From: Liam Lawless (lawless@ic24.net)
Date: 06/18/01-04:23:31 PM Z
Hi Bob,
As I understood it, the problem is lower contrast with high intensity (i.e.
sunlight), which I believe my reciprocity failure theory supports: the areas
where the exposure is heaviest would suffer most. It's just an idea that
occured to me after thinking about it for a few minutes, and I don't deny
that other factors may also be operating, but I don't think self-masking
alone provides a satisfactory explanation: I'd expect the self-masking
effect to be the same with high or low intensity exposure, if the two
exposures are photographically equivalent. (If the two exposures are "the
same", why would the self-masking effect differ? Further explanation is
needed!) Maybe the same laws don't apply to non-silver, but the same effect
is observed with other processes, so maybe they do.
Clerc mentions in passing that a weak light generally gives greater contrast
under the heading of "Silver Print-out Papers", but nowhere else that I can
see. However, under "Pigment Processes", he says, "It may be said that, as
a general rule, printing on carbon tissue by artificial light lessens the
contrasts in the print, all other conditions remaining equal. This
reduction in contrast is specially marked when arc lamps on a high voltage
are employed, as their light is very rich in ultra-violet rays." I've no
idea whether high voltage arc lamps are more or less actinic than sunlight.
But, as far as cyanotype is concerned, I wonder if bronzing of the shadows
might not have prompted Shannon's original question. Clerc again: "On
exposure to light (chosen as strong as possible), the unprotected parts of
the paper turn progressively a deeper and deeper tone of bluish or purplish
grey, finally becoming metallized with a silver-grey or bronzed appearance.
If the exposure to light is considerably prolonged, the exposed parts become
lighter and lighter, while the incompletely protected parts darken. The
image then appears reversed. If this reversal is only partial, washing in
water is often sufficient to restore the image to its ordinary condition.
On the other hand, various reagents can be applied which will restore the
print to a normal condition. The more vigorous prints are usually those
which have undergone slight reversal."
Further on, he names some of these reagents: a very weak solution of
hydrogen peroxide, eau de Javelle, potassium bichromate, persulphate,
chlorate, etc., neutral or slightly acidified. (Such treatment should be
deferred until after the second washing at least, and should be followed by
washing in several changes of water.)
There's also a footnote about the cause of reversal: "This inversion seems
to be due to the progressive reduction of the ferricyanide (less sensitive
than the ferric salt) giving rise to a white ferrous ferrocyanide which is
easily brought back to the state of Prussian blue by oxidizing agents or,
more slowly, by the simultaneous action of air and water."
Clerc's interjection on the nature of the exposing light, "chosen as strong
as possible", would appear to confirm that a strong light will reduce the
contrast of cyanoypes, since cyanotype is an inhrerently contrasty process.
Liam
-----Original Message-----
From: Bob Kiss [mailto:bobkiss@caribsurf.com]
Sent: 18 June 2001 12:16
To: alt-photo-process-l@sask.usask.ca
Subject: Re: contrast control, the sun, and cyanotypy
DEAR LIAM,
I feel the problem may be more complex.
For Silver Gelatin materials High Intensity RLF usually compresses the
high exposure areas while leaving the middle and lower exposure areas (in
any given scene) at "normal" contrast with an net decrease in contrast
necessitating an increase in both exposure AND development. Low Intensity
RLF usually does the opposite, i.e., it reduces the effectiveness of the
lower exposures while leaving the middle and higher exposure areas (in any
given scene) at "normal" contrast resulting in an effective INCREASE in
contrast necessitating an increase in exposure but a decrease in
development. All of this results from the time and intensity dependent
conflicting processes of latent image formation, interstitial silver ion
migration, of electrons to the interior of the AgX crystal and
rehalogination.
Hence, speaking strictly of RLF (not factoring in Auto-masking) higher
source intensities should result in reduced contrast and lower source
intensities should result in higher contrast *if they are outside the "flat"
region where reciprocity doesn't fail*.
This seems contrary to what people are experiencing...yes? Even the
instruction sheets with Chicago Albumen Works POP state that contrast can be
increased by leaving the negative/POP sandwich in the printing frame under
normal room fluorescent lights for very long exposures. I haven't tried it
so I cannot say.
So I am afraid the problem is more complex...perhaps spectrum as Jeffrey
suggests, or some other causes. And, of course, this may be totally
academic as Cyanotype is NOT a silver/gelatin process.
CHEERS!
BOB KISS
----- Original Message -----
From: Liam Lawless <lawless@ic24.net>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Saturday, June 16, 2001 9:21 PM
Subject: RE: contrast control, the sun, and cyanotypy
> As we all know, silver emulsions are most sensitive to light intensities
in
> a certain range, within which exposure = time X intensity. Outside this
> range (i.e. for very low or high intensities), the relationship no longer
> holds (reciprocity failure): the photographic effect of a given amount of
> exposure is less than it would be when reciprocity obtains, and exposures
> need to be increased acording to the emulsion type and how far the light
> intensity falls outside the normal range.
>
> Seems pretty likely, therefore, that sensitisers for alt processes suffer
> similarly. If a source such as the sun is bright enough to cause
reiprocity
> failure, this could explain the loss of contrast. Remember, too, that the
> light intensity will be highest under the clearest areas of the negative
> (the areas which print darkest), and that this is where the effects of
> reciprocity failure would be greatest.
>
> Just a guess.
>
>
>
> Liam
>
> -----Original Message-----
> From: shannon stoney [mailto:sstoney@tnaccess.com]
> Sent: 15 June 2001 03:47
> To: alt-photo-process-l@usask.ca
> Subject: contrast control, the sun, and cyanotypy
>
>
> Hi. Another cyanotype question, which may apply to other processes as
> well. I have been using the sun to expose my cyanotypes. When there are
no
> clouds, the exposure takes about three minutes, and I get about eight
steps
> on the step tablet. But, if it's cloudy, the exposure may take seven to
> eight minutes, and I get more steps! Has anyone else observed this
> phenomenon?
>
> --shannon
>
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