I had this idea to try making Black and White gum prints using a single negative but with multiple exposures.   Recently I experimented with using gray toned Rives BFK with a negative and a positive. I printed the negative using black pigment and the positive using white pigment—the gray would provide a neutral ground.  This idea was inspired by old master drawings where charcoal or ink was used on a “non white” surface (often tan) for shadows and white Conté (or similar material) for the highlights. The results were promising as evidenced by the test print below where Black and White pigments were printed with a negative and positive over Van Dyke that was printed with a negative.

I approached the new B&W workflow by eliminating the positive with white pigment and the gray Rives BFK.  Instead I use a single negative that I register on a piece of white Rives BFK with multiple exposures. 

Making the Print: Cyanotype

 I use Cyanotype as the first layer to add detail and enhance the D-max.  I do a 20-minute exposure (no greater than step 3 to avoid altering the color balance) before any gum layer is put on.

*I use Winsor Newton Lamp Black for sets 1-3

SET 1: 

I expose (about 1 min 40 sec) and develop enough to achieve slight tone in step 3.

SET 2:  I then increase the Gum to Sensitizer ratio to 13:11 (for greater contrast) and expose (about 6 min 30 sec) and develop to achieve slight tone in step 8.

SET 3:  I increase the Gum to Sensitizer ratio again to 14:10 and expose (about 1 min 15 sec) and develop the long enough to achieve slight tone in step 3.

Final layer (gouache) Using the same 14:10 ratio and the Cyan negative I do a short exposure (about 30 seconds) with Lamp Black.  Repeat if necessary for greater D-max and greater depth in the print.

In my last post, I discussed how the old curve I used with Gray Scale negatives was too contrasty for RGB negatives when making my gum prints.  The old curve was based on creating eight distinct points (representing the 8 printable steps I can achieve with gum) and altering the input and output of percentages of black ink.  

In Gray scale mode, the Input levels and Output levels are represented in terms of ink percentages rather than light values in “Pigment/Ink %” (on left), where white represents 0% ink and black is 100% ink.  In RGB mode, by default, Photoshop displays the Input levels and Output levels gradients in the Curves dialog box from black to white in “Light (0-255)” (on right): black represents areas of no light, a brightness value of 0, and white, the maximum amount of light possible, at level 255.

To come up with a new curve, it was easier for me to work with the “Light (0-255)” graph.  I got the new input numbers for the new curve by using the old curve in RGB mode.  I just needed to find the new output numbers.

Fortunately, much of the preliminary work was done when I came up with the old curve so it wasn’t necessary for me to “start from scratch.”  Originally I used a densitometer to measure the percentages of black ink on a grid of 100 shades of gray printed on Pictorico Ultra Premium OHP Film and compare those readings with measurements from a Stouffer 21-Step Sensitivity guide. See post from July 4, 2011 for more information: 

I enlisted a former student of mine from Queens College and aspiring gum printer, Vanessa Fischer, to help come up with a new working curve. I made a printout of the 100 shades of gray for the purpose of writing notes.  

I identified those squares of percentages of black ink that corresponded to the old curve along with the correlating densitometry readings.  Using the eyedropper tool in Photoshop, I recorded the number that appeared in the “INFO” window in RGB and repeated for all 8 squares corresponding to the 8 steps in my curve.

In the example square below, the bottom number (0.87) represents the densitometry reading, the middle number (85) is the percentage of black ink from the old curve and the top number (54) is taken from the “INFO” window—which has now become the new output numbers. 

Below is a comparison of the old and new curve. The new curve is shown on the right in red below. 

Vanessa was able to try out the new curve before I had a chance to print.  Once I saw her result (below), I was convinced it was going to work.

The new curve.

While experimenting with making Gum Prints over Van Dyke this past fall, I realized that RGB negatives contain more information than same image as a Gray Scale negative (subject of my last posts).  I decided to explore this further and see how it would apply to my making full color gum prints.  

Below is the curve I’ve been using when making full color prints using gray scale negatives.  Complete workflow can be seen in my January 25, 2012 blog post http://tonygonzalezartist.tumblr.com/post/16491648522/gum-bichromate-workflow-2012

Below is a print made using the curve and gray scale negatives.

Next I made a new set of negatives using the same curve but with RGB mode negatives.  Below are the results.  Immediate observation revealed that although the curves translated identically on the graph, the prints however were different.  The RGB negatives produced a much more contrasty print which resulted in loss of highlight detail (as seen in the elimination of clouds and sky in the RGB print).

What was needed was a new curve for RGB negatives that would replace the old curve with the gray scale negative.  The graphs below shows the new curve (in red) compared to the old curve (in black).  

The following prints show the comparison of the old curve with gray scale negatives (right), the old curve with RGB negatives (middle) and the new curve with RGB negatives (left). 

The results of the new curve with the RGB negatives regained the highlight detail lost with the old curve using RGB negatives.  The color, tones, density and contrast are also well balanced.

Next blog post, the new curve…