Contact printing emulsions
I've been getting a lot of e-mail over this in the last week.
Let me trace some history for you all.
Kodak Azo paper and Agfa Lupex paper have common roots as pure chloride contact printing papers which originated in the early part of the 20th century. At one time, Azo was available in 19 varieties, samples of which are in the archives of the George Eastman House.
These chloride emulsions were basically UV sensitive and were rendered more sensitive to visible light by addenda. These addenda were also used to control contrast. In fact, the book "Silver Gelatin" describes two of the common methods of increasing contrast which include use of copper salts and use of citric acid. Copper however can affect the tone of the developed silver, and citric acid is chancy sometimes. My methodology uses neither copper salts nor acid for normal contact grades with a neutral to cold tone. I use another method for this, but I do have a warm tone Azo type emulsion that is unique and uses copper salts to help get both the warm tone and the contrast needed. Neither copper nor citric acid affect the light sensitivity of silver chloride to any significant degree.
Well, anyhow, the basics are held in a fundamental chloride emulsion originally used at RIT in the photo science course as taught by my friend Dr. Bruce Kahn. His formula was derived from the original as used in Azo many years ago, and modifications were needed to fix it up for use with modern gelatins. I have posted his formula here on APUG and you may be able to find it on backup copies of his web site. His web site no longer exists.
There are two routes to fixing the problems with the formula Bruce published, and which are primarily due to the use of modern gelatins AFAIK.
With his permission, his entire course syllabus is made available to my students at great cost to me (lunch for Bruce 2 years ago at the Dead Lobster) and he repaid it with lunch at the Macaroni Grill last spring when I showed him what I had done with it. I now owe him! We trade off!
In any event, this is a starting point for the real Azo type emulsion for those of you out there astute enough to follow this.
One thing for the sake of those following the work of M&P and no disrespect to them either. My formula, and the real Azo formula as well as Lupex coats exactly at the speed and contrast desired at most any scale. No keeping is required to 'adjust' the curve shape or speed. What I coat is what is usable for the next year at least with no major change. It is a matter of the ingredients used and the methods of using them. Their efforts to 'fix' the problem and use keeping may work but would not be needed with the 'real' formula I am very sorry to say. Alex Hawley has used some fresh coating that I can repeat time after time at just about grade 2 or 3, your choice. I also have a grade 1 and a grade 4 with custom half grades.
I hope this answers some of the questions.
I feel that I cannot post the entire formula, nor the custom adjustments of contrast as that would detract from the course material and from the book which will include far more than just this minute bit of information.
This has been a costly venture for me. If you have patience, I will eventually publish all in some form or another.
Now, over to you. I encourage all who say they are making emulsions out there to post samples of their prints. It is one thing to say you are doing it and another to show results. We need more results. I have shown mine here on APUG and my students get to handle the actual prints. Then they make their own and go home with the prints. I'm truly getting tired with some of the smoke and mirrors that goes on. (sorry for that tiny rant)
Thank you for the very detailed explanation. It is filled with more hints than I expected to see, but it does clear up some of my misconceptions about Azo and papers like it, and fills in more pieces to the puzzle.
Truth be told, the information you have provided thus far on emulsion making has always been clear, easy to follow and leads to an immediate understanding of how these processes work, and the history behind them.
Thank you for taking the time to share these techniques, the many examples of the results of your work, and your knowledge from decades of experience as an emulsion engineer, with all of us.
Many thanks for your posts and your experience Ron. As I have said before (and I'm sure I'm not the only one), I don't understand everything that is posted, but with each reading I absorb a bit more and I enjoy each reading just a bit more than the previous one. Some day it will all come together for me in an emulsion that I coat myself with your (virtual) help.