The Step Tablet

This is the first in a series of entries planned to give some insight into the craft of platinum and palladium printmaking. Some of the observations will be purely technical, some related to my own working habits, and others to the experiments, successful or otherwise, that contribute to making me a better printer. I hope in explaining the more technical aspects, not to overcomplicate my points.

Pictured is a step tablet, a table of known density values which is created on a computer. When inverted and printed onto a clear film for use in the darkroom, the darkest step is heavy with ink. This acts as a mask, preventing light from ‘exposing’ the coated sheet of Platinum and Palladium paper. To simplify, black on the negative makes white on the print.
Conversely, where there is no ink on the negative, light can pass through easily to expose the light-sensitive paper. This results in the darkest values of a print and is known as ‘maximum density’ or D-Max.

Logically then, the steps in-between black and white produce various shades of grey. More ink on the negative, lighter on the print. Less ink on the negative, darker on the print. Given this fairly simple relationship, it might seem that we could invert a photograph, create from that a digital negative, and then return to the darkroom expecting a faithful reproduction of these tones on the print. Unfortunately it doesn’t quite work that way, which is the reason for having a tablet of ‘known’ values to test against. You see, as a chemical process, the relationship between negative density and print brightness is not linear, and this must be corrected for to produce a well balanced print which shows a full range of tones.

We do this by making a platinum print from the step tablet, knowing full well that it will not look good. This is then processed normally and dried, before scanning to bring it into digital form. We then measure the density of the various steps and notice immediately a disconnect between the reading, and the number printed in each of these steps. A linear process would show these numbers as the same, but this is not a linear process. The next step aims to make it so.

The curve palette in an imaging application is opened, and the numbers written on our tablet inputted. On the opposing axis, the actual readings we got from our print are plotted against these target values. A correction curve is created to accommodate these differences, and will now be applied before printing future negatives. This in theory has given us a linear process, and the technical foundations required to make an expressive print.

This curve needs testing as anomalies can easily creep in. We do this by repeating the above. Secondary correction curves can be used, rebuilt and retested until the correction reliably produces a smooth, linearised negative. For platinum printing this tends to look very dense and quite ‘flat’. So long as our conditions and working practices do not change, the curve will work. For those of us who test new papers, mix platinum and palladium in differing ratios to alter print colour, and change developer to affect print contrast frequently, the building and refining of curves is a never ending process in the pursuit of fine printmaking.

Thanks to Ron Reeder for this step tablet.

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