----- Original Message ----- From: Mick Maguire Subject: RE: B/W Paper Developer?
> Can I ask a dumb question? > > What is "bronzing" what does it look like and why does it happen? Bronzing is when bronze coloured blotches appear on black and white prints. The cause is quite a complex chemical reaction. This is what Ctein has to say about it: <Ctein> "The source of the trouble was the opaque whitener used in RC papers....... RC paper manufacturers use titanium white (titanium dioxide, TiO2). Titanium white is a photo active compound. A molecule of it can absorb a photon of light and go into an energetically excited state. It can transfer that energy to other molecules, including oxygen. The excited oxygen is highly reactive and forms peroxides and ozone, a highly oxidizing molecule containing three atoms of oxygen. The oxidants attack the polyethylene layer under the print emulsion....., making it brittle and, when enough of the polymer bonds are damaged, producing cracking and crazing. The oxidants also attack the fine silver particles in the image and convert them into mobile, unstable silver compounds. When these compounds migrate to the surface of the print, they readily react with reducing agents. The silver either plates out as a shiny layer of metallic silver, breaks down into superfine particles (colloidal silver), which look brown or yellow instead of black, or reacts with sulfur compounds in the air to produce brownish silver sulfides." </Ctein> William Robb - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
