You are correct!
You are correct!
Old emulsion technology dumped Silver Nitrate or Silver Nitrate + Ammonia into a pot of active gelatin + a salt mixture. This was heated from 1 hour to 2 days at temperatures from 40 - 90 deg C (the 2 day 90 deg emulsions were called BOILED emulsions). The gelatin was active gelatin and the sulfur sensitization came from the gelatin. There were as many as 3 grades of gelatin for this type of treatment. The wash was essentially the last step.
In the 40s - 60s, the gelatin was inactive, and the addition rate was timed at a given speed. Temperatures were 40 - 60 deg C, and sulfur sensititization or sulfur + gold took place after the wash was complete.
After the 60s, emulsions used inactive gelatin, precise flow rates, measured vAg, and precise steps for digestion, ripening, and other addition steps. I have given the steps elsewhere but I include them below:
1. Nuceation - the preparation of the small seeds on which the emulsion is built.
2. Ripening and dilution - the step at which an emulsion is diluted to the optimum gelatin content and vAg. It is held to adjust grain size and composition. *
3. Digestion - the step at which a silver halide solvent is added to modify crystal shape and size. It is held to allow this to take place. *
4. Growth - the step at which more silver and salt are added in a linear or quadratic fashion. Any modifiers may be added here.
5. Hmmm, IDK what to call this, but here the vAg is adjusted by adding silver and salt to the optimum point for grain size, halide content and vAg.
7. Sulfur or sulfur + gold sensitization.
* = optional step.
I hope this clarifies how complex things can be (as if adding complexity can clarify anything :D )
When was the technique of controlling pAg introduced?
Well Ed, that is difficult to say.
When Kodak began making dual run (salt + silver), those were in the 40s and they were controlled by hand to give identical (or nearly so) flow rates. Later a conductivity meter was added and the salt flow was adjusted to keep the needle centered. Then they found that it was a property of the halide, and they worked with pAg, but it was not accurate enough and finally they went to full control and used vAg. This process took place over 30 years, and involved designing several versions of lab and production equipment, each one better than the last.
It was a slow gradual evolution that slowed down in the 90s with the final EK models and automation. I am given to understand that Agfa never completed their automation, and IDK about Fuji or Ilford.
have anyone used hand-applied emulsion on gold leaf? I'm looking for suggestions and guidance.
Jill - I can't help with your quesiton, but posting it in a thread of it's own may help others find your question. There's a "New Thread" button near the top of this page:
Her website is www.ReverseGlassPainting.com
Note! You cannot reach her via the Toll Free number. Best to email.
So does the concentration of silver added to the salt+gelatin have an effect? In otherwords, If I add 1mol at 10Ml per minute, is that the same as 2 mole at 5ml per minute? or would this change the crystal structure?
It would seem that less concentrated solutions, added at a higher (and probably more controlable rate) would result in a more monodisperse solution.
Is it advisable to "dump" part of the ag at the start of the proccess, to nucleate tiny crystals in solution, then slowly add the remainder to grow the nucleated crystals?
The concentration of reactants change the nature of the precipitated silver halide.
The absolute vAg (or pAg depending on equipment) determines the crystal habit (see a previous post with chart for this) and the control of flow rate of Silver + Salt to maintain that starting vAg will control dispersity.
Addition rate determines overall size and crystal dispersity as well. Shorter addition times give more monodispersity and smaller cyrstals, on average. But, it depends.
Older emulsions used initial dumps of silver. In fact, early German translations use the word "gekipped" (tipped or dumped) to describe the initial step of silver addition.