The problems I have encountered so far include highlight staining, unblocking shadow detail, resolving mottling in mid-tones and highlights and achieving a smooth tonal gradation. I've been discussing these problems over e-mail with Mark Nelson, Sam Wang, Chris Anderson and Beth Moon, and re-reading Mike Ware's and other's technical notes on coating methods and problems. These problems and their solutions will be discussed further in the next few posts.
I have been using three test images during my calibration of Herschel's classic cyanotype, and those images suggest an approach to test image choice when tackling a new process.
The above high key image challenges the process to distinguish tone and detail in highlight. It also brought to light the mottling of the emulsion when coating Arches Platine that my low key image hid.
Much of my work when tackling a new alternative process involves calibration with a step tablet, determination of exposure time, and construction of curves using Mark Nelson's Precision Digital Negative system. At some point towards the end of that calibration process you want to print a test image that exhibits a range of tones from shadow to highlight with mid-tones present. Also, the image should have detail to allow judging the sharpness of the image. I have been using the image of the figure in coiled irrigation duct for that step.
For checking the calibration and handling of shadow detail I use the low key image to the left. The interior of the black bowl has a lot of subtle detailed variation in the darkest tones of the scale and revealing those details in a print requires a careful calibration and adjustment curve for each alternative process.
Paper choice, emulsion formula, coating method, development, humidity. These all factor into the calibration of an alternative process. Variations in any of these will affect the final result and ultimately mean the difference between a mediocre print, a good print and the final goal of an exquisite one.