The use of well-made digital negatives for Platinum/Palladium printing has the substantial benefit of standardizing many of the variables involved in contrast control. In other words, digital negatives can and should be tailored to eliminate many of the variables of the process that are concerned with controlling contrast.
The goal of all approaches to making digital negatives is to create a negative in which the density range (DR) matches the exposure scale (ES) of the printing process. Methods that rely on the standard printer driver are limited to using the Media Type selection to control the amount of ink (ink limits) applied to the film. A Photoshop curve is required to do the heavy lifting of pulling and pushing the tonal values to where they need to be in order to produce a print with a linear tonal scale. QuadToneRIP (QTR) allows for much finer control of how the inks are used. A good QTR profile, before the addition of a process specific correction curve (Gray Curve), will produce a negative that prints fairly close to linear. The Gray Curve needed to linearize the profile will be very gentle, especially when compared to the very steep toe and shoulder section and flat mid-ranges of the curves required in other approaches.
The QTR Curve
More properly called a profile, the QTR Curve is a set of instructions that determines how the printer applies ink to the substrate when printing with the QTR driver. In the Mac version, QTR works as an alternative to the Epson printer driver. It appears in the printer list as a separate item with a name such as Quad3880, and is accessed through the Print menu from any application such as Photoshop or Print Tool. In the PC version, QTR is a stand-alone program. The PC version also includes a Curve Creator tool, which the Mac version lacks. In the Mac version, QTR Curves are created as text files using TextEdit. The text file for the QTR Curve referenced in the screenshot below looks like this:
- The # at the beginning of the first line indicates that this line is not part of the code. It is a note where I have specified the purpose of this file. This corresponds to the Notes tab in the PC Curve Creator tool as do each of the other sections.
- Ink limits are determined in part by testing with the calibration target to find the maximum load that the substrate can hold, and in part by trial and error to find what is needed for a particular printing process.
- Number of Gray Parts and the other variables associated with it is an area with many possibilities for experimentation.
- The settings in the example above began as a file I obtained from someone else (probably Kerik Kouklis or Clay Harmon) in 2008. I have modified it substantially over the years since then to arrive at something that works well for my system and methods.
- The GRAY_GAMMA and GRAY_CURVE fields are of special interest.
- GRAY_GAMMA allows you to alter the slope of the entire tonal scale. A setting below 1 add density over the entire tonal range and reduces contrast, making it useful for long scale processes such as Pt/Pd and salt printing. Settings above 1 have the opposite effect and are useful for short scale processes such as traditional cyanotype and gum bichromate.
- The GRAY_CURVE field can contain either
- a set of input output numbers as in the example above
- a path to a Photoshop curve saved on your computer such as: /Users/…/3880/QTR/Diginegs/3880pK-UPOHP-BL-Pd_3a.acv
- Note that you do not apply the correction curve to the image in Photoshop. Rather, it is built in to the QTR Curve.
- The beauty of this method of making digital negatives over any of the methods using the Epson driver, is that if you have done a good job with the ink limits and the gray variables the actual curve will be very gentle and smooth unlike the curves needed to linearize other methods.
Making the Gray Curve
Because the correction curve is specific to each set of printing conditions, first you must decide on some standard procedures in your Platinum/Palladium printing process. Paper, contrast (via chlorate, Na2, or other method), ratio of Pt to Pd, developer and temperature of developer all should be considered. The curve above was designed for Arches Platine, a minimal amount of Na2 to ensure fog-free highlights (1 drop of 1.25% per 1ml of ferric oxalate + 1ml Pd), and pure palladium developed in Potassium Oxalate at 120°F.
- Make a 21-step digital test negative using your QTR Curve with the Gray Curve field empty (or with “0;0 100;100” as a placeholder).
- Print that negative using your preferred coating mix at your pre-determined exposure time (minimum time to achieve maximum black) and develop using your standard procedure.
- When dry, use a densitometer or spectrophotometer to read the 21-steps. Enter those values into the spreadsheet.
- Some printers do this step using a scanner. I feel this introduces an opportunity for error since you are reading a scan of the test print rather than the print itself.
- Until recently, I used an X-rite 400 reflection densitometer to make density readings, but sadly it is no longer working.
- Fortunately, I found that I can use my DataColor Spyder3 Print spectrophotometer manually as a densitometer.
- Do one of the following:
- Use the Curve Input/Output numbers (highlighted in yellow) to create a custom .acv curve file in Photoshop to be linked to the Gray Curve field in your QTR Curve file.
- Enter the Input/Output pairs directly into the Gray Curve field
- Save the text file with a new name and run the install script.
- Repeat Step 1 using the new QTR Curve that includes the Gray Curve.
- Repeat Step 2
- Repeat Step 3
- The graph of the 2nd test print should be very close to a straight line.
- Tweak the curve if desired, save with a new name, and repeat the tests until satisfied. Usually one tweak of the curve is sufficient.
Prepare the image file in the same way you would if your goal was to produce a high-quality black and white digital print. I work with scanned black and white film in 16-bit grayscale mode. If your originals are color film or files, they can be prepared in any way you like but will need to be converted to grayscale for printing. I use Gray Gamma 2.2 for the document profile. Whatever profile you choose, be consistent.
It’s not a bad idea to proof your file on paper with QTR before printing your negative. When you are satisfied with the digital print, you are ready to make the negative.
Printing The Negative
Recent versions of Photoshop, CS6 and CC, lack the ability to print with No Color Management which was an option at least through CS3. This unfortunately introduces the possibility of unexpected interference with the operation of QTR from the operating system or from Photoshop. It is possible to print negatives using QTR from within Photoshop but not without using some form of color management. Print Tool (by Roy Harrington, the author of QTR) is a simple stand-alone printing utility that restores the ability to print with No Color Management.
- Select correct Quad Printer
- Page Setup
- Open image file(s) in Print Tool
- Set desired Position and Scaling
- Set Printer Color Management to No Color Management
- Check Negative — Invert & Flip
- Alternatively, this can be done in Photoshop before opening the image in Print Tool. Do it in Ps if you do not want to print a black border around your image.
- Click Print…
- Open QuadToneRIP menu, make the following selections, then save as preset
- Mode: 16-bit or 8-bit
- Curve 1: [name of curve]
- Paper Feed: Manual Front
- Resolution: 2880 dpi
- Speed: Uni-directional
- Black Ink: Photo
- Insert transparency film in printer
- Click Print
- Let the negative cure overnight or use a gentle flow of warm air to speed up the drying/curing process. This is important!