Continued-Photochemistry-Pyro
Developer by Robert Chapman (Return to begining
of article)
Maximum
energy pyro developers
If a pyro developer contains little or no sulfite,
two development reaction byproducts contribute to the final image, namely,
silver metal and stain. One such developer (RAF pyro-metol) is shown below.
As you will note, the working strength (metabi)sulfite level is only slightly
above 2 g/l. This is very low for conventional black-and-white developers.
It is, however, typical for processes (both black-and-white and color)
where the oxidized developing agent is employed for further image formation
reactions.
| RAF
pyro-metol developer |
part
A
|
|
| metol |
3.9
g |
| potassium
metabisulfite |
4.4
g |
| pyrogallol
|
4.4
g |
| potassium
bromide |
1.6
g |
| water
to make |
1
liter |
| |
|
part
B
|
|
| sodium
carbonate |
108
g |
| water
to make |
1
liter |
| For
use: Use equal parts A and B (one shot only!) |
| |
|
| Robert
John formula |
| metol
|
2.4
g |
| sodium
sulfite |
10.4
to |
| |
15.6
g |
| pyrogallol
|
2.8
g |
| potassium
bromide |
1.2
g |
| sodium
carbonate |
52
g |
| water
to make |
1
liter |
| |
|
Another
early pyro tanning formula is shown next3. It is similar to
the RAF formula except that the sulfite concentration is varied to control
the degree of stain (and hardening). This developer should also be used
one-shot.
If the sulfite level were increased in these formulas by roughly 5–10x,
both developers would perform as superadditive types with the formation
of only a silver metal image. This would occur because the oxidized pyrogallol
would react preferentially with sulfite to form soluble, colorless byproducts
(just as oxidized hydroquinone reacts with sulfite to produce the weak
developing agent, hydroquinone monosulfonate). Thus, no stain and no gelatin
hardening. Formulations optimized for pyro stain and hardening, unfortunately,
lead to a very short developer working life due to the low sulfite (anti-oxidant)
concentration.
In short, a high energy pyro developer enhances shadow detail that might
otherwise be lost in another developer. Thus, a pyro developer increases
effective film speed— but not by the usual mechanisms (e.g., longer
development, more active developer) that often increase film contrast
as an undesirable side effect.
Practical high energy pyro developers are usually superadditive—meaning
that two developing agents are employed rather than pyro alone—to
increase the development reaction rate. The familiar superadditive developers
outside this discussion are either MQ (metol/hydroquinone) or PQ (pheni-done/hydroquinone)
types. In the case of pyro developers, commonly used super-additive agents
include hydroquinone, chlo-rohydroquinone, amidol, p-aminophenol,
catechol, and color developing agents such as CD-3 and CD-4.
Pyro tanning developers
In some instances, only the hardened gelatin byproduct is required for
the final image. Examples include the dye transfer process and the related
Technicolor process. In these cases, a silver metal image is not necessary.
Indeed, bleach/fix steps are often used to remove it. These processes
are discussed in detail below using (initially) a green-colored original
subject to illustrate the principles.
©2006
Preston Publications. All rights reserved. No part of this
material may be reproduced, transmitted, or stored in a
retrieval system for public or private use without the written
permission of the publisher.
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