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

RE: haunted GUM (related to judy's favourite pet peeve: the pigmentratio test)

You are talking different pigments here and you are not quite all that specific to the end, so it is a bit difficult to draw conclusions. For example you are talking lamp black and then switching to iron oxide black, etc. 1 gram of carbon black (or lamp black) is definitely not the same as 1 gram of iron oxide black. Pigments have different densities and covering power.
I do a lot of gum printing with lamp black, so let me point to you what works for me. 2.5 to 3.5 grams of powdered lamp black mixed with 100 ml of 14 baume gum is about as much pigment as the gum can handle. I make stock solution to last me for months. To this you can add water and dichromate to your taste. It is the amounts of solid pigment and solid gum that will establish the properties of the final image. If I read you correctly you are using 0.5 g of pigment in 5 ml of gum (I assume standard strength). That is 10% and I could never get an image with so much pigment. At 3% carbon you can get vere, very dense blacks.
Print us a step wedge, even if it is one printed on your printer.

Date: Tue, 6 Oct 2009 01:28:42 +0200
From: phritz-phantom@web.de
Subject: haunted GUM (related to judy's favourite pet peeve: the pigment ratio test)
To: alt-photo-process-l@usask.ca

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?

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