Good question Claudio,The silver grains are "sensitized" not "filtered". This means that where a grain used to be "sensitive" to blue light it is now "sensitive" to green light when the dye is added. And the grain becomes low contrast as well. The blue sensitive emulsion had no sensitivity to green light and "filtering" it with a green filter would do nothing. Sorry for using emulsion chemistry terms.
Now I have attached the spectral sensitivity curves for Polymax paper to show everyone exactly what is happening. The three curves are density curves. 0.3 and 1.6 are the furthest ones and show the contrast. When the curves are close together the contrast is high. ...when far.... the contrast is low. Expose the paper with the color of light shown on the horizontal axis and you get the contrast at that light wavelength. So you can get variable contrast in your printing by ratioing the blue and green light exposures OR using filters that let certain ratios of blue and green light through.
By the way, even though the curves stop at about 550 nm the sensitivity continues into the red but at a lower level than shown. Just put a straight ruler on the slope of the line and extend it down....a foot or two.
Dave----- Original Message ----- From: "Claudio Bonavolta" <claudio@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx> Sent: Saturday, November 10, 2007 7:21 AM Subject: [pure-silver] Re: multigrade paper is amazing
Dave V a écrit :Sorry Richard, Your question and Shannon's are completely different. Your is "How" and Shannon's is "Wow"!Regarding Richards question. I like to keep answers simple. You can get there many ways. The order suggests history of sophistication. Here's three or four.1. Make a high contrast, blue sensitive emulsion and coat it. Make a slower blue sensitive emulsion and coat the grains in the kettle with a low contrast green dye. Coat this on top of the blue emulsion. Phase two > use laminar flow coating and coat at the same time with thin layers.2. Make the same emulsions as above in two different kettles. Mix them together and coat them in one layer.3. Make a big batch of high contrast blue emulsion and add just enough low contrast green dye to the kettle to coat half (or some other ratio) of the blue crystals but not all of them. This is called 'starved' sensitization. Coat this in one emulsion.Note: You can not take a high contrast blue emulsion, sensitize it with a low contrast green dye and get a variable contrast system. You get what ever contrast you sensitized it to with the green dye. That's how you get graded papers.<snip> DaveThanks Dave but just to be sure I understood it fully ...By "coat the grains in the kettle with a low contrast green dye" does is mean you coat the surface of each crystal with a green dye before putting it in the emulsion, like putting a micro green filter in front of every grains ?-- Claudio Bonavolta http://www.bonavolta.ch =============================================================================================================To unsubscribe from this list, go to www.freelists.org and logon to your account (the same e-mail address and password you set-up when you subscribed,) and unsubscribe from there.
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