Re: self masking and POP

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From: Sandy King (sanking@clemson.edu)
Date: 01/31/03-10:42:40 AM Z


Chris wrote:

>I've come across a couple books that conflict on self masking. One book
>says cyanotype is not self masking because the purple/blue/grey color does
>not hold back actinic light and shadows block up, but that VDB, platinum,
>and palladium are all self masking (Van Keuren). Another book says
>kallitypes and VDB are not self masking but salt is, and hence shadows don't
>get as deep (Crawford). Another says VDB is not self masking, too (James).
>Who is correct? I think all people agree that platinum is self masking, but
>how about:
>palladium
>kallitype
>brownprint
>van dyke
>cyanotype
>salt?
>And furthermore, which of these are POP? Cyano, VDB, salt??
>Chris

The confusion and/or inaccuracies that exist in the literature may
result from the fact that in addition to processes that are POP and
those that are DOP, there are also processes that are POP to varying
degrees. In another response to your message Richard Knoppow notes
that there are two mechanisms for self masking: the increasing
density of the photolytic silver holding back
further light, and desensitization with increasing exposure.

There are two another mechanisms which may also result in some
self-masking. In colloid photography, the conversion of dichromate to
chromate will produce a printed out image, even in the absence of
any pigment in the colloid layer. And in iron processes that do not
use silver the iron itself will produce a slight printed out image.

 From the above one can assume that there is some self-masking going
on with virtually all of the processes you mention. However, for me a
true POP process is one where the image appears completely, or almost
so, with exposure. With this understanding salted paper and albumen
are perhaps the best examples of POP processes. Vandyke is mostly a
POP process but the image goes through some intensification with
further treatment. Ziatype is also mostly POP.

With most forms of kallitype, as well as platinum and palladium,
exposure forms only a whisper of an image, much too faint to cause
much self-masking. The same would appear to be true of cyanotype.
And, even though there may be some masking caused by the printed out
image that results from the conversion of dichromate to chromate in
gum and carbon this does not appear to result in any significant
self-masking.

Finally, look for the proof in the pudding. In a real POP process one
will often see a very long toe that results in low contrast and
reduced separation in the shadows. This will happen to some extent
with any POP process where the negative is of greater density range
than the exposure scale of the process.

Sandy King


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