[pure-silver] Re: Cleaning a Drum Dryer


----- Original Message ----- From: "Bogdan Karasek" <bkarasek@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, April 03, 2007 6:10 PM
Subject: [pure-silver] Re: Cleaning a Drum Dryer


Hi,

When I was doing the sepia toning, it was written on the instructions that a hardener should be used after toning because the bleaching process renders the paper soft. Do they mean that the bleach "eats" away the protective layer of gelatin coating in order to get at the emulsion?????

I did not use a hardener and noticed that on several of my prints, there were motes of dust imbedded in the paper. I took one print and tried to rub off the dust but to no avail. That's when I realized that it was imbedded in the paper, but which layer, emulsion layer or was there still some gelatin left, despite the bleaching?

Cheers,
Bogdan

Richard Knoppow wrote:

snip

The gelatin is not eaten away but rather softened. Some toners are pretty alkaline. Gelatin does not of itself have a pH but does have a preferred pH called the isoelectric point. At this point the swelling is minimum. For most photographic gelatins this is just on the acid side of neutral. When the gelatin is bathed in an acid or alkaline solution it swells and softens, the further the bath is from the isoelectric point the more the swelling. An auxilliary hardener changes the way the gelatin swells but the white alum hardener in fixing baths is removed by an alkaline bath. So, sometimes it necessary to harden again after treatment in a toner or reducer or intensifier bath. Many modern papers have emulsions containing a mixture of gelatin and plastics which are considerably harder than the old style emulsion and less affected by pH or temperature, but some papers probably still need extra hardening. If you are getting embedded particles when ferrotyping hardening may not help much. The tin must be scrupulously clean as must be the print. Otherwise any particles will become embedded or leave some sort of mark on the print.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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