Platinum/Palladium Printing on Wood
The challenge for anyone attempting to print photographs with the photochemical methods of the pre-digital age lies with the dilemma that the individual tones of gray of an image do not translate from the original to the final print in a linear manner. That is to say, for example, that an image area of 20, 40, or 60% gray in the original or as loaded onto the computer display may vary widely in tone from its original in the final print unless corrective measures are undertaken during the printing process. The dilemma can be addressed by adjusting exposure in the camera or photochemically in the darkroom by adjusting exposure and contrast grade of paper. By the same token, meticulous digital printers employ digital color profiles tailor made for the specific printer, ink and paper they use.
Happily, for printing with the age old and hallowed process of photo-chemical printing in platinum/palladium (as well as other so-called alternative processes), contemporary methods of printing negatives digitally obviate many of the photochemical manipulations required with traditional methods of printing from large format film negatives. Application of a digital adjustment curve, specifically constructed for the process, to a digital image file, allows one to obtain consistent linear translation of a digital image from the monitor screen to the final print, even a photochemical print in platinum/palladium. These methods allow the photographer to produce consistent and tonally accurate platinum/palladium prints on watercolor paper, such as Arches Platine, that has been properly sized for the platinum/palladium process. An important difference between hand-applied processes such as platinum/palladium and the silver emulsion processes of the more recent past is that the platinum/palladium sensitizer is an aqueous solution of metallic salts that absorbs onto and into the paper substrate rather than a suspension within a gelatinous or resinous coating that lies over the surface of the paper. Thus, the substrate must be somewhat absorbent of aqueous solutions, and uniformly so throughout its entire surface.
Based, I suppose, upon a long standing fascination with wood working and enjoyment of hand made wooden objects, I have long aspired to apply the process of platinum/palladium printing to wooden surfaces, not just to apply the image to the wooden surface, but to incorporate the wood as an integral component of the final piece, its grain contributory, other natural defects visible. Like many papers, wood is organic, relatively absorbent of water, and aesthetically pleasing to both sight and touch. Most woods however are darker than paper contracting the potential dynamic range of the ultimate print and have variable absorbency both within a single surface and among the myriad of wood species. Fortunately, most wood is slightly acidic favoring the platinum/palladium precipitation necessary for the printing process.
Finding no literature for platinum/palladium printing on wood, about 10 years ago, I casually began a series of experiments designed to best simulate on wood the printing process typically used on paper. Within the past two years, I have refined the process sufficiently to be able to make satisfactory platinum/palladium prints with a consistency comparable to that I experience with printing on watercolor paper. Although I have used a variety of species of wood, I favor lighter colored woods with visible but not overwhelming grain and fairly substantial absorbency. I have printed on very hard woods such as maple, but generally favor a softer open grain wood such as baltic birch.
In addition to the usual issues and vagaries of hand printing in platinum/palladium, wood introduces some unique challenges of its own. Firstly, in order to improve the paper white or lightest aspects of the image, I slightly whiten the wooden substrate by lightly applying a white pickle stain. Secondly, to achieve consistent absorbance of the metallic sensitizer, I apply a polyvinyl alcohol size available from Gamblin®. Thirdly, each piece must be examined carefully to plan in advance how the grain and other characteristics may best be incorporated into the final work. Because wood is very rigid and much thicker than paper, it does not fit into traditional contact printing frames. This has required construction of custom presses slightly larger that the desired print size by which I can clamp a heavy piece of plate glass over the negative and sensitized wood panel. The same issue of the wood thickness mandates that the wood be treated in the trays face down in order to maximize exposure to developer and, especially, to clearing agents and the water bath. For the final prints, I primarily print on commercially available birch wood art panels, but have printed successfully on ordinary wood planks with no or minimal warping.
Of course, as with conventional platinum/palladium printing with digital negatives, one must determine a new standard exposure time and then a new adjustment curve for each variety of wooden surface. I find in my laboratory I need a substantially longer exposure time for wood, around 2.5 times my usual exposure time for paper. Once the exposure time, digital printer settings and digital adjustment curves are established, the process for printing in platinum/palladium on wood is as follows.
I use cradled birch panels for most of my work. The quality differs from brand to brand, unfortunately proportionate to price, especially for the larger panels. The cradling on the back of the panel prevents warpage during development and also, after varnishing, makes an easy to hang or otherwise display final piece. I recommend using baltic birch plywood cut into small pieces for calibration and practice sessions but you may get some warpage during the clearing and water bath treatments.
Birch panels usually require some light sanding but the commercially available cradled birch panel are pre-sanded and ready to go.
The first step is to apply a light white pickle stain to the unfinished wood. I find the MinWax® products work well but many other products are available. The stain will go on more easily, smoothly and be wiped off more cleanly if you first apply a stain preparatory solution first. This can be brushed on and allowed to dry for approximately 1-2 hours until the surface does not feel cool. After the stain prep has dried, the grain will be slightly raised, so the surface should be lightly sanded until satiny smooth. Then, apply the white pickle stain to the piece with a foam brush. After covering smoothly, quickly wipe off with a paper towel or smooth rag allowing the grain to be clearly visible. The goal is to slightly lighten the overall wood tone without obscuring grain and other characteristics of the wood. Then, allow the residual stain to dry overnight.
After the stain has dried overnight, lightly sand with find grit paper (I use sponge back sanding pads) until satiny smooth to touch. Carefully apply Gamblin® PVA Size with a foam brush. Cover the entire surface but do not allow the size to pool within the printable are or on the edge. Pooling on the edge can be remove by gently wiping with the edge of a foam brush held at about a 45o angle. Pooled size blocks absorption of the sensitizer and cause spots on the final print. Allow the PVA size to dry for several hours or, preferably over night. After dry, again sand the surface lightly until satiny smooth.
The wood substrate should now be ready to accept the metallic sensitizing solution to make a platinum/palladium print. The coating process is very similar to that used for paper. I print with the Na2 method of Richard Sullivan, mixing equal parts Ferric Oxalate, 27% and 15% Sodium Chloropalladite, adding one drop Sodium Platinum Na2 solution. I use 1 drop Na2 for each 10 drops of the each of the others, all mixed in a small shot glass. For the first print, I add a tiny amount of Tween-20@ by touching the dropper to the inside wall of the vial and allowing the sub-drop portion of surfactant to mix with the sensitizer. I repeat the touch of Tween application about every third print. Using an ultrasonic mist generator (humidifier), I briefly wave the wood surface in the mist. Then, apply the sensitizer in the normal manner and spread carefully with a fine 2 inch watercolor brush until the printing surface is a uniform mustardy hue. Set aside to dry. I place my coated panels in a print dryer and allow at least one hour to dry. I then place the panel to be printed in a heated film dryer, turned to the lowest setting, for a few minutes to be sure the coating is completely dry.
The contact printing process for Pt/Pd on wood mimics that on paper with some necessary practical modifications. As with the print on paper, one sandwiches the sensitized substrate (in this case, wood) and the negative under glass to achieve uniform contact of the negative with the substrate and expose to ultraviolet light. The wood art panels that run approximately one to two inches thick, do not fit in a conventional contact printing frame. However, the problem is easily overcome by laying the wood panel, sensitized side up, on a plywood base and using a 1/4 thick sheet of plate glass to hold the negative tightly in contact with the sensitized wooden panel. I use plate glass sheets cut to overlap the wood panel on all sides by about one inch. I hold them firmly in place using screen clips held down with screws on each corner. This plate glass press press apparatus, holding firm the sensitized panel and negative are then placed within the UV lightbox. I find the required exposure time to be somewhat over twice that required by paper, about 2.5 times. The exact standard exposure time must be determined with the calibration process and with my equipment runs around 12 minutes. In the face of an overly flat, under-exposed or over-exposed image, one is sorely tempted to adjust exposure time or vary the photochemical composition of the sensitizer. Invariably, however, a far more effective and efficient approach is to return to the digitized image, carefully measure k-values in crucial areas and make appropriate adjustments to the k-values on the digital image, then a new negative, before printing a new image in platinum. Often seeming small local shifts or separation of shades of gray produce dramatic improvements in image quality.
The process of development of the exposed image on wood is identical to that of the platinum/palladium print on paper, with some notable practical exceptions imposed by the physical characteristics of the wood panel. The three dimensional character, its capacity to float, and tendency to warp in water all mandate some accommodation. In addition, I find the platinum/palladium print on wood much more difficult to clear than a similar image on paper regardless of the clearing method. I use potassium oxalate developer and clear in two or three successive baths of hypoclear, having found other clearing agents equally effective and difficult.
For development, lay the exposed wood panel face up, its latent image clearly visible, in a tray only slightly larger than the wood itself. Wearing protective gloves, rapidly pour the developer over the entire surface to instantly bring forth the near final image in a manner well familiar to platinum printers. Then, immediately turn the image face down to float on the developer for about 30 seconds, checking its quality now and then. Then, manually remove the developed panel and lightly hold, by the edges, under under gently streaming cool water over the developed surface for about 30-60 seconds. This process will minimize warpage as may happen if the piece floats in a water bath. Then, place the developed panel face down in hypoclear for 10 minutes, agitating occasionally, approximately once a minute. Again rinse under running water and place in a second hypoclear bath for another 10 minutes. I use large trays of hypoclear holding about 4.5 liters of solution for a 10x10 inch print, and, even so, the first tray becomes exhausted after three or four uses. I discard both hypoclear solutions after each days use and mix fresh from stock before each printing session. After clearing, place the panel, again face down, in a running water bath for 10 minutes. Avoid prolonged washing to minimize warping, but the image should be bright and clear after that time. I also use a gentle clamp to hold the panel against the side of the wash tray so that water does not splash onto the back, further minimizing warping. Then shake of any any excess water and place the wood print in a print dryer face up to dry. I allow the panels to dry for three days or longer before finishing.
If the wood is not completely dry before varnishing, cracks appear along the edges of grain. If in doubt, I place in my heated film dryer on low heat. Assured that the newly printed woodprint is thoroughly dry, I apply two coats Archival Acrylic satin spray varnish to the print surface, allowing a day or so between coats. The edges, I varnish with Liquitex Satin varnish using a foam brush approximately the same width as the cradling thickness, normally one inch.
Once the varnish has thoroughly dried, the final piece may be mounted by suspending a standard picture wire between the inner sides of the cradling, supported with a French Cleat or placed to stand alone on a shelf or table, depending on the size and aesthetics of the print.
Michael Van Buskirk