I am new to both digital negatives and alternative processes. I have chosen ziatype for my initial dive into alternative processes.
My question is simple. If I calibrate using a formula should I be able to use my curve if I alter my formula slightly; say an extra drop or two of A or B, or by adding a drop or two of <span style=“font-family: ‘Open Sans’, sans-serif; font-size: 14px;”>ammonium dichromate, or change exposure time by a minute or two. Or, should I create a new curve for each combination?</span>
You are tailoring your negatives to the process, so any changes you make to your process (including paper) will likely require at least relinearizing to achieve linear results.
Exposure time is something that you will standardize as part of the calibration process. It’s really crucial to do this, and get it right, before getting into the PiezoDN linearization process because changing it more than a very small amount will have a significant effect, especially in shadow rendering. Your goal will be to find the minimum time to reach maximum black. It’s really easy to misjudge this and end up using a longer time than needed which results in over-inking the negative in order to compensate.
In my opinion, the best practice is to get a 4x5 Stouffer step tablet to use as your test negative, and try a range of variations with that to figure out what you want to standardize on. Ziatype has a lot of variables. Longer tonal scale is always better so keep the dichromate to a minimum if you must use it at all. The various sensitizers and palladium solutions all have different image color and contrast characteristics, so changing them will almost certainly mess with your digital negative curve. Alternatively, if you have a large format negative that is very contrasty for silver printing you might be able to use that as a test negative. The Stouffer step tablet give you useful information about exposure scale that you can’t get any other way.
I changed to Ziatype from the B&S Na2 Pt/Pd system just over a year ago and I had a problem trying to determine correct exposure user a Stouffer step wedge.
Using 100% LiPd with no sodium tungstate or contrast booster I got a time of 12 mins with ‘traditional’ exposure steps but even after an hour exposure with a Stouffer, the steps had not merged. Going beyond 12 mins did not improve the dmax at all.
Adding sodium tungstate to warm the image needed dichromate or Part B to boost the contrast but the exposure times rocketed for both and I never got a good dmax.
I now use CsPd to warm the image (33% for neutral and 50% for warmish tone), with exposure times 14 and 18 mins respectively and both without contrast booster. However, very small changes in the moisture content of the paper before exposing will swamp the effects of the CsPd.
As Keith says, different curves are definitely required for each variation of chemistry.
To avoid the use of contrast booster (for digital negatives), two things are essential:
Use a Piezography system with Photo Black Shade 1 or denser - I wasted so much time trying to get paper white and good dmax with an older Piezography system that worked fine with Pt/Pd
Use Bostick & Sullivan products - Ziatype chemistry available from Europe doesn’t give as much contrast as B&S, as I found out to my cost.