A real formula
Here is a real emulsion formula. At Kodak, we called this an SRAD emulsion (Single Run Ammonia Digest). This is adapted from one by Baker in his 1940s textbook on emulsion making. Comments will follow.
Potassium Bromide 132 grams
Potassium Iodide 4.5 grams
Gelatin 30 grams
Water 1 liter
Silver nitrate 130 grams
Water 500 ml
Heat A and B to 45 deg C
Add 28% ammonium hydroxide to B with stirring until a clear solution results.
Add B -> A over 10 minutes
Hold for 30 minutes at 45 deg C.
Let stand for 2 hours or until at room temperature.
Shred into noodles and wash. (make sure all salts and ammonia are removed)
Remelt and adjust gelatin percent to the desired level (5 - 10%)
Add spectral sensitizing dye and hold at 45 deg for 15 mins.
Coat with a hardener and surfactant.
This can achieve up to ISO 40 speed.
Now, here come the caveats. This formula assumed, as they did at the time, that you were using standard (ACTIVE) photo gelatins, and you will be lucky if you get ISO 3 - 6 with it using modern oxidized photo grade gelatins. You cannot get active gelatins that are any good today, for the most part.
The only way to get speed is by chemical sensitization, or finishing. This involves the addition of any one of a variety of ingredients. The original was allyl thiourea, another was thiourea, and then finally they added sodium thiocyanate. Modern emulsions use either sodium thiosulfate or sodium thiosulfate plus a gold salt. It is done after the wash step, as excess halide represses this sensitization. This finishing step varies for every emulsion and sometimes for every batch of every emulsion.
The problem is that the quantity, time and temperature must be determined by trial and error as it is based on the surface area of the emulsion. This is a very complex procedure. This type of emulsion varies from batch to batch quite a bit in speed, contrast and fog.
BTW, this emulsion is polydisperse and the iodide that would otherwise be in the core and not very useful is churned by the action of the ammonia to be more uniformly distributed and therefore increases speed. This gives a rather high speed negative film with a long latitude and an upwardly bowed mid scale. (sound familiar?) Yes, this curve is very similar to some very revered products of the 40s, 50s and 60s.
Enjoy and have fun.