Re: Dichromate dilution and speed

<large snip, Katharine Thayer said>
My results do not support the idea that there is
> little difference in speed between diluted ammonium dichromate and
> saturated ammonium dichromate. I found instead that reducing
> concentration from saturation to 1/5 of saturation, following
> Christina's description, required 5X the exposure time to make a
> properly exposed print. (Prints exposed for the same time, or even 2x or
> 3x as long as saturated dichromate prints, were so grossly underexposed
> the gum simply ran off the paper leaving no image at all.) I ran this
> test on two different gums and got the same result both times. This is
> too little data to prove anything in a greater sense of course, but it
> certainly casts doubt on the assertion that was made.

Hi Katharine et al,

Happy Thanksgiving to all! Nice to come back to a bunch of gum postings,
even if we all disagree still!

First wanted to clear--I made not an assertion, only an inquiry--I've
learned to add lots of caveat emptors to anything I say here :)-, but maybe
I didn't clearly do so with a last post. NO ASSERTION THAT THE SPEED
WAS THE SAME. (is that clear enough??) Just that the speed wasn't so
incredibly slow (or truncated) with dilute dichromate to make it unusable. I
went back and
reread my posts to see how this got unclear. So here I reiterate:

1. Using a lesser dichromate with diginegs resulted in a 1-4 (usually 1)
minute exposure. Before this, I had no acceptable exposure, IMHO. Too low
contrast, too long development.

2. I exposed my Direct Dupe to ONLY a 1 minute exposure just to be really
drastic and compare it with a typical digineg exposure--this negative
normally exposed with saturated dichromate for about 6 minutes. The 6%
dichromate produced a tack sharp print in the 1 minute exposure, but I would
expose longer because it developed in too quick a time for my liking--under
10 minutes. I am not assuming from this quickie experiment that therefore
the Dupe neg will expose in exactly 6 minutes with the 6% solution as it did
with the saturated 30% solution--just that a 6% solution works fine for a
regular neg, and I would recalibrate my times accordingly. No scientific
questioning here, just an off the cuff experiment to answer Charles Ryberg
when he asked if I had tried the 6% with a regular neg.

My questioning was exactly as Sandy says:

"The main issue as I see it is the one posed by Chris. That is, what
is the minimum amount of dichromate one can use and maintain good
speed and contrast?"

My inquiry was if gum exposure is not a straight line "characteristic
curve".
There is no way I can, nor do I want, to
spend time proving this, it is only interesting to me in practice.

That's all I'm interested in, and it was in reference to digital negatives,
which weren't working **for me** until I drastically reduced the dichromate.
Apparently with **my setup** (caveat emptor), the saturated dichromate
solution was too fast, and too low contrast, to produce a print that I felt
made diginegs worthwhile. I couldn't add more pigment to adjust contrast---I
had enough color already. I use an Edwards lightbox--maybe this is it?
but no, because Sam uses a NuArc. Apparently in your practice, Katharine,
the "...minimum amount of dichromate one can use and maintain good speed and
contrast" is not a 6% solution. Whether that applies to all, or all out in
the northwest area, or whether the PCB's floating down here in the water
have added speed to our dichromates, who knows. It may be that at different
times and locales I have to switch from the 6% to something else stronger,
who knows? Even then, I can only offer my practice that at this time and
place--it works for me.

I am so happy Suzanne Izzo posted to say she, too, used only a (gasp) 2.5%
solution and got 6-8 minute sun exposures! And with potassium dichromate at
that! That is half the 6% I was using, and in my mind her "6-8 minute
exposure"
is still within the realm of acceptability. In fact, sometimes I wish my
gum exposures were longer so I wouldn't have to stand there, time them by
the minute, and rush. Anything under 10 minutes is great!

I'd like to say that my motivation for the lesser dichromate was
environmental, but it wasn't, alas. It was just to make diginegs work.
However, finding it worked, I know that the profs I taught with at MSU will
be THRILLED to hear how little dichromate one can actually use. One prof in
particular really had a hard time even thinking to teach gum.

Whether this works for anyone else or not, so be it. If one's contrast is
too high or one's exposure too long, add more dichromate. Changing the
pigment to adjust contrast is not always, in my book, a viable method, if I
want my color to be a certain depth. Actually, I already have my pigments
mixed into the gum arabic, in separate 50ml bottles, which I sometimes
dilute with plain gum to get a particular color at a particular time, not to
adjust contrast..

Far be it from me to say this method is gospel--this list has proven no gum
gospel. I have gone from ice cold sulfury well water and an incredibly low
(semi-arid) humidity in MT, to warm city water and high humidity here,
around 40-60% inside (or, at least, high for me; my paper never "crackles").
These kind of changes alone have made my practice change somewhat.

But I do mix my own gum from dry powder and find it a cinch, contrary to the
old books that have you suspending it in muslin. I have 2 mixed up, 1+2
water and 1+3 water. I find the latter a bit watery, frankly. And I do use
a hair dryer to dry coats.

Actually, too, none of this info is attributable to me in the first place,
but Sam and then Sandy. I'm so glad to have found something that works well
for me, and hope that in talking about it, it helps others, too, but I
cannot take credit for it, right or wrong. It is soooo exciting to finally
get diginegs to work!! I now scan my negs on my flatbed scanner (made a
little "mask" to keep them above the scanner to prevent moire), print, and
go.
Regards,
Chris
Received on Sun Nov 30 20:29:14 2003