ABBREVIATED GUIDE TO MAKING DIGITAL NEGATIVES FOR PLATINUM PRINTING
The fundamental challenge for the traditional photographic printmaker lies with the phenomenon that light sensitive solutions or suspensions of metallic salts applied to photographic media do not respond to tonal gradations of light in a linear manner. Rather, some ranges of tone show a disproportionately greater deposition of silver, platinum, or palladium than others causing a given gray value in the scene to yield a different shade of gray in the print. This is true for all light-sensitive photochemical solutions. This non-linearity can be anticipated by adjusting exposure of the photographic negative in the camera, and the disparity can also be corrected, to some extent, in the darkroom both chemically and by dodging and burning selected regions of the print. With the contact printing techniques used in Pt/Pd printing, most tonal adjustments traditionally have been achieved by manipulating the ratio of chemical agents and by adjusting the ultraviolet exposure time. When working with a negative printed on transparent ceramic-coated media with an inkjet printer, it becomes easier to compensate for the nonlinearity by digitally creating an adjustment curve that forces the tonal conversion to approximate a linear relationship between the subject and the final platinum/palladium print.
A daunting array of variables confront the printer of traditional platinum/palladium prints from digital negatives. These range from alterations of the photochemical process through a whole new set of parameters that can be adjusted within the digital process. It soon becomes clear that for consistency and piece of mind, one must settle upon which set of variables to manipulate and which can be held constant. By confining adjustment to either the photochemical side or the digital side of the process, each new image will not require a whole new process of calibration. It is possible to develop, or even to purchase, a fairly standard digital protocol with which to print acceptable digital negatives. One can, then, manipulate the parameters of photochemistry based upon time honored visual assessment of the negative and a progressive evaluation of test prints. However, keeping the exposure time and photochemistry constant and confining manipulations to the digital image file before applying a constructed standard conversion to the digital negative will ultimately prove more predictable, more efficient and vastly more powerful.
Standardization of the digital negative making process to yield predictable platinum prints requires that the three steps be undertaken for each type of paper used. Firstly, establish the standard print time and, simultaneously, the black point/D-Max by determining the minimal exposure time to assure printing the blackest black possible without darkening shadows. This is accomplished by making a contact Pt/Pd print of the commercially available Stouffer Transmission Projection Step Wedge* sandwiched with a blank sheet of Pictorico Transparent Media** at an exposure time well beyond the estimated Standard Print Time, perhaps 10 minutes or so, with high power UV bulbs. When dry, carefully examine the array of numbered progressively lighter bars to identify the border at which the black tones last merge. The highest number of the last two bars to merge together (D-Max) corresponds to the number of stops to correct the exposure time. This will become the Standard Print Time for all subsequent Pt/Pd prints with that particular paper.
Secondly, establish the paper white point using the Maximum Optical Density Matrix within the digital inkjet printer software. This process allows one to adjust the printer to lay down the minimal amount of black ink necessary to achieve paper white, (ie, to block all metallic deposition in the whitest area of the print) at the standard exposure time.
Thirdly, having established a standard exposure time to produce reliable black and white points, one can then measure, compare and correct each gray value to assure accurate and smooth tonal transition throughout the final platinum/palladium print. This third crucial step begins with making a platinum/palladium print of a digital step wedge printed with the proper maximal optical density as determined in step 2 and at the exposure time determined in step 1. The Pt/Pd digital step wedge so printed is then scanned and the k-value of a full range of 10-15 squares are then measured. The measured k-value of each square is compared to and ultimately corrected to its corresponding digital value by constructing a digital adjustment correction curve to that forces the complete array of tones in the digital image to translate linearly to the appropriate tones in the final Pt/Pd print. After making the first adjustment curve, it should be applied to the original digital step wedge and a new negative made. Then, make a new Pt/Pd print and repeat the scan and measurement. Areas that are still not closely corresponding can then be tweaked to make a second adjustment curve and the process repeated until a curve results that will closely reproduce the k-values of the original step wedge onto the platinum print. This can now be applied to any image knowing that it will accurately reproduce all the tonal values from the digital black and white image on the screen to the final platinum print. An additional advantage of this approach lies in the ability to alter focally individual shades of gray within selected regions of any image, using digital image software to measure, and the curves image adjustment layers to change, the percentage of gray desired at specific locations within the image.
Michael Van Buskirk
The specific details of preparing digital negatives and using them for platinum/palladium printing are available within the text and citations at www.platinumprints.org.
Michael Van Buskirk
* Stouffer Industries, Inc., Mishawaka, Indiana. www.stouffer.net
** Mitsubishi Imaging (MPM), Inc. 555 Theodore Fremd Avenue, Rye, NY 10580