U of S | Mailing List Archive | alt-photo-process-l | haunted GUM (related to judy's favourite pet peeve: the pigment ratiot

haunted GUM (related to judy's favourite pet peeve: the pigment ratiotest)

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  • Subject: haunted GUM (related to judy's favourite pet peeve: the pigment ratiotest)
  • From: phritz phantom <phritz-phantom@web.de>
  • Date: Tue, 06 Oct 2009 01:28:42 +0200
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hi all,

my gum is acting strange again. the only reason i can think of is an increased pigment load.
my standard pigment is lamp black, which is a very strong pigment. 0.5gr are enough for a very thick and opaque layer (before exposure). since i was used to this strong pigment, i was generally using too little pigment for all the other colors, resulting in very thin layers. so, i made a comparison sheet with dabs of all the different pigments (all are powder pigments) in various strengths. i was quite surprised to see that for example 2gr (+5ml gum + 5ml saturated pot-di) of my burnt terra di siena produces a coating that is neither thick, nor opaque.

at first everything went fine, then suddenly a very thick blue black coating (1,5gr iron oxide black + 1gr phthalo blue +5ml gum + 5ml pot-di) didn't come off at all during development. ok, i thought the reason was that i increased the exposure time as well to compensate for the bigger amount of pigment. later: the same with a short exposure of 1 minute. the next day: again, with a layer with 2gr of burnt siena.

it was time to search for errors. i coated a sheet with 1,2gr of iron oxide black (not my favourite pigment), again with 5ml gum + 5ml pot-di, ripped it in three parts and made a comparison of the two different sheets of glass i use as printing frames and put the  third  one for 10min under the desk lamp that i often use during registration and such. the first two printed fine and pretty much the same. but with the third one, i noticed something strange. not only that there seems to be some uv present in the light of the desk lamp, but also: i left part of the sheet covered and it received zero exposure. and this part stayed completely black, not a whiff of pigment came off in the appr. 20min of development.

here's a scan of the test strip:

the part on top with the white stripe received ZERO exposure. i scratched off a little bit to show that the pigment is wet and soaked. it can be removed, it just doesn't want to come off on its own (nor did i have any success with brushing or sprinkling of water, only nothing or everything comes off)
i'm sure this is somehow related to my problems. i'm just getting too confused here. it probably means that my images were severely underexposed. i did extensive testing for negative colors lately and determined with a step wedge (unfortunately not a stouffer one) that my minimum print time is 50seconds. i printed the thick layers with up to 2:30min. still nothing.

(sorry for my total inability to write succinctly in english... my apologies)
can anyone put some sense in this? i'm completely lost. any tips, except trying even longer exposures?