[alt-photo] Re: Gum Printing: Looking for some wisdom

[Christina Anderson]

Hi Jennifer,
I am coming into this conversation very late, being on vacation, but one thing (please forgive if I haven't followed correctly or someone already said this) when calibrating gum printing, the dichromate will get browner and browner and give a false sense of exposure well past the time needed to harden the gum. Dichromate goes to a dark brown ultimately (and when cleared with potassium metabisulfite etc. turns pale green) and the steps go on and on darker and darker so you see that line of demarcation still, well into overexposure of gum, so I think that might be confusing. For instance, where you have the test strips with the strip of, I assume, Pictorico down the center, if well-cleared that result might change. But Pictorico is 1/2 stop exposure, too.

For the record, as Marek, I think, said, dichromates don't go "bad." They are inert until in contact with an organic. So it isn't the dichromate. I've used 6--year old solution with no problem, and this is also well-documented in the literature.

I expose a step for no more than 15-20mn, develop, dry, and then clear the heck out of it so little dichromate stain remains. Then I can better judge the step wedge. From this I find my times for thalo blue very minimal (5ish), magenta about 5-6, and yellow 6-9 under a bank of UVBLs. I don't expose any colors longer than this except a thicker black. But who am I to know, as all things gum are not cut in stone!

Looking at all your colors, too, I think you have pretty much nailed your dilutions if you are going to use the strongest ones you've shown, and not the weakest ones.

Happy gumming!
Chris

Christina Z. Anderson
christinaZanderson.com

On Mar 11, 2013, at 2:26 PM, Darkrooms, Department of Art wrote:

> Hello all,
> 
> I am reporting back with the tests I made last Friday.  I was going to mix
> new Ammonium Dichromate, but I did not have a new bottle.  So, I decided
> to retest my original test of Ammonium Dichromate.  You can view the tests
> here: http://www.flickr.com/photos/93779173@N03/sets/72157632974249458/
> 
> I think that I now have a good jumping off point with these tests.  This
> week I hope to work on the calibration of the QuadTone Rip negative.
> 
> Thank you for your help!
> Best,
> Jennifer Gioffre
> Teaching Support Specialist
> 
> Architecture Art and Planning
> Cornell University
> 120 Tjaden Hall
> Ithaca, NY 14853
> 
> Office: 607-255-4207
> Fax: 607-255-3462
> jmg393 at cornell.edu
> darkroommanager at cornell.edu
> 
> 
> 
> 
> 
> 
> 
> 
> On 3/7/13 1:56 PM, "Kees Brandenburg" <workshops at polychrome.nl> wrote:
> 
>> Hi Jennifer,
>> 
>> As we are talking about gum, let's not forget this variable. What gum
>> arabic do you use? Premixed or self mixed, and at what concentration and
>> baumé? I'm using a 14-14,5 baumé self mixed gum. This is about 26-26,5%
>> 
>> Looking at your exposure times for cyano and van dyke, compared to mine
>> (6 and 4min) I would say that you would end up somewhere around 10
>> minutes with your bank of UV and normal gum sensitivity. I find your
>> pigment concentration rather low. With 'normal' strenght gum I am using 1
>> gram per 20ml gum for winsor blue and lamp black on gelitin/glut sized
>> paper and never see any stain.
>> 
>> With dichromate I'm a 'replenisher'. When the bottle seems to empty with
>> a coming workshop or project, I make some new and fill the bottle up.
>> From time to time it stands for several weeks or more.
>> 
>> kees
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