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)



dear marek, loris and david,
thanks for the replies.

david,
i'm not printing with the sun. i use a philips sunlamp with 4x 15w uv tubes (those small ones, as thick as a finger). no significant generation of heat- at least not with the times of gum printing. i recently did proper testing for exposure times and negative color and the resulting base time was 50sec (the time to get the darkest two steps to merge minus 25%). surprisingly this was less time than i was using before.
at first i thought the heat could be reason for the strange behavior, but still it doesn't explain why the part not covered by the cardboard dissolved better than the covered part. both should have gotten the same amount of heat, most likely the covered part got less than the part in the open.

loris,
unfortunately i don't know the humidity. i never did anything about it (humidifying or the other way (dehumididfy?)). the temperature was a little over 20c the last days, 22-23. the humidity today (overcast and rainy) in the area is 76%, yesterday probably less. i coat with a brush and dry for about an hour with a fan (no heat). the coated paper was used the same evening,i think within 2 or 3 hours.


marek,
sorry, for the confusion. but you got me there. i mentioned the lamp black only to explain why i suddenly wanted to increase the pigment load. but you were right, 0,5gr is way too much. i just coated a sheet using the the amount i always use and it's only 0,1gr of lamp black. sorry, my memory failed me there and i didn't really re-check before writing.
as i said in the other mail, i printed the test strip in iron oxide black (a pigment i rarely use and significantly weaker than lamp black), because it (the layer not coming off at all) happened first with this pigment and i wanted to re-create what happened.

the next steps:
(please, if you want me to do anything else or need more info, just say so)
i just coated 4 sheets (unfortunately unsized paper this time. i only have one sized sheet of the paper left and hardening with chrome alum takes at least 24h. i can later re-do certain steps, if necessary) : one with 0,1gr lamp black, one with 1.1gr iron oxide black, one with 2gr burnt siena and another one with half the strength of the siena. i will print 3x the chart throb scale with different times and again leave part of it covered to receive zero exposure.
i also coated a piece of newspaper with each mix, to show how strong the layers are. loris, your monitor might be calibrated, mine is not. but i'll do my best.

regards and thanks,
phritz


Marek Matusz schrieb:
Phritz,
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.
Marek
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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:
http://i31.photobucket.com/albums/c367/phritz/teststrip.jpg

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?
thanks,
phritz


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