Emulsion making technology
With the rapid decline in analog photography, I have decided to begin writing up notes on emulsion technology to share with others. The posts I make are more limited than what I teach in my workhsops, due to the lab work and details in the steps required but here is a starter.
Modern emulsions are very complex, requiring hours to precipitate and very advanced computerized process control equipment. Usually, they consist of up to 25 or 50 steps that define an emulsion. Each step has a distinct function necessary to the making of a good emulsion.
A modern emulsion consists of the following steps. They include:
In nucleation, silver and halide are run into halide with dilute gelatin at about 2%. This is not the optimum for preparation of the final emulsion. The small 'seeds' of the future emulsion are made here.
After the nucleation, the emulsion is 'adjusted'. This includes changes in gelatin concentration, silver concentration, halide concentration, temperature, and dopants.
Dopants may include metal salts such as Rhodium chloride or other addenda to adjust reciprocity and latent image keeping. It may also include addenda to rebalance halide ratio and chemicals to adjust curve shape.
After this is the growth stage. This is an addition of silver and halide at a controlled rate adjusted such that the rate of addition corresponds to the growth rate of the surface of the emulsion. This is often a quadratic equation that requires a ramped addition of silver and salt and is very precise and different depending on whether one wants a cube, a t-grain or an octahedral grain. There are others possible.
This step is very precise, and in spite of previous posts to the contrary relies on very precise control of halide addition rather than silver addition. The measure of this is pAg similar to pH. It is the negative log of the silver ion concentration.
Finally, the emulsion is adjusted for the final concentration and ratio of silver and salt. Again, this is monitored by the measurement of the negative log of the silver ion concentration, pAg.
This entire procedure may requre over 1 hour to accompish.
This emulsion is then washed by a method called ultrafiltration which is akin to dialysis. It removes the extra salts from the newly made emulsion.
The emulsion is then chilled and set, and made ready for the folowing steps.
After precipitation, the emulsion is chemically sensitized, spectrally sensitized, doctored (prepped for coating) and then coated.
This is a start for you to understand current emulsion technology.