Load the image saved earlier that we had used for our initial test. The image values should still be positive and not negative.
Now that we have constructed a curve to create a customized inkjet negative it is time to put it to the test. In all likelihood, your printing process will vary from print to print from all the variables that will affect gum printing. Relying solely upon a single test for our ideal curve will probably not work well for all prints. By completing more tests we can accumulate more data as to how our negatives will print and refine our curve into an ideal and useful one for gum bichromate printing.
Refinement Stage 1
Apply the curve we previously made in Photoshop by going to Image>Adjustments>Curves... selecting the curve from the preset drop down box at the top. Mine was saved as '2-4-8 NEG1 Smooth'.
In Gimp, similarly go to Colors>curves.. and select the appropriate preset.
The image and scale will become inverted according to our very specific scale. Note that the negative is much darker than the first negative we made.
Before we print this however, it is important to gather the rgb values each of our zones has become. Use the eyedropper tool and the info pallet to measure each zone and half-zone's rbg value after the '2-4-8 Neg1 Curve' has been applied. Place these numbers into a second chart in excel in place of the inverted values in the first chart, as shown here in blue.
After the values are recorded, print the negative. It is a good idea to use a pen or sharpie marker to label the side of the transparency with the same name as the curve that was applied, in this case, again '2-4-8 Neg1 Smooth'.
In the exact manner outlined in the last section, print a layer at your shortest time, mine was 2 minutes.
When the first layer is dry hours or a day later, print a second layer of your middle time. My time, again, was 4 minutes.
And yet again another many hours or days later print the 3rd layer at the longest time with half as much pigment (my 3rd time was 8 minutes of exposure). If you chose to use 4 layers or more continue to over print the layers.
In my first refinement print you can see it turned out my curve may have been too dark, or my print did not expose as much. In other words my curve isn't quite right yet and all of my highlights appear to be blown out into white starting around zone 6!
Data and Curve 2
As outlined in the previous tutorial, scan your print with all the settings disabled just as before. Again use the selection tool and the histogram palette to measure the rgb values of each zone from the two scales. Average the values and use the formula to compute the relative scale. My values are shown above.
Sometimes, fishy things seem to happen. In mine the brightest rgb value was 193 and the darkest was 70 (outlined in red). Use these values in the formula for the relative scale. My zones 9 and 10 seem to get darker and darker. This is probably due to a slight staining of my highlights.
Again, open any image in Photoshop or Gimp, open the curves dialogue box, and enter the new data from our 2nd print.
(note that I could not enter the data from higher than zone 8, where input is 255 and output is 12, as the measured values begin to decrease.)
To ensure that the pure whites print as dark as possible on the transparency, use the draw curve option (the pencil circled in red) and draw one little pixel where the input is 255 and the output is 0. It is tricky, but when you get it, you will see a single pixel in the bottom right corner.
Hit the smooth button only once, and save the curve with a name like '2-4-8 Neg2 smooth'.
As we had seen in this test, my highlights had printed too bright, white in fact. This meant my highlights were printing too dark in the 1st curve. In this second curve we can see in the circled area of the curve the highlights have been brightened a bit in the negative. I call this little bump in the curve the 'safety catch' of the curve. If you do not get a bump in your curve I would advise printing with shorter exposure times and starting these tests over, although this is not absolutely essential.
Having the 'safety catch' on the curve will allow us to print longer than our highest exposure time if we ever wish to and not worry about over exposing our brightest highlights. The highlights are safe, in other words.