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.
Even when it was new, AZO was an ugly, green paper. I don't understand why anyone would want to reinvent it. Good ridance that it's gone, i say.
What is the purpose of the rhodium chloride? The quantity is minuscule, but probably significant. A good thing, considering the price.
Ron, this is almost exactly the Agfa Lupex "type" formula, Normal Grade, scaled down by a factor of 200 :-)
Except in the Agfa Formulae the Rhodium Chloride is in Part C (3) rather than in Part A (1) and the gelatin in Part D (4) has KI added, and is dissolved in water.
In addition for the Normal grade paper both ripening times are 10mins at 60°C.
The Rhodium Chloride sharpens the toe of the paper.
I'm aware of what you say and that is why I said that it is similar to both Azo and Lupex, as this formula is a workable hybrid of both and by either using sulfur sensitization and oxidized gelatin or adjusting addition times and temperatures, the 'correct' result will be gotten. So, based on the above formula, I have seen Azo type results from 4 distinct formulas. Three of them keep less well and are harder to control for speed and contrast than the one I have chosen.
I have seen comments about the Rhodium Chloride being added just before pptn starts and others that state it should be run in partway into the pptn. This is another difference that I find in various formulas.
This can be simplified considerably (as we did at Kodak). My own version is quick and simple.
You may be right but everyone asks me about Azo type contact papers even though I can make many other types. And a lot of money is riding on M&Ps efforts.
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I have posted lots of them with my formula right here. It is a tweak and a simplification of the one above. The prints Alex Hawley did and his review used a very similar emulsion to this. It was altered using time and temperature as noted above to give the desired results with modern gelatin.
Just out of interest, why were active gelatins phased out?
Originally Posted by Photo Engineer
They were too variable and uncontrollable. In fact, EK was almost ruined by the famous (infamous) story of cattle eating mustard which made the gelatin extremely active and fogged the film. It was only then that Mees et. al. discovered that it was the allyl thiourea in the gelatin that caused the actual sensitivity increase in emulsions during making.
Originally Posted by Lachlan Young
The use of sulfur compounds and inactive (intert oxidized) gelatins are what ushered in the modern range of photo products.
Ron, With all of your knowledge of Azo, why are you not offering your formulas and your services to Michael Smith?