----- Original Message -----
From: "Peter Badcock" <peter.badcock@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, October 28, 2005 6:16 AM
Subject: [pure-silver] Re: Defender Photo Bulletin
The article and further comments can be seen in this thread
on APUG:
http://www.apug.org/forums/showthread.php?postid=207286#poststopregardsPeter Thanks for posting the link. There are two addenda:First, individual silver grains are microscopic, whatappears under a grain focuser is clumps of grains due to thestatistical distribution of grains throughout the layer ofemulsion. In addition there can be physical clumping due tophysical migration of grains during development when theemulsion is softened sufficiently. High pH developers likeRodinal tend to encourage this kind of clumping, howevermany modern B&W films have hard enough emulsions to resistthis kind of clumping. He talks about finer grains being dissolved. I presume hemeans in the developer but he may be conflating solventaction in the developer with a phenomonon which takes placein certain kinds of ripening. Long ripening tends to enlargesome of the halide grains at at the expense of others sothat the entire emulsion tends to become larger grained.Modern emulsion making methods te
nd to prevent this fromhappening. Solvent developers do not dissolve entire grains,they strip some of the surface and affect the way themetallic silver is generated. Solvent developers tend towardthe generation of fine filements of silver rather thansilver crystals of the same shape as the original halidecrystals. The filimentary grains look like little wads ofsteel wool under the electron microscope. They presumablychange the covering power of individual grains and, hence,the statistical coverage. Sulfite also tends to reduceemulsion swelling which, as noted above, can contribute tograininess. BTW, sulfite at an optimum amount also increasesfilm speed slightly due to the solvent effect by exposingmore development centers to the developer. When the solventaction goes further there can be a reduction in film speeddue to the solution of some development centers and hencethe loss of some of the latent image. The solvent effect ofSulfite is dependant on factors other than just i
ts amount.For instance D-76 and D-25 have the same concentration ofSulfite but a difference in pH and activity so D-76 is amedium-grain, full speed, developer where D-25 is anextra-fine-grain developer with a loss of some speed. All this is covered in many texts including some fairlyelementary ones.---Richard KnoppowLos Angeles, CA, USAdickburk@xxxxxxxxxxxxx
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