A very high speed film formula
I've gotten requests to post a very high speed film formula. Well, I can't post an exact formula for obvious reasons, but I can speak in generalities about an ISO 800 - 3200 emulsion that I know about.
So, here goes. This will be a double run graded iodide core-shell emulsion.
First you must start with a low gelatin kettle of about 2% and high iodide, adjusted to between -200 and -250 mv in vAg.
Run into this dilute silver nitrate and dilute potassium iodide to maintain the vAg at the initial value throughout this nucleation. This can take from 1 - 10 minutes depending on flow rate and the amount of nuclei you wish to make.
Hold this kettle of nuclei while dumping in a gelatin solution such that the volume of the kettle is not changed much but the gelatin % rises to 5 - 10%. This is necessary for the next steps.
Run in some dilute silver nitrate to raise the vAg to the value selected to give either cubes, octahedra or t-grains.
Run in concentrated silver nitrate (about 10x the value of the first step) and concentrated potassium iodide (also about 10x the value of the first step). Ramp the flow rate up over about 1/2 hour in either a parabola or a linear ramp flow rate while keeping the vAg constant at this new, higher value. The type of ramp is dependant on the crystal type (cube, octahedra or t-grain).
While this ramp proceeds, ramp down the potassium iodide flow and begin ramping up the potassium bromide flow until at the end of the ramp there is no iodide going in and the salt flow is all bromide. During this ramp, the addition of Rhodium, Iridium and/or Osmium salts are done to tweak overall stability and reciprocity.
Then, dilute silver nitrate is run again to adjust the vAg to the final value. This is usually about +50 mv. The emulsion is washed and kept at about +50 mv.
The aim is to have a pure iodide core with a graded iodide shell and an overall iodide content of about 8 - 10%. The emulsion aim size would be about 8 microns. The final speed would make this useful as the fastest component of the fastest film you would ever want to make. This would place it in the range of up to ISO 3200. Speed is dependant on the type of sulfur and gold finish and the final addenda during finish.
Hope this gives more information to those requesting this type of formula, and also shows why it is difficult to do this type of thing at home.
You need a very well calibrated meter, and at least 3 if not 4 pumps for the solutions. Just the effort to fine tune such an emulsion will eat up a lot of silver nitrate.
Have fun thinking about this one.