-step exposure test with it, adjusting exposure as necessary. From this you can determine the exposure scale as well as the correct exposure. (You may even be able estimate whatever limiting is needed.) Then make a 129-step using the “goldmaster”, print it using the same pigment-dichromate-gum mixture at the just determined exposure, process, and evaluate. You’ll probably need to limit it if you didn’t already make a guess at that. Do that and make another negative. Print, evaluate, linearize, &c. Repeat as necessary.

Cheers,
Keith

1 Like