Pigments and DR (was: Re: solarized gums?
On Feb 15, 2007, at 1:10 AM, Loris Medici wrote: Sorry, I skipped over this part to correct a misconception about what I was saying about overpigmentation, but I think it deserves coming back to re-address the question of whether it makes sense to relate density range of the negative to the number of steps printed in gum, using a formula derived from other processes. A year or two ago, someone showed a step tablet that showed 12 steps on a 21-step for a particular gum emulsion, and someone else said, wow, that's ... 12 X 0.15 = 1.80! Gum will print a negative with a DR of 1.80! Well, yes and no. That may be true technically, but it doesn't mean that the print will remotely resemble, in tonal range, a print you'd print from another process with the same negative. You'll start from the darkest dark that particular coating mix is capable of, and go up 12 steps from there. Regardless of the negative, the tonal range and the especially the DMax of a print are determined more by the pigment and concentration of the pigment than by the negative. For example, during the period of time when I was interested in printing only very high-key images, I could get a lot of very subtle steps with a very lightly pigmented mix, probably 16 or 18 steps, but the gradation from one step to the next was very subtle, and the darkest step was very light. The pigment ( IME) that will give you the darkest DMax in gum, lamp black, won't give you 12 steps at its maximum saturation; it will only give you 4-6 at best. I don't know whether you could achieve 12 steps with a lighter mix of lamp black, since my tests using lamp black have been at maximum strength, or overpigmented in order to produce stain for demonstrations; I don't print with lamp black as a rule, because I don't care for the opacity of the pigment. But we'll just assume that a lighter pigment mix would give you 12 steps, for sake of the argument here. In that case, you could print that coating mix with a negative with a DR of 1.8, but you wouldn't be able to get the darkest DMax that the pigment is capable of with that mix. in order to get the deepest DMax possible, you'd have to print a second layer with more pigment in it to go darker. So you're always constrained more by the coating mix than by the negative. Burnt umber, the example referred to above, is weak (has low pigment strength), but it also doesn't print very dark, which is a different issue from pigment strength (for example, pthalo and lamp black both have high pigment strength, but pthalo can't print nearly as dark as lamp black). There are pigments that are good for making a dark brown one-coat gum, such as ivory black or a mixture of burnt umber and a healthy dollop of black, but burnt umber by itself just can't do it, as someone who consulted me several months ago after trying this, discovered. Burnt umber by itself is good for adding color to a gumover or to a multiple gum print, but is too puny to carry a one-coat by itself. You have to work within the limitations of the pigment. And bringing the DR of the negative down to match the characteristics of the pigment isn't going to change the characteristics of the pigment. One of the clothesbaskets full of test prints from my gum-curve- testing period is full of prints of density patches using most of the pigments in my arsenal. I was curious whether different pigments would require different negative colors. . Being a statistician and understanding the dangers of sampling error, I printed each test print three times to be sure the test print came out the same way each time, making it more reasonable to assume the result is robust and not an anomaly. What I found was that except for two oddballs, all of my stock pigment mixes printed the same pattern of density patches, giving a particular red-orange color as the best negative color for my printer, for all three of the three test prints made for each of the pigments. The two oddballs gave inconsistent results across the three, with one or two of the patterns being the same as all of the other pigments, and the remainder being different. One of those oddball mixes was known to be underpigmented, as I had started mixing a new stock mix before I realized I didn't have another tube of PR 209, and I already had the gum in there and couldn't pull some of it out. That one, at least, really should be redone now that I've replenished the PR 209 supply. But on balance, the result of this series of tests indicates that the DR for gum, as reflected by the color that gives minimal white, is the same across pigments. This suggests to me that the DR for gum is determined by the gum and not by the pigment. But those mixes, that all give the same negative color, print with very different numbers of steps, almost by definition; pigments print differently, even when they are carefully standardized to give 100% color saturation for each pigment, as I do. A saturated mix of burnt umber is going to print a different number of steps than a saturated mix of pthalo, is going to print a different number of steps than a saturated mix of lamp black (and each of those step prints will start from a different absolute tonal value for the DMax). So, since in my observation the different pigment mixes seem to want to print with the same DR, although they will print a different number of steps, I'm not sure how much sense it makes to think of density range of the negative in terms of the number of steps printed by a particular coating mix. Katharine As someone
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