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Photo Engineer
08-31-2006, 09:09 PM
After an emulsion has been made, it can be washed by 3 methods. They are:

1. Noodle wash: The chilled hard emulsion is run through a device like a potato ricer. The noodles are run directly into a moderately hard water with calcium sulfate in it to reduce swelling. The emulsion is washed either until a drop of silver nitrate solution into some of the wash water gives no precipitate or using condictivity, the emulsion has a given specific conductivity to electrical current.

This type of emulsion swells during washing, and therefore becomes more dilute. Exact concentration cannot be predicted except by analysis.

2. ISO wash: The emulsion is precipitated in a phthalated gelatin or a mixture of regular and phthalated gelatin. The phthalated gelatin is not as good as regular gelatin in some ways, in "protecting" the precipitating crystals and therefore you usually lose some speed, as the grains are slightly changed.

After the precipitation, the emulsion is cooled and acidified and it forms a lump similar in texture to silly putty. This mass is washed with distilled water over and over just as above until the wash water is either free of salt or at the right conductivity.

At this point, dilute base is added as the emulsion is heated and the emulsion goes back into solution, but now has no extra salts. This emulsion is purer than noodle washed emulsions and can be adjusted in concentration by addition of the desired amount of distilled water.

3. UF wash: By means of ultrafiltration (similar to dialysis) salts can be removed from an emulsion and during that process, the concentration of the emulsion can be adjusted to the desired level by addition of distilled water.


After this is all done, the emulsion goes through a process called chemical sensitization or finishing. This involves a process as long and involved as the making process including the addition of a number of organic chemicals and metal salts. It improves the inherent blue and ultra-violet sensitivity of the emulsion.

In older emulsion formulas, the active gelatin used caused this step to take place at the same time as the precipitation. This fact is omitted in most textbooks and discussions on emulsion making and leads to emulsions, made with modern gelatins, to be slow and low in contrast.

After this step, the emulsion is spectrally sensitized. This involves adding the spectral sensitizing dye at the correct temperature, for the correct time. This is a rather simple direct step that takes little time, but sometimes nothing happens. That is why you have people select dyes for you ahead of time. They must be tailored for the surface of the emulsion, otherwise the wrong things happen. For example, I tested 5 sensitizing dyes and found only 2 that worked, and I had to use special methods to get one of them to work.

After this, the emulsion is diluted to the appropriate concentration and then the surfactant and hardener are added. This process is called doctoring, and the chemicals are called finals. This is done shortly before coating.

Hardeners include formaldehyde, glyoxal, glutaraldehyde and succinaldehyd. Another hardenr is chrome alum. All of these work, but many chemistry texts state that the reaction of aldehydes with gelatin is quantitative and rapid. This is NOT SO!!! In practice if this were so, you could not have a coating session that lasted hours or you would coagulate the emulsion. The reaction actually takes place over hours, and in the dry coating continues for YEARS! This is explained in both Mees and James and in Haist. It is contrary to posts on this forum and elsewhere, but is well known and is called "AFTERHARDENING".

Other hardners exist but they are quite toxic and not recommended for home darkrooms.

Lastly, the emulsion is filtered and optionally, degassed to remove air bubbles and particles. At this point it is in the coating room, and connected to the pumps ready to go, or, in your beaker ready to be hand coated. Or, it is ready to be poured into your silver recovery can if you have found you made an error along the line. I've done all of the above.

Good luck to you all.

PE