Some thoughts on the use and management of salts in making Silver Halides.
In the early days of chemistry, “researchers” used all manner of things to create a name for themselves. An example is the early isolation of the element Phosphorous which was first prepared by the distillation of decomposed horse urine from a bed of sand. Well, it works notwithstanding the probable odor and mess not to mention the difficulty of obtaining the sample. So, great effort was expended for expected great reward.
The same is true of the early days of photographic emulsion making where any and all chemicals were used in hopes that the experimenter would have that “ah hah” moment and discover something great. And, if they failed to produce anything significant, well they could still write a paper or book about their “super dooper” method of emulsion making. Some were good and some were bad. Here are some thoughts on the matter.
1. Potassium vs Sodium salts.
I saw both used at EK with no rhyme nor reason. I saw no real logic so I went to the best emulsion maker that I knew, and who was at the peak of his career. If you went any higher you might suffer nosebleed. Anyhow, I asked him which was better and why. His answer was that there was no best. Each gave a slightly different result, but there was no best and no easy way to predict what might happen if one used one salt over the other. In fact, he pointed out that most formulas originating in Europe (Kodak Harrow and Kodak Chalon) used Potassium salts and most from the US used Sodium salts. His reasoning was thus: Na salts are easier to get here in the US and are less expensive but the opposite is true in Europe. IDK if this is true and I never followed up, but after that I did see the trend of formulas from Europe did indeed use K salts more than Na salts. And, I also came to realize that there was no real way to predict what one would give compared to another, and what changes would be needed to convert from one salt to another. The bottom line here is use what you prefer, have the most of, what costs less or whatever, but stay consistent.
2. Lead, Cadmium and Mercury salts
These salts were used in many early warm tone emulsion formulas, but what goes in must come out. This means in the wash or in the process. Cadmium was the biggest offender being used anywhere from 100 mg per mole of Silver up to about 18 GRAMS / mole of silver. Cadmium and Lead have been history since about 1970, but Mercury (as a very toxic Methiodide salt) is still in use in some cases, but at very very low concentrations. It is now used as a keeping agent more than anything.
3. Ammonium salts
It had been known for years that Ammonium hydroxide dissolved Silver Nitrate to form a complex soluble material. I had been used for years in making emulsions up to the ‘60s when better methods were put into play. You see, this method causes fog and the effect varies with pH which varies with concentration and also during the pptn, As the Silver is precipitated, Ammonia is released and pH goes up. If it goes up too much or too fast, then fog forms. The clear advantage is that you can form uniform rounded cubic crystals which is nice. You also form a lot of fog which is bad and must be avoided by some means such as time, temperature or the use of antifoggants.
Now, some propose that Ammonium Halide salts be used, but with them, pH goes down with time during pptn. Since you are already slightly acidic, and pH goes down a bit, what you see is a decrease in the effect of Ammonia as the pptn proceeds. Well, in this case it was never very large anyhow. So, over my time at EK, I never saw a practical emulsion that used Ammonium Halides. Either Ammonium Hydrixide was used or better solvents or methods of handling the Ammonia were used. The trick, what you want, is to have the Ammonium ion present in a slightly alkaline medium and at a constant amount for the desired time. You will find this hard to do with Ammonium Halides.
4. Other positive ions
Almost every positive ion on the Periodic Chart has been used, via their halide salts or otherwise, for making Silver Halide emulsions. AFAIK, even after studying over 1000 emulsions over 15 years, only good old Na and K are still in there and still being used. All of the others have vanished due to lack of interest, cost or pollution not to mention lack of some overriding utility.
Finally, lets go back to the opening paragraphs of this post. When Phosphorous was first isolated as a pure element, the original worker considered himself a “scientist”. But, in view of today’s chemistry he was not. In a similar fashion, emulsion chemistry was published by what we might call “dabblers” in the proto science of photographic science. All of the texts up to that time were done with a less than scientific method when compared with today. And so, even with the publication of the Agfa formulas in the ‘40s, they were published as-is with no explanation or qualification. Since then, everything has been cloaked in secrecy and all books including Glafkides and Haist have been redacted or contain outright errors in the text.
I will pass this way only one time, and during that time, I have determined that I will publish modern and correct descriptions of the science of photographic engineering as far as I am able. I had much more information before I retired, but I returned that material to EK archives when I retired. I had stacks of data that were nearly overwhelming. They would have been useful today, so all I can present is a pale comparison compared to what exists.
Attached is a set of electron micrographs that depict an SR (Single Run – Silver Nitrate into Halide salt + gelatin) emulsion (Na or K – not much difference) and an Ammonia digest using the Silver complex of Ammonia rather than NH4Br (Silver + NH4OH run into Halide salt + gelatin. The third electron micrograph is of a modern dual run emulsion in which salt and silver were run together. Note that the use of the Ammonia complex of Silver ion only approaches the quality of a dual run emulsion. Note also that an ammonium salt alone such as NH4B4 will only approach the SRAD (Ammonium complex) version very very slightly if at all!
A curiosity question: Picturing in mind's eye a batch-run of 35mm Plus-X as an example, I would suppose the emulsion, (if only a single-layer is done); is mixed up in a vat or vats and brought up to the coating machine and hooked up to the feed inlet. Then it begins coating what was previously a cleaned-up and empty machine (fountains, rollers, and drums, whatever). Then at some point the base begins its feed, and the run is on, as long as the run was planned for. Not knowing how many shifts a run might last, at some point the run is over and the machine is washed-up. The whole process could be, or must be not unlike an offset web press, in my imagination. In the event that I do have a reasonable grasp on how film is made, comes my question: In such a run, about how many pounds of actual pure Ag went in to that batch run? And how much in grams or ounce fractions of Ag would there be in a 36 exp. roll?
Sorry to say this, but your fundamental concept is a bit off. The machine is already threaded with about 1 mile or more of film support (that is 72" x 5000 ft at Kodak Park). The hopper is running with the many layers of the film substituted by water at the coating temp. The emulsions, overcoats, undercoats and all begin feeding and a stable bead is established in the light and then they go dark. Meanwhile the coating machine (support) and the feed pumps are ramping up to operational speed and then they go dark and run until that job is finished.
A typical kettle can contain from 10L (Research) to 50 - 100L for prototyping up to 1000L for some applications.
Silver in most films run about 3 grams per square meter or more. That is a very rough approximation.
The coating machine is left threaded with film after the job. Rethreading a mile of film support is a job you do not want! Never let the machine run out!
So, when I put some Tri-X into my camera it is coming from a process like this? That boggles the mind! Just think of the production scale necessary for developing such a thing to be feasible! Not to mention the films and emulsions. I have a strange sense of "retroactive luck"... I'm glad film had such a golden age so that we can still benefit from all that was gained. I'm over 50 years old so I lived through some of it, but I don't think it was commonly appreciated. ( I know the product was appreciated.. I mean what was behind developing and making that product. )
I realize computer technology is nothing to sneeze at, or belittle. I concede it does indeed take some fine minds and collaborative efforts in that field. That said, I simply do not have the awe or respect for Apple or Microsoft, et al; as I do for companies like Kodak, DuPont, and the likes of them. Probably never will.
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Thanks for the meaty post. Fun!
To paraphrase a movie I watched WAY too many times when my son was young: I do not think you are as old as you think you are. Little surprise that EK wasn't using ammonium bromide when you were there.
A couple of times recently you have alluded to ammoniacal emulsions being obsolete technology. I've never run into that tidbit anywhere else, so I assume it is one of those proprietary things (??) Another silver solvent? I don't read much beyond the mid-70's. After that date, the literature becomes pretty much meaningless for the home darkroom, so I can easily believe I've just missed the "breaking news". Anyway, would love to hear all about it.
For the micrographs of crystals you've posted, what are the indicators of "quality"? I see differences in crystal structure. Is there something else? The third image is very uniformly octahedra. That's cool, but a little boring. Is this likely an emulsion that is intended to be used with other, different-structured emulsions in a final, multi-layer (or blended) product?
In point #1, I'm guessing you mean Na and K chlorides, as for, in general, paper emulsions (??). Are there many NaBr negative emulsions? Also, in point #4, most regular readers of this forum know you mean the halides used for the bulk of the precipitation, but don't forget iodide on your list of "survivors."
Potassium and Sodium salts in optics have long story. When the Europe accepted the Christianity , they wanted to make churches and decorate with glass panes. Constantine was the first Roman Emperor who accepted the Christianity for Roman Empire and than he came to Istanbul and built East Roman Empire than the Byzantine Empire. At AD 573 , Byzantians built Hagia Sophia - the biggest building in the world for coming 1000 years - and decorated the walls with Gold leafed , Red , Green and Blue mosaics. The secret of Byzantine mosaics and their long life for the coming 1500 years was potassium salts were produced by Istanbul sea weed catched from Bosphorus. There is only one place you can find that weed is Venice another glass capital. Potassium salts lowers the melting point of glass batch and protects the glass from elements for a long time.
At that time Europe was using sodium salts to lower the melting temperature of glass batch and sodium harmed the glass and all european church glasses start to deteoriates.
Until mid ages , they could not solve the problem and source of potassium. Later they discovered if you burn some mid european alges and put the burned ashes to the glass , they are the potassium source.
Sourcing potassium was a battle for Europe for 1000 years until the mid ages.
Originally Posted by dwross
Since his opening sentence says "salts in making Silver Halides" - it would seem to me that he was NOT limiting his comment to chlorides, but halides in general.
Indeed, Fred is correct. I meant all Na and K halides to be included as possible examples. And, unlike glass making, the Na or K is not included in the crystal or molecule. In some cases, Cd, Sr, Hg and Pb are included in the crystal.
To continue with the halides though, there are NaBr and KBr halides used in emulsions aplenty out there as well as NaCl/Br and other mixtures. I even know of a make that uses NaCl, NaBr, and Methyl Mercuric Iodide to make a 90% Br, 9.9% Cl, 0.1% I emulsion. The Mercury is also there, retained in the crystal. AFAIK, this latter make was abandoned in about 1990 or thereabouts for a better method.
Now, as for the use of Ammonia. I have said that NH4 halides are restrictive and less effective than NH4OH. I've also said that the use of Ammonium Hydroxide also can cause fog. I've also said that it has been abandoned. Well, here s the modern way:
Kettle - Gelatin, Halide salt mix (Na or K salts). Feed lines are AgNO3 and Salt in 2 lines as needed.
Begin the run. When DIGESTION is to start, add the desired amount of (NH4)2SO4 (Ammonium Sulfate) and then add KOH or NaOH solution to get a pH of about 8. Digest as desired (your time and temp) and then add an equivalent of H2SO4 to bring the pH back to the starting point! NH4 Halides and Sulfate are so ineffective at the acid pH, that you effectively turn on and then turn off the ammonia digest along with any fog. This is not only the common modern method if Ammonia Digestion, it is controllable and illustrates how acid can turn off the digestion properties of the Ammonium ion.
I hope this answers those questions. BTW, the above Acid / Base cycle is about out of date now having given way to an even newer method that we really cannot use in our home labs very easily.
Science marches on.
I can only imaging what a undertaking it would be to thread such a monster machine. I suppose the entire works has to be kept absolutely dust-free and temperature/humidity controlled. Does the coating machinery have some sort of "self-cleaning" apparatus?
Originally Posted by Photo Engineer