U of S | Mailing List Archive | alt-photo-process-l | Re: Exposure times in tricolor gum

Re: Exposure times in tricolor gum

Neat that you find this out, too, Henry. And that you find the exposure times to be longer for Y, less for M and less for Thalo because that corroborates "across the pond" and with very low humidity here, etc. etc.

Yellow and carbon black seem to be pretty "straight line" for me, fairly close, and magenta and thalo are very close. Thalo is such a fast color, even when deep. It goes to show that the old timers were correct when they said colors printed at different speeds.

That never made sense to me (why colors would have speeds) until seeing the effect through PDN that different colors of negatives printed faster and slower, and then realizing that this would be true of pigments. But it is surprising, the curve shapes of the different pigments are so drastically different.
Chris
__________________

Christina Z. Anderson
http://christinaZanderson.com/
__________________
----- Original Message ----- From: "Henry Rattle" <henry.rattle@ntlworld.com>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, November 18, 2008 6:24 AM
Subject: Re: Exposure times in tricolor gum


Yes, the Y curve is quite a lot closer to the 45 degree line - less sigmoid
than the cyan and magenta curves, which are close to identical.

Afraid I mix the pigment by eye - small blob, larger blob... So can't be of
much help there.

Best wishes

Henry


On 18/11/08 12:50, "Loris Medici" <mail@loris.medici.name> wrote:

Thanks Henry,

By "...Y is much flatter..." you mean that it's closer to the ideal 45
degrees linear transfer function or it's more drastic? I'm currently
printing using a single curve devised from M tests.

What can you say about the pigment concentrations you use?

Regards,
Loris.


18 Kasım 2008, Salı, 2:43 pm tarihinde, Henry Rattle yazmış:
Hi Loris,

My exposure times (50% gum/pigment, 50% saturated Potassium dichromate)
are
7 minutes for C and M, 10 minutes for Y. These times are "standard
printing
times" from precision digital negatives (PDN) testing, all exposed through
R255B60 colorised negatives, with separate curves generated for each
colour.
(The C and M curves are very similar to each other, the Y is much
flatter.)

Best wishes

Henry


On 18/11/08 12:20, "Loris Medici" <mail@loris.medici.name> wrote:

This goes mainly to tricolor gum printers printing from digital
negatives
separately calibrated for each color layer: do you experience any
exposure
time variation? If yes, can you please tell me your exposure times (and
dichromate strenght, only if it isn't kept constant) for each layer? I
just want to see if there's a correlation / connection...

I balanced the pigments according to 2Y + 1M + 1C. In other words, my Y
stock paint:gum solution contains 2x paint compared to both M and C
solutions. Using the same coating solution formulation (which is 1 part
paint:gum solution + 1 part gum solution + 2 parts 10% ammonium
dichromate
solution), I find that yellow requires the most exposure whereas C
requires the less (M in between). Is this similar in your case?

Thanks in advance,
Loris.