Many thanks and a Question to Denise Ross

[chris77]

Hello emulsionistas, Bonjour Denise!

Your book is very very helpful and it was a pleasure reading !
Chapeau!

I have got a question concerning the bromide paper emulsuon recipe.

In fact 2 questions.
First, why is it ammonium bromide and not potassium bromide?

Second, where does the potassium iodide come into play? Its mentioned on top, but in the recipe it seems to not be used..

Thanks for enlightening me further

Cheers,
Chris

 

[dwross]

Hi! Chris. Thanks!

First, I use ammonium bromide in that recipe rather than potassium mainly because I like the resulting emulsion better. The bromide paper recipe is an adaptation of my favorite BrI dry plate/film recipe. It has a slightly expanded contrast range. In other words, you can add more exposure for shadow detail before highlights are blocked. That is effectively more speed. You can substitute KBr if you'd like and see which variation you like better. It's not a 1:1 substitution, so you may need to look up the correct ratio on The Light Farm website. I don't have them on the top of my head.

Second, iodide is added to a bromide emulsion to increase speed. It's not so much that iodide is slightly more sensitive, it's because the iodide creates "flaws" in the bromide crystals, sometimes called "light traps." If you are interested, there is more information in Duffin's book, Emulsion Chemistry: http://www.thelightfarm.com/BookImages/Duffin.pdf

Unfortunately, the first printing of my paper book ran into a glitch in the recipe formatting publishing software. Hopefully, all the errata was corrected in the second printing. The errata corrections for the first printing are here (scroll down past the slide shows): http://www.thelightfarm.com

Good luck and fun!

 

[chris77]

Hi! Chris. Thanks!

First, I use ammonium bromide in that recipe rather than potassium mainly because I like the resulting emulsion better. The bromide paper recipe is an adaptation of my favorite BrI dry plate/film recipe. It has a slightly expanded contrast range. In other words, you can add more exposure for shadow detail before highlights are blocked. That is effectively more speed. You can substitute KBr if you'd like and see which variation you like better. It's not a 1:1 substitution, so you may need to look up the correct ratio on The Light Farm website. I don't have them on the top of my head.

Second, iodide is added to a bromide emulsion to increase speed. It's not so much that iodide is slightly more sensitive, it's because the iodide creates "flaws" in the bromide crystals, sometimes called "light traps." If you are interested, there is more information in Duffin's book, Emulsion Chemistry: http://www.thelightfarm.com/BookImages/Duffin.pdf

Unfortunately, the first printing of my paper book ran into a glitch in the recipe formatting publishing software. Hopefully, all the errata was corrected in the second printing. The errata corrections for the first printing are here (scroll down past the slide shows): http://www.thelightfarm.com

Good luck and fun!

Thanks for your quick reply and all the good information.
Best wishes to you too.
C