Re: gum curves
I am going to preface my remarks with the understanding that they apply only to making realistic three color gum print, i.e. you want the colors to be as true to nature as possible.
I am not currently working with color, either in gum or carbon, but I have made many full three-color prints with both processes and have a very good understanding of the mechanisms involved. When I was making color gums and color carbons in the 80s there was no PDN but we still had to deal with film curves since each of the individual color separations, the RGB records, has to be developed to the same contrast if you are to avoid color cross-overs, which result in the inability to balance color in either the shadows, mid-tones or highlights. This is true of any assembly color process, be it color gum, carbon or dye transfer. The first and most important step is to experiment with exposure and development that gives you three records that have the same curve shape and contrast. In commercial three-color carbon and carbro this was considered of such high importance to get good color that special bleach and re-develop procedures were used to balance the separation sets.
All of the is predicated on the proposition that your color tissue set (or pigment or dye set) is balanced so that if you were to combine cyan, magenta and yellow reliefs developed from the same negative you would get true realistic color. If the tissue set (or pigment or dye set) is not balanced, there are still ways to balance color with some of the processes, for example, decreasing or increasing time of development with carbon or gum to increase or bring down the density. However, the best way to make a realistic color print is to start with a balanced color tissue set (or pigment or dye set) and balance your color separations so that they have the density and contrast, and a similar type curve. If you work with a step wedge a balanced tissue set with a balanced set of separation negatives should produce a gray scale. If one of the colors dominates in the shadows, mid-tones or highlights it will be very difficult to make prints with realistic colors.
So in essence, with any color assembly process the key to success begins with evaluation of the individual Red, Green or Blue separation, not with the final print. If you begin with a balanced tissue or pigment set, and produce a set of color separations with an identical curve, you will have color balance. At that point, you further balance for density by increasing exposure equally for the three records. If your tissue or pigment set is balanced, this will give you a balanced final.
One of the advantages of making separations with digital negatives is that you can actually make curves that will give true color from what are essentially unbalanced tissue, pigment or dye sets. However, this involves making curves for each color that correct or compensate for the higher or lower contrast of the sets. I would not claim that this can not be done without a sophisticated method of making curves, but it would certainly be very hard to do so.
At 12:55 PM -0600 10/30/06, Jeremy Moore wrote:
"because curves are developed for each individual pigment separately,