Re: Contrasting agents

["Jeffrey D. Mathias" <jeffrey.d.mathias@att.net>]

Monnoyer Philippe wrote:
> Is a contrasting agent in Pt/Pd processes (or even cyano) just working
> as an oxidizer ? Meaning that it reduces the density evenly on the
> print, and as the concentration is low, the reduction is low too. This
> gives burned highlights. Isn't it more a reducer than a contrasting
> agent ?

Sorry I do not have curves.
But I do suspect that many of the so called contrast agents work as you
suggest.  Better overall quality is to be had by adjusting the contrast
in the negative.  I have not really found any contrast agent that works
linear over the density range (they seem to work mostly in the
highlights).

I have sometimes found that switching papers provides more realistic
contrast control than chemicals.  For example, Bienfang 360 provides
lower and Cranes Cover-90 higher.

Each of the contrast agents or methods seems to adjust contrast in their
own way, but each also brings some detriment to overall print quality. 
I have not seen any information that quantifies or compares various
contrast scenarios.  However, there are many claims that particular
contrast agents do it better.  This may be better for their particular
situation.

Note also that a particular agent may provide other impacts beneficial
to the image.  For example potassium dichromate can give a boost to
clouds (even reported by Weston in his daybooks) or in combination with
potassium chlorate can give a sharpness to rock or brick.  These seem to
be more local contrast effects rather than overall contrast adjustment.

Adjusting contrast of platinum prints can be like spicing a soup, and
there may be just too many variations and preferences to precisely
quantify.

Bottom line is: get it close in the negative and fine adjust with
contrast agents or methods.

-- 
Jeffrey D. Mathias
http://home.att.net/~jeffrey.d.mathias/