Re: Tonal inversion (was (Gum) Tonal scale)

Dec 7 2005 1:25 am

Joe,

Say we define pigment density (PD) or pigment concentration if you prefer as
the weight of pigment versus the volume of gum plus dichro used (g/ml). It
could also be defined as the number of pigment particules versus the volume
again but this could be harder to measure.

I think we could easily "predict" what the results of your test show with
the following reasoning: since both gum and pigment are not perfectly
transparent to light and both acting together will "interfere", block and/or
filter out some part of the light acting on the dichromate, this will have
the effect of increasing the "net optical density" (NOD) of the negative you
are using. In other words the less PD you have in your emultion the less the
NOD of your negative will be and this implies more of the light will get to
the dichro which translate to more exposure and more steps that show. All
this of course assume the same 75 units of exposure for all these test
prints as you mention below (hopefully I got this right).

This NOD effect could explain part of the reasoning for multiple exposure
and I suspect also it can be used to actually modify the response curve to
suit ones need. Kind of the analogue counterpart to digital curve
adjustment.

I did try a few test of my own using only dichromate a bit like you did and
I got all the way up to 21 steps but before I got there I change my light
source. The first light was a 500 watts photoflood light at 12 inch above
the print, a small 5x7 in this case. As soon as I exposed above say 30
minutes, the staining started then I used a 75 watts bulb for which I have
the spectrum which showed a promissing level in the UV range and used it at
12 inch also. I started at 30 minutes and doubled this time each time until
I got out of steps for a 2 hours exposure. No inversion at all that I can
see, I checked a few time if the glass was warm or hot and I would say it
stayed more warm then hot. This kind of suggest that heat may play a
significant role in this inversion stuff.

I didn't measure the value of each steps but I can say this "qualitatively"
the shadows contrast is low for the first few steps then it increases at a
much higher rate up to about step 13 then the contrast drops to very low ie
from step 13 on, the tonal change appear as deacresing gradually and without
perceptible steps from 16 and above. I'll try to put this on my web site
later.

I understand pretty well that as soon as I'll add gum and pigment to the
dichromate the number of steps I'll get for a 2 hour exposure will change
dramatically especially so if I use a dark and opaque pigment. I kind of
compare this to the filter factor, for exemple a Wratten #29 (deep red) as a
20x factor and I wouldn't be surprised if I needed this much or even more
exposure for some pigment. I don't know if I'll have enough patience to get
21 steps out of a more "normal" mix of gum, pigment and dichro. I wouldn't
be surprised though if the shape of the response curve presented much of the
same features I discribe above....

To be continiued
Yves

----- Original Message -----
From: "Joe Smigiel" <jsmigiel@kvcc.edu>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, December 05, 2005 3:55 PM
Subject: Re: Tonal inversion (was (Gum) Tonal scale)

> OK gang, here's a couple more scans from yet even more gum tests.
>
> First is a test using a pthalocyanine blue (Damiel Smith Thalo Blue) at
> different concentrations of pigment. Starting dilution the same as I
> posted the other day for other colors, namely 1 gram powdered pigment
> per 10ml gum arabic solution and 5 ml saturated potassium dichromate.
> Then cutting the pigment mix with additional gum and dichromate to get
> ratios of 1gm:20ml:10ml, 1gm:40ml:20ml, and 1gm:80ml:40ml.
>
> http://my.net-link.net/~jsmigiel/images/technical/gum/pthalo_test.jpg
>
> This color printed out about 5 steps on average but all tests are very
> heavily stained. This is typical for pthalo blues for me even though
> others report printing it without staining. Again, the only time I've
> ever printed a pthalo blue without appreciable pigment stain was usnig
> Linel Hortensia Blue and even then there was a perceptible overall color
> shift caused by very slight staining.
>
> Now here's the interesting (to me) part. I also retested the Daniel
> Smith Bone Black pigment that produced the tonal reversal the other day.
> For today's test I used the same pigment concentration as before, (1gm
> powdered pigment in 10 ml gum + 5 ml saturated potassium dichromate), as
> well as the same paper and exposure source. I reduced exposure from 600
> to 75 exposure units based on the previous test to give me maximum
> density at step #1 today. I based today's corrected exposure on last
> week's intentionally overexposed print which first exhibited the tonal
> reversal. (There's a little variation there probably due to mixing the
> emulsion slightly different today, but the maximum printed density is
> close to before.) I ran two tests with the same emulsion batch & paper
> the difference being I intentionally coated both at the same time,
> printed and processed one sheet immediately, and let the other sit in
> the dark for an hour before exposing it. The latter print is just
> barely darker and this difference could probably be equalized by letting
> the print soak a bit longer. (Both were autodeveloped for 1 hour in 3
> changes of water.)
>
> The image linked below show last week's test adjacent to the two tests
> run today:
>
> http://my.net-link.net/~jsmigiel/images/technical/gum/black_reversal.jpg
>
> Tom showed last week that the reversal shifts along the stepwedge scale
> with exposure and I can confirm that observation today. (So, heating of
> the emulsion under the most opaque areas of the light attenuator is
> probably not the cause. It must be pigment and/or pH related.) As my
> scan shows, the reversal effect still occurs even with reduced (and
> probably optimal) exposure for this mixture. But, take a look at the
> numbers! Not only has what should be the white field around the numbers
> and the higher steps reversed, the numbers and letters have reversed as
> well in the tests run today! The numbers should print as maximum
> density yet they have printed as minimum density. Weird with a beard!
>
> I'll let y'all discuss this amongst yourselves. I'm bailing. I'm off
> to devote my time to wetplate collodion now.
>
> Joe
>
> >>> gauvreau-yves@sympatico.ca 12/03/05 1:53 PM >>>
> Joe, et. al.,
>
> Your absolutely right, in fact I would add that every things
> (conditions,
> variables, parameters or whatever) in play in a test like this will
> influence the result in some way, may be it will be a very small
> influence
> not to say an insignificant one but it will.
>
> Yves
>
> PS. Many of you use from time to time acronyms I think there called,
> like
> here "IIRC" below. Would there be some place I could go to find out what
> they mean if this exist???
>
>
>
> ----- Original Message -----
> From: "Joe Smigiel" <jsmigiel@kvcc.edu>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Saturday, December 03, 2005 1:05 PM
> Subject: Re: Tonal inversion (was (Gum) Tonal scale)
>
>
> > Yves, et. al.,
> >
> > I just realized this effect is probably also related to specific
> > pigments. (This would also support what Tom has observed IIRC, and
> > perhaps also to Chris' observation regarding pH.) The reversal effect
> > appeared when using bone black as the pigment but not cobalt violet.
> > Each was given the same exposure which presumably would lead to
> similar
> > heating of the test wedge (assuming the integrator is working
> correctly
> > on the exposure unit). Since the bone black reversed and the cobalt
> > violet didn't, something else must be contributing to or causing the
> > effect.
> >
> > You are correct in that it may be very difficult to isolate the exact
> > cause of the effect without introducing some other variable. I
> > certainly don't have the means to do so.
> >
> > Still, it appears I have overexposed the bone black test by roughly
> > 3.5-4 stops so I'll run a test at a much shorter and more normal
> > exposure to see if the reversal effect dissappears at that level.
> >
> > Joe
> > ---
> >
> > >>> gauvreau-yves@sympatico.ca 12/03/05 12:10 PM >>>
> > Joe,
> >
> > This may well be the way to check this idea out but as you say it may
> > introduce something else in the process. I think it would be somewhat
> > complicated to verify this, with a high degree of certainty in
> practice.
> >
> > In my own test, this happened to me as well and as soon as I reduce
> > exposure
> > a bit it didn't happen. This makes me believe that there could be
> > somekind
> > of threshold involved but again this is all very speculative on my
> part.
> > Maybe someday I'll have the time and the money to investigate crazy
> idea
> > like this one.
> >
> > Thanks
> > Yves
> >
> >
> > ----- Original Message -----
> > From: "Joe Smigiel" <jsmigiel@kvcc.edu>
> > To: <alt-photo-process-l@sask.usask.ca>
> > Sent: Saturday, December 03, 2005 11:44 AM
> > Subject: Re: Tonal inversion (was (Gum) Tonal scale)
> >
> >
> > > >>> gauvreau-yves@sympatico.ca 12/03/05 11:06 AM >>>
> > > >>... I think it could be caused by heat...<<
> > >
> > > I agree. This seems to be plausible explanation for the effect.
> I'll
> > > run a couple more tests reducing exposures or perhaps doing
> > intermittent
> > > ones to keep the negative (test wedge) from overheating and see if
> > that
> > > has any impact (although I might be introducing some sort of weird
> > > intermittency effect by doing so).
> > >
> > > Joe
> >
>
Received on Wed Dec 7 03:18:17 2005