“The Book of Photography” (Hasluck, 1907) chapter on “Plates and Films” includes two formulations for a silver (bromide) – gelatine emulsion. Starting with the simplest [formatting and numbering is mine]: ”
- gelatine, 30 grains, [dissolved in] water, 1 [fluid] ounce ;
- silver nitrate, 175 grs., water, 1/2 oz. ;
- potassium bromide, 140 grs., water, 1 oz. ;
- gelatine, 240 grs., water 2 oz.
… each of the ingredients [1, 2, 3, 4] dissolved separately[, and then combined ((1+2)+( 3+4))].” There’s also more sensitive (“more rapid”) emulsion given: ”
- Nelson’s gelatine No. 1 soluble, 30 grs., water, 1 oz. ;
- silver nitrate, 175 grs., water, 1/2 oz. ;
- potassium bromide, 130 grs., water, 1 oz. ;
- potassium iodide, 5 grs., water 1 oz. ;
- hard gelatine, 240 grs., water 2 oz.”
– which, I’d have thought, ought to give a slower emulsion, as AgI is nominally less photosensitive than AgBr. I presume the additional sensitivity has something to do with differing crystal habit.
Update: I’ve cracked it. Mixed AgI + AgBr grains are more sensitive because the iodide:
- introduces additional irregularities into the cubic AgBr crystal lattice,
- interacts with longer wavelengths of light than AgBr, extending its spectral range,
- accelerates crystal growth, and thus accelerates “ripening” of the emulsion.
(Source)(Another)
Filed under Alternative processes, Emulsions, Recipes
Tagged as AgBr, AgI, Alternative, DIY, Emulsions, Recipes, Silver bromide, Silver iodide, Silver-Gelatine