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)

Hi Phritz,
It looks, by your exposure times, that you are using the sun to make your exposures with. It looks to me as though you have baked our gum onto the paper, a problem when exposing in direct sunlight. Find some open shade and try about 45 secs exposure and then develop. More than 2 mins in the sun in my part of the world (south west Turkey) is far too much. I'm looking at 45 secs max. You can't let the glass get warm or it will totally screw up your results. Hope this helps,

On Oct 5 2009, phritz phantom wrote:

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

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?