From: Christina Z. Anderson (zphoto@montana.net)
Date: 02/26/03-09:19:00 AM Z
Pete,
Feel good: your book is available, 4 copies, for $75 on abebooks.com!
You've grown more valuable since 1980.
> However I must have missed something could you clarify and explain what
you
> meant by the discussion a while back of chromium vs. dichromate stain,
I will send, below, all the notes I have compiled on the two stains.
Katherine was the one, btw, who clarified that sulfuric was used to clear
chromium stain not dichromate stain.
What I figure from the below list compilation is that there are two
stains that remain, the dichromate stains (more prevalent with am di than
pot di, sometimes not there at all) which can be removed by pot meta, sod
bisulfite, sod sulfite, etc. Pot meta disturbs/softens the emulsion the
least, so some say.
Then there is the chromium stain which is caused by the dichromates
being reduced to that, a greenish more permanent stain that doesn't budge
well, unless you clear with stronger solvents, such as 1% sulfuric. It
seems that the reason for this stain is overexposure, and it doesn't happen
often.
I am not an expert on this, and I have not experienced chromium stain,
so feel free to refute/correct this.
List notes (I think yours, Pete, first?) and a few of my own:
"If I have to print very hard to burn in dense highlights from a
pinhole negative, the resultant brown of the ammonium dichromate in the open
areas of the negative turns a blue-gray color when I 'clear' in sodium
bisulfite. Now that I have tried sodium sulfite (after getting wind of it
from P-F and the List and rejoicing that it didn't make me choke), I find
that the brownest areas remain warmer and could perhaps be considered
greenish. Now I have the option of a cool clearing or a warmer clearing or
no clearing at all. The brown stain was a problem, degrading the cyan to a
muddy grey knocking the edge of the magenta, and affecting the yellow only
to a slight extent . This problem was quickly solved by using any of the
following clearing procedures: 5% sol bath of either sodium sulfite, sod/pot
metabisulfite, pot alum, liquid acid hardener, or a long soak wash in
running water 12 hrs /24 hrs. All of these baths gave much cleaner color
the most noticeable being the effect on the cyan which went from a muddy
grey to a fully saturated blue. However I noticed as time went there seemed
to a slight green cast appearing which was a will of the wisp effect hard to
predict. At first I thought it was badly mixed pigment. I was using acrylic
colour so I did a series of tests without any colloid present and mixed them
physically with a paint brush, the result no green cast. So it had to be the
colloid /bichro mix. Working on this surmise I did a further series of tests
with out pigment the results of which were most interesting. With a full
exposure the colloid went brown using different papers. The shade of brown
was different. I also used three different colloids to cross check on a
control sheet of fabriano artistico 1) W&New gum, 2) egg, 3) gloy and I got
the following results: 1) strong brown 2) green brown 3)medium brown.
These tests were then subjected to the clearing baths and as expected they
each turned a soft green grey, the colour of turquoise that has been let
down with a slight grey tone. So how could I get rid of this residual stain?
A number of other chemicals were tried with no effect. Then I remembered
work I had done on bromoil years before-many bromoil bleaches contain a
small amount of sulphuric acid, which had the effect of turning the gelatine
colourless before inking, and as similar chemistry is employed I decided to
give it a try. BINGO it worked the brown stain disappeared and no green
stain reappeared, however the stain did not in fact go away. On heavy
exposure a light neutral grey tone was created, but this did not pose any
problems as it just made the colours in the shadow regions slightly darker
and in effect undetectable, in normal practice."
Chromium stain:
"Dichromate stain is a yellow stain that can be soaked out (it can be
bright or faint, but it's definitely yellow and can be seen in the back of
the paper. As I said before, I almost never get it and when I do, I don't
soak it out in water but in sodium bisulphite, OUTSIDE). But the greenish
tanning that occurs with long exposures is another matter. I've never seen
it except once when I experimented with using exposed gum as a size. It's a
yucky green color, it is in the gum itself, and it does not wash out. I
abandoned the idea (of using gum as a size) because I found this sickly
green completely unacceptable as a background color, and wasn't interested
enough in pursuing the matter to experiment with different exposure times to
see if the color could be eliminated. I pulled out Luis Nadeau's book on
gum printing and found that he also makes a distinction between the two
types of "stain". After a discussion about clearing the yellow dichromate
stain he adds, "In some cases, especially with long printing times, a
greenish stain will remain in the paper. This chromium oxide is basically a
permanent pigment and is the result of light acting on a dichromate
compound. It should be of no concern." but in a footnote says that a dilute
solution (.5 to 1%) of hydrochloric or sulfuric acid would probably dissolve
it."
Clearing it with sulphuric:
My notes: Some use a 1% solution of sulphuric acid to clear hard to
remove residual dichromate stain, but this is very toxic and dangerous, so
don't try this unless you know how to handle chemicals. You can also use a
solution of sodium sulfite (0.5%) with some sulfuric acid added (0.1%). The
clearing time is about ten minutes, then wash 5 minutes in tap water, then
5 minutes in 0.5% sol. of sodium carbonate (to be sure of neutralizng the
sulphuric acid), then two rinses in tap water.
Others use sodium bisulfite, but it is harsher on the print:
Katherine's research: "The clearing step is intended to remove dichromate
residue, not hardened gum, but as a matter of fact sodium bisulfite does
soften the hardened gum, too. The "various sulfites and sulfates" which
clear dichromate stains have different qualities, being different compounds,
and while they may be roughly interchangeable for the purpose of clearing
dichromate stains, they are not interchangeable in all respects. Sodium
bisulfite, by many accounts, is the strongest and most likely to soften and
lift the hardened gum, which is why I recommended it for gum removal, and
why something like potassium metabisulfite, which removes dichromate stains
but isn't so likely to disturb the hardened gum, would not be useful
removing hardened gum. Sodium bisulfite, having rather strong fumes, should
be used outside or in a well-ventilated area."
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