Re: haunted GUM (related to judy's favourite pet peeve: the pigmentratio test)
dear marek, loris and david,
thanks for the replies.
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
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
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 20°c 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.
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
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,
Marek Matusz schrieb:
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
Subject: haunted GUM (related to judy's favourite pet peeve: the
pigment ratio test)
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
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
can anyone put some sense in this? i'm completely lost. any tips,
except trying even longer exposures?
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