Re: Gum calibration (was: Paper negatives- Ink Selection)
now, we're comparing exposure time where dichromate concentration is similar, lightsource is similar, negative substrate, paper, gum and stock gum solution strengths are the same, what is considerably different between Keith and I (which you are ignoring) here is the printer (therefore paper profile & inks) and I'm completely sure that Keith's negatives DR are greater than mines and if he calibrates for a lower DR, he'll get shorter exposure times...
This last is THE (not simply A) good idea. Better if instead of (or in addition to) a true image you use (or add) a grayscale with known UV densities.OK, I decided to take another road, since the people doesn't get it with the current one: Keith, can you please send me a small image (say 4x4" @ 300dpi, 16bit grayscale) privately, if you like? I will make a negative for you from that one and send it to you allowing you to test it (along with the negative you personally made from that file) and see what happens? (Or, you can just have it measured with an UV densitometer - that also will give an idea...)
"Similar" does not mean "equal", so that if all you listed (concentration, lightsurce, substrate and so on) is not measured or strictly kept constant you (and some other chiming in) will never be able to compare the results or infer any proven deduction for or against these types of comparison.
For example, the type and age of the tubes, the characteristics of the surroundings reflective surfaces, the distance from the lightsource to the sensible surface, and the thickness and composition of the glass are the variables contributing to define the light flux to the negative. So, [you keep constant all these variables] OR [you measure the actual flux to the negative *under* the glass].
Think adding to this all about sensitization, paper, humidity, temperature, etc...
This is why many of us frequently say "for my lightsource and my paper and my...." (Judy or Mark perhaps will add also "and my fortunate pants").