----- Original Message ----- From: "Jeff Kelley" <jeff@xxxxxxxxxxxxxxxxxxxxxxxxx> To: <rollei_list@xxxxxxxxxxxxx> Sent: Monday, February 25, 2008 10:17 PM Subject: [rollei_list] Re: Rollei Retro Film > Richard Knoppow wrote: > Even though some modern B&W films have very hard > emulsions not all do and there is still some advantage to > the hardener in preventing damage to the film when its wet > (the hardener has little or no effect on dry film). > > > Richard, is it true that staining developers like Pyro > harden the emulsion & > therefore eliminate the need for adding a hardener to the > fix or stop bath? > > Jeff > Pyro and some other developers, hydroquionone under some conditions for instance, cause differential hardening of the emulsion. While the same reaction products that cause tanning are not the ones that cause the stain image they are similar in that the effects are proportional to the image density. Pyro developers leave a sort of relief image which can be seen when the negatives are viewed by reflected light. When used on unhardened gelatin Pyro developer can be used to form a matrix image of the sort used for dye transfer work although I think the developer supplied by Kodak for dye transfer used a different developer. Because Pyro has relatively little effect on those parts of the emulsion bearing a low density image it probably does not protect them from wet abrasion. BTW, a side effect of the differential hardening is a small change of the index of refraction of the gelatin. This can produce a small acutance effect at the borders of high and low density areas. There is also a variation in the swelling of the emulsion which can cause a further acutance effect due to a variation in developer penetration, plus there can be some slight geometrical distortion of the image due to the variation in shrinkage. For the last reason Pyro developers designed for aerial photography are usually of the non-staining type. Note that Pyro in the presense of enough sulfite will not generate enough of the reaction products of development to cause either staining or tanning. While both effects are considered desirable now they were not at one time and a considerable amount of work was done in devising non-staining Pyro formulas especially for motion picture use where the stain could be misleading in judging the contrast of negatives (development by inspection was the rule in the silent period) plus the exagerated edge effects caused by the tanning could result in quite noticable "edge crawl" on prints especially if a duplicate negative was used. All ancient history. --- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org - Online, searchable archives are available at //www.freelists.org/archives/rollei_list