From: Sandy King (sanking@clemson.edu)
Date: 07/05/03-11:36:24 AM Z
Chris,
>
> I'm glad you got cleared up through Sandy's expertise, but Sandy, could
>you please answer my former question: is number of steps only related to
>contrast or can I safely say it is also an issue of exposure? In my mind
>it has got to be! At a certain point, exposing both am di and pot di at
>their dilutions will show more steps up the scale the longer the exposure
>is, I would think (not talking greater number of distinct steps, or lower
>contrast--just more squares toward the 21 that have image color, you know?).
>Anyway...it may be clear in Jack's mind but not mine--yet. Your 3% to 4.5%
>is absolutely invaluable information to me. You've saved me lots of time.
The point here is that my reference was carbon printing, not gum,
though I believe many of the same principles apply, at least in so
far as hardening and tanning of the colloid is concerned. Clearly the
actual theory of how gum printing works is very different from carbon
so no direct correlation is possible. However, for carbon printing
the answer to your question is yes, the number of steps you can get
on a print is more a question of contrast than exposure, assuming
that you have at least one or two maximum blacks (or reds, blues,
etc.). Beyond that point increasing exposure will not increase the
exposure scale of the process.
>
> Judy, in reference to dilutions, I cannot imagine why not mix the
>chemicals at their saturation point... in practice it doesn't make common
>sense to dilute am di to pot di's strength in your stock solutions. If you
>need less speed, you may use water in your coating mix, or less dichromate.
>And Sandy himself has said that you'd have to have pot di at 4.5% to match
>am di's speed at 3%, so it is not just a question of dilution.
The question of using either ammonium or potassium dichromate in
carbon printing is a moot point since in practice there are no
negatives that would such a strong dilution. A potassium dichromate
solution of 8% or an ammonium dichromate solution of 5% will in
practice give such a long exposure scale that we could print all 21
steps of a wedge that ranged from about 0.05 to 3.5. A saturated
solution of ammonium dichromate would produce a carbon image of
extremely low contrast.
>
> My theory is it relates to acidity and alkalinity, but I'll shut up
>with my testing. However, anyone interested, just use acid and alkaline
>drops and test it yourself. AND, I found this info out from arcane books.
>But, all this testing doesn't make a better gum print, content wise anyway.
Acidity is an important issue. Again, in carbon printing the practice
in former days was to add ammonium to potassium dichromate, which
converted the solution to a chromate of much higher pH. This would
result in less sensitivity but greater contrast. no
>
>
> But, Judy and Jack, I beg to differ with both of you! I've been mulling
>this over for the last two days, and in agreement I will say that the most
>valid way to learn gum is by doing. Period. The time I spend researching
>if spent gum printing would be better for my understanding of the process,
>since I happen to spend thousands (no lie, over the last 3 yr) of hours
>researching photography!
I absolutely agree with you on this point. Research is important in
virtually every aspect of learning and it can can a lot of time and
effort in directing one toward a more productive and efficient use of
time.
Finally, take with a large grain of salt the comments and opinions of
all gum experts, because you will find that they disagree on many
things. Some people say ammonium dichromate is better than potassium
dichromate, others say the opposite. Some swear by sunlight, others
by UB fluorescent banks, others by HID lamps. I know someone who
bought a very nice UB fluorescent bank for a pittance from some poor
soul who had been convinced by one of the experts that you can't make
quality gum prints with fluorescent tubes. And I know for a fact that
one of the best known gum printers in the country recommends against
the use of ammonium dichromate because it stains.
As for my own work with carbon I know for a fact that it does not
make a rat's hair of difference to the final product which of the two
dichromates is used, so long as the percent solution is adjusted to
take into consideration the extra sensitivity of ammonium dichromate.
There are, however, reasons why I might use one over the other in
specific operating conditions or with certain light sources, though
that is another matter.
Sandy
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