From: Michael Heath (heathmg@hotmail.com)
Date: 01/31/03-01:04:57 PM Z
>In a real POP process one will often see a very long toe that results >in
>low contrast and reduced separation in the shadows.
To make things more confusing, my Coming Into Focus book by Barnier says POP
papers have an uncharacteristic curve "that results in greater shadow
contrast and lower contrast in the highlights." What gives??
Mike
>From: Sandy King <sanking@clemson.edu>
>Reply-To: alt-photo-process-l@sask.usask.ca
>To: alt-photo-process-l@sask.usask.ca
>Subject: Re: self masking and POP
>Date: Fri, 31 Jan 2003 11:42:40 -0500
>
>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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