Thank you so much for all the information you have provided. Such a great resource. I was wondering if you could share a little more about your vacuum table? Is it something that you bought or made?
I acquired the vacuum table from a college buddy of mine. It’s a “Multilith” vacuum table made by the ADDRESSOGRAPH-MULTIGRAPH CORPORATION. According to Wikipedia “The original company which manufactured the Addressograph, Addressograph International, merged in 1932 with American Multigraph of Cleveland, Ohio, to form the Addressograph-Multigraph Corporation manufacturing highly efficient addressograph and duplicating machines. In 1978 the corporate headquarters moved from Cleveland to Los Angeles, California, and the corporation name changed in 1979 to AM International. In 1982, the firm filed for bankruptcy under Chapter 11.”
Although I liked the shadow density in my last print, I felt it may have been getting too dark and losing some detail. So, I changed the curve slightly to bring the numbers up in the first four steps. The new output numbers below appear in red below with the new curve to the left.
Here is the result:
I like the depth of shadows, quality of description and overall contrast of the print.
Here’s a side by side comparison of the print from the 2011 curve on the left and the new print on the right with the corresponding curves below.
One final test will see if this will be the new 2012 curve:
In an ongoing effort to improve the quality of my gum prints I’ve spent the better part of last year adjusting the curve that I use to make the negatives. Below is the curve used for my last show “Naiads” in 2010 next to the new “2011 curve” I’ve been working on (shown in red).
Keeping the lower end of the curve (representing the shadows) the same, I “pulled” up on the upper portion (representing the highlights) in order to get better separation in the lighter tonal values.
The results were successful in achieving the desired tonal definition in the highlights and this “2011 curve” what I gave my students to use to make their gum prints last fall semester.
Afterwards, I wondered if similar results would occur if I “pulled down” on the curve for better tonal separation in the lower values to produce more “sculptural” shadows. If it is true, then the question is how much do I “pull down” on the curve?First, I started by keeping the D-max the same output level as with the 2011 curve (22% black) and lowered the next three points (shown in orange), which would allow for greater density in the darker values of the print.
I also tried keeping pulling down on the D-max to an output of 19% black while slightly adjusting the other three points.
I achieved more density in the shadows but at the risk of losing some detail (both color and tone) as shown in the print comparison below (“2011 curve” on the left and the “19 % pulled down curve” on the right). The shadows appear too abruptly with the “19 % pulled down curve” and doesn’t possess the subtle nuance of transitional values of the “2011 curve”.
In an attempt to find an acceptable middle ground between the two above prints, I will next try “pulling up” the curve so that there isn’t such a dip in the first 4 points.
Use the highest resolution file for best results (in the past, I was using 4×5 negatives and making high res scans, now I’m using the Canon 5D Mark II).
In order to get the curves I needed to print CMY negatives in Gum I came up with an ink jet gray scale using the Epson 1400 printer that was equivalent to the Stouffer 21-Step Sensitivity Guide. Knowing that Gum can only print a total of 8 steps (from step #2 to step #9), I used a densitometer (the X-Rite 331 Densitometer) to measure each of the steps (from 2-9) on the Stouffer 21-Step Sensitivity Guide and came up with a numerical value that I could use to match against an ink jet gray scale using Pictorico ULTRA PREMIUM OHP Transparency film. Making wedges in Photoshop and filling in each wedge with different percentages of black ink I came up with a ink jet gray scale that is equivalent to that of the Stouffer 21-Step Sensitivity guide (give or take .02). To apply this ink jet gray scale, I used Photoshop CS5 and in curves, I made 8 points (to represent the 8 steps that Gum can print) and changed the input and output accordingly:
Input 0% – Output 22%
Input 16% – Output 52%
Input 30% – Output 65%
Input 44% – Output 72%
Input 58% – Output 76%
Input 72% – Output 80%
Input 86% – Output 83%
Input 100% – Output 85%
“Pulling up” on the “top” of the curve (even 1%) to an output greater than 85% will increase the contrast of the negative while “pulling down” on the output below 85% will reduce the contrast. Using a different printer is one of those variables that will alter the results of the negative.
Making the Master Template
In Photoshop, I start with a “canvas” the size of the media I’m using (13×19 inches). I fill the background with 100% black (which will print white). In four corners I place the registration marks.
Then I place the four step tablets (YMC, Y, M, C) on the top of the negative making sure each one (except the YMC step tablet) is a separate layer that can be removed later.
Then I place an image as a “place holder” for positioning the negatives (also a separate layer to be removed later).
In alternative processes, the print is only ever as good as the negative (film or “desktop” negatives). Gum prints require the negative to be the same size as the final print; which was a challenge (to say the least) using traditional darkroom methods. With film, density (the dark and light portions of the film) and contrast (distinct separation of tonal values) are controlled by exposure and development. The “Characteristic Curve” of film illustrates the increase in negative density as exposure and development increases. The three portions of the curve represent the different areas of the negative that shows detail: the “TOE” portion are the light areas in the negative and represents the shadows, the “STRAIGHT-LINE PORTION” are the midtones and the “SHOULDER” portion are the dark areas in the negative and represent the highlights in final image. The overall “curve” of each negative could be manipulated by adjusting (increasing or decreasing) the exposure and/or development of the film.
Digital technology (Adobe Photoshop and Epson desktop printers) has made it possible to produce quality, consistent, predictable negatives effortlessly. Before I began printing out digital negatives, I had to create a gray scale with ink on Pictorico OHP Premium Transparency Film that was the equivalent to the gray scale with film. Using a Stouffer 21-Step Transparent Guide, I measured the 8 steps that the Gum Bichromate process can print (from #2-#9) with the X-Rite Densitometer Model 331C and came up with a numerical value for each step. I then created a gray scale with wedges of varying percentages of black ink (from 2% to 100%) and with the Densitometer I measured each wedge to come up with a numerical value for the different percentages of black ink. Finally I identified the percentages of black that come close to steps #2-#9 on Stouffer 21-Step Sensitivity guide (give or take .02).
I am now able to apply this information in order to alter or manipulate the density and contrast of digital negatives by simply adjusting the “input” and “output” using “CURVES” in Adobe Photoshop (similar to increasing or decreasing exposure and development with film).
Gum is susceptible to the many variables that go into the process. In order to identify the variables I can control, I find it easier to separate the process into two parts: the negative (digital darkroom) and the print (wet darkroom).
The process I’ll be describing is what I found that works for me under my specific conditions in order to achieve the results I’m looking for.
Computer: MacBook Pro
Printer: Epson Stylus Photo 1400
Ink: Claria High Definition Ink
Adobe Photoshop CS5
I made my exposure unit using eight Sylvania 20 W 24-inch bulbs (F20T12/350BL/ECO) about 3 inches away from the print. I use a 20×24 inch vacuum table for making the contact prints.