A real Azo type formula
This formula is similar to the Agfa Lupex and Kodak Azo papers when they used the old active gelatins. This would be about 1940.
Gelatin (medium ripening) 3.5g
Water 152.5 ml
Egg White 7.5 ml (albumen - to add gloss)
Sodium Chloride 2.5 g
Sulfuric acid 0.5 ml (H2SO4 diluted to a density of 1.18)
Rhodium chloride (0.1 g/ L) 0.1 ml
Bring to 45 deg C.
Silver Nitrate 5.0 g
Water 80.0 ml
Bring to 45 deg C.
Add B -> A over 20 minutes at 45 degrees C.
Add to this emulsion:
Gelatin (hard ripening) 15 g
Gelatin (medium ripening) 5 g
Bring to 60 deg C and hold for 25 minutes so that these gelatins will melt fully.
(NOTE: these two iodide steps can be combined in my experience)
Add 1 cc of 0.65% KI and hold for 10 minutes
Add 1.5 cc of 0.65% KI and hold for 10 minutes.
(NOTE: this antifoggant/stabilzer below is very subject to crystallization in the emulsion causing problems. I have demonstrated it to my students. I have found better substitutes than this ancient compound, but hope to learn how to use it properly as it is good because it is very very inexpensive.)
Add 0.5 ml of 1% Mercaptobenzothiazole in alcohol
Stir 2 hours while slowly chilling and then wash.
This yields a grade 2 Azo type paper.
Now, the problems are these:
1. We no longer have the 3 kinds of active gelatins avaialble to us.
2. Silver Chloride emulsions were sensitive to these active gelatins and would keep poorly if not made correctly.
Today, with inactive gelatins, this formula must be finshed with sulfur (from hypo as I described elsewhere on APUG) and even though this will be a useful substitute, the time and temperature must be carefully determined, and the emulsion is subject to poor keeping. Usually it turns out to be high in speed and contrast and will vary from batch to batch.
The solution is to find the condition of precipitation in the absence of a sulfur finish which gives the right speed and contrast right from the outset with inactive gelatin and then it will keep very very well. This is what Kodak did with the 'ultimate' Azo paper, and what I have tried to do. Therefore, my formula and Kodak's do not resemble this older version of an Azo type paper.
They both have good keeping and good speed and contrast from the time they are coated.
But, this formula is a good starting point. At some future time, I will describe all of the reasons for the things that go on in the above formula and all of the things that can be done to improve on it.
Oh, and BTW, this shows that the early contact papers were AgClI emulsions, not pure AgCl emulsions.