Re: Platinum and reciprocity

Luis Nadeau (awef6t@mrburns.mi.net)
Mon, 11 Nov 1996 20:54:21 +0300

.
>It is not a matter of reciprocity failure.
>
> When you are dealing with papers where the light sensitive materials sit
>at the
>surface of the paper and within it, rather than in a layer of gelatine on the
>surface, self masking comes into play. The light sensitive salts at the
>surface
>of the paper darken first at the surface under the thinnest parts of the neg to
>act as a mask slowing down the action of the light on the salts under the
>surface of the paper so that more time is available for the light to affect the
>salts under the denser parts of the neg.

The same phenomenon can occur with the right emulsion *in* a layer of
gelatin. P.O.P. (Printing Out) paper was/has been available in both
collodion emulsion (ca. 1885-1920) and gelatin emulsion (ca. 1885 - right
up to today with Chicago Albumen Works papers)

Luis Nadeau
nadeaul@nbnet.nb.ca
Fredericton, New Brunswick, Canada
http://www.mi.net/dialin/awef6t/