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

RE: Exposure times in tricolor gum off list

Currently not, but I did before (for multilayer gums, not tricolors) and
in the end will probably revert to this strategy (for tricolors also)
since I feel that I'm not completely satisfied with the results I get with
all those painstaking / long calibration steps...


P.S. I cc'ed this to the list...

18 Kasım 2008, Salı, 3:13 pm tarihinde, John Brewer yazmış:
> Hi Loris
> So you are using a curve from another process that has a similar exposure
> scale and adjusting the pigment/gum/dichromate ratio to fit it?
> Yes, sharing with the list would be a good idea.
> Best
> John.
> -----Original Message-----
> From: Loris Medici [mailto:mail@loris.medici.name]
> Sent: 18 November 2008 13:03
> To: John Brewer
> Subject: RE: Exposure times in tricolor gum off list
> Hi John,
> I see that our patterns match, that's fine...
> About calibration, frankly I think gum is a PIA to calibrate with PDN
> and/or ChartThrob. It's just too hard to be as consistent as these
> workflows require. I currently use a single curve devised from M tests
> (which is not a perfect solution) and may opt to design color-specific
> curves later depending on the results. I'm not after a super-balanced
> perfect color representation, just an approximation...
> My recommendation would be to use a predictable / repeatable process which
> is close to gum in terms of exposure scale (such as 2A + 1B cyanotype),
> and use the curve devised from these tests for printing gum. Of course,
> you'll have to adjust your coating solution to offset the longer exposure
> scale of cyanotype (= stronger dichromate, maybe 1 + 1 + 2 sat. PD or 1 +
> 1 + 1 15-20% AD...) IME, the best results comes from negatives optimized
> for Cyanotype (much better shadows separation / detail).
> My reasoning is = if it's OK to use the same in-camera negative (or
> similarly exposed / developed in-camera color separations negatives), and
> still get good results (as shown by previous practitioners), perhaps we
> shouldn't struggle much with color-specific calibrations...
> Maybe we should share this with the list?
> Regards,
> Loris.
> 18 Kasım 2008, Salı, 2:49 pm tarihinde, John Brewer yazmış:
>> Hi Loris
>> I'm replying off list as although I'm a competent gum printer with film
>> negs
>> I'm trying to do tri-colour with digi negs and am struggling a little.
>> In
>> the last couple of days I've made a couple of prints and my exposures
>> are
>> 4
>> mins C, 5.5 mins Y and 4.5 mins M. I use 2Y + 1M + 1C too. However my
>> paint/gum/dichromate mix is 1 part paint gum solution + 1 part gum
>> solution
>> + 1 parts saturated potassium dichromate solution.
>> There is still some tweaking to be done and I think this is a curve
>> problem.
>> How do you arrive at getting curves? I'm using Chartthrob. I can't use
>> PDN,
>> (because of the cost/licence restriction), as I'm hoping to teach
>> tri-colour
>> in the summer to students who have financial difficulties - I work for a
>> mental health charity who use art as therapy.
>> Best wishes
>> John.
>> -----Original Message-----
>> From: Loris Medici [mailto:mail@loris.medici.name]
>> Sent: 18 November 2008 12:21
>> To: alt-photo-process-l@usask.ca
>> Subject: Exposure times in tricolor gum
>> 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.