From: Katharine Thayer (kthayer@pacifier.com)
Date: 02/23/03-12:44:07 PM Z
Christina Z. Anderson wrote:
>
>
> Gum questions:
> 1. Which colors absolutely do not work? I have these notes: hooker's
> green (washes out), emerald green, and any chromium based colors.
I've been told by someone I generally find reliable that the truism that
chromium based colors don't work with gum isn't actually true. I haven't
tested this assertion myself but just pass it on for whatever it's
worth, and maybe someone else has better information on this. The person
who told me this is unfortunately not a member of the list so can't
speak for himself on the matter.
I'm sure I've used hooker's green without a problem, but have never used
emerald green. Of the many colors I've used, I've never found one I'd
say doesn't work. The thing is that each pigment has its own
characteristics and you have to learn the proportion of gum to pigment
and the exposure that gives the desired results for each individual
pigment, but I haven't seen a color that can't be brought to heel. I
know of someone who uses the Daniel Smith moonglow very successfully,
for example.
> 2. Do you use am di or pot di? I learned on am di and am wondering if it
> is worth it to buy some pot di. Webb/Reed say only use am di if you can't
> get pot di. Hirsch says use am di. Go figure. I just want to know what
> you guys use, and I do have the notes from the past about am di being lower
> contrast/muddier but then those saying that isn't the case. Does it boil
> down to what you are used to?
After a lot of observation and discussion, I've become convinced that
how well ammonium dichromate works is a function of the intensity (and
possibly the heat production) of the light used. Some people using very
strong lights (including the sun) may find that the ammonium dichromate
is too fast for their lights and will result in overexposure and
dichromate "staining" which can change the appearance of the colors and
result in a "muddy" appearance which isn't seen with cooler lights. So
the answer is, it depends. In my practice, ammonium dichromate gives
clear colors and no excess dichromate deposits, but as we always say,
your mileage may vary. It's true that potassium dichromate gives more
contrast (fewer steps) all other things being equal; whether you like
that or not will depend on personal preference, although to some extent
the contrast can be adjusted by changing the pigment/gum proportions. In
my tests, ammonium dichromate can be printed with the same sharper
contrast as potassium dichromate by using more pigment in the ammonium
dichromate mix than is in the potassium dichromate mix.
> 2a. If you diluted am di to the same percent as pot di would it be
> equivalent in speed?
Not precisely. To reduce ammonium dichromate to the same speed as
potassium, you have to dilute it more than to the same percentage
solution as the potassium. Assuming that a saturated solution of
potassium dichromate is about 13% by weight, a solution of ammonium
dichromate would have to be mixed at about 10% by weight, if my
calculations are correct. There's a chart somewhere, maybe in Kosar,
that gives these percentages. It's between 70 to 85%; that is, the
percentage by weight of the ammonium dichromate should be somewhere
between 70 to 85% of the percentage by weight of the potassium
dichromate to get the same speed. I hope that's not too convoluted to
make sense.
> 3. What clearing agent do you use? I have come across sodium sulfite,
> sodium bisulfite/metabisulfite, potassium bisulfite, and sulfuric acid 1%
> solution. Does sodium sulfite truly do the trick so I don't have to choke?
> the metabi literally gives me instant asthma.
I've always used sodium bisulphite but I do it outdoors because the
fumes, as you say, are really noxious. I'll be interested in hearing
other's experiences on the alternative agents. It's my opinion that
sulfuric acid is necessary only for seriously overexposed prints which
contain a lot of excess reduced chromium products, and shouldn't be
necessary for a properly exposed print. And I disagree respectfully with
those who say that prints with ammonium dichromate require more
clearing; as I explained above I believe that's only true for those
using certain kinds of lights.
> 4. Has anyone used lemon juice in the sensitizer to insolubilize it,
> decreasing exposure and giving better midtones? (Demachy, Photo-Aquatint,
> p. 39).
Not me.
> 6. How many gummists are there? I know Judy, Dave, Katherine, Joe, but who
> else isn't speaking up? Even part-timers?
You mean on the list? I see Julian Smart and Hamish Stewart have already
spoken up. Sarah Van Keuren used to be here but I think she isn't now.
Ernestine Ruben watches the list but doesn't speak up as much as I'd
like, and I hope she won't mind my mentioning her name. Suzanne Izzo is,
or was, here, ditto Mary Pat McNally and Harry Bonham. Dick Sullivan is
a gum printer from way back. And there's the group of guys who started
by printing gum over platinum, some of whom are now printing gum by
itself. Kerik Kouklis, Clay Harmon, and others. That's just a few of
the gum printers I'm aware of that hang out here, besides the ones you
already mentioned, and it would be great to have more of the lurkers
pipe up.
Your class sounds great, and I hope you'll keep us posted on how it
works.
Katharine
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