View Full Version : How to make the best emulsion, or why "Silver Rich" is a myth.

Pages : [1] 2

Photo Engineer
02-22-2007, 12:26 PM
Ok, here goes a long description of emulsion making.

Most all emulsions published fall into two classes, SR (Single Run) and SRAD (Single Run Ammonia Digest). These are emulsions in which silver nitrate is run into salt and gelatin either with our without ammonia.

First, I'll explain some problems inherent in early emulsion making.

To make a silver halide that is uniform in grain size, and therefore has good contrast and optimum speed, the pAg should be held constant (pAg is the negative log of the silver ion concentration. Elsewhere on APUG, a member has insisted that pX be measured (negative log of the halide concentration), but this is wrong).

As you add silver nitrate to salt, the pAg changes because the silver and salt concentrations are changing. Therefore, to keep pAg constant, silver and salt should be added to salt and gelatin. This is the only way to control the situation properly. These are called RS emulsions (Run Salt). The only posted example of this is on Jim Browning's web site.

However, this method is messy and very hard to control because a thing called 'ionic strength' is changing due to the buildup of sodium or potassium nitrate as a by product of the emulsion making. Therefore, the rates must be controlled by a computer and constant measurment.

Why is this so? Well, a cube is made at one pAg, an octahedra at another and a t-grain at another pAg. If you don't control the pAg, then you get 'K' grains or 'Klunkers' which is a mix of all possible types of grain of all possible sizes.

So, the 'perfect' emulsion is one type of grain at one size.

Ammonia digest is an attempt to rectify this in an SR emulsion. Ammonia dissolves the grains and rounds them off and makes them more even in size. However, this creates its own problems such as fog, and spherical shaped grains along with rods and needles, again 'K' grains. It is also hard to control the digestion step. Some modern emulsions use ammonium sulfate with a pH cycle step to carry out a moderate quick ammonia digest. It can be used but will round off any grain. So, a pure cube may end up with clipped corners and an octahedral grain may end up as a sphere. You have to be careful.

Well, where does silver rich enter into this? Simply that the old SR and SRAD emulsions themselves contain 'dead grains'. These are grains that have no light sensitivity at all. In addition, the old active gelatins did not activate them very well during the finishing step.

As emulsion scientists learned how to use sulfur or sulfur + gold in a post precipitation step called 'finishing' or 'chemical sensitization', the number of dead grains decreased by a big amount. In addition, making grains all of one size also decreased dead grains. This is because the sulfur or sulfur + gold tended to react more evenly with the more uniform grains.

Therefore, in todays modern emulsions, you can coat less silver and still get the same density. At one time, as an example, about 50% of the silver might expose and develop, but today nearly 100% will expose and develop. If this is true, then about 1/2 of the silver will give the same image with no sacrifice in grain and will give improved sharpness due to lower turbidity.

In addition, the curves of the films and papers can be more carefully fine-tuned, and batch to batch variations will be less.

Modern emulsions are RS emulsions with many controlled jets of salts combining with the silver under computer control. In addition, jets of metal salts and organic chemicals are being added to control raw stock keeping, latent image keeping, and reciprocity. I have seen 5 or more jets cycling on and off during the make of a modern emulsion.

So, there is a thumbnail sketch of making a better emulsion and then finding you can reduce the level of silver. Silver rich was necessary in the old days simply because of an imperfection in the emulsions due to the methods used in making them. As long as you achieve the same measured dmax with no sacrifice in grain and shaprness, there is really no difference in quality.

Now, there is a difference, but that is another story. It is due more to modern papers than film or paper curve shape or even to silver level, but silver level plays a certain part in this. You do see that FB paper is better than RC paper in many ways that can only be seen to be appreciated. That is part of the rest of the story which will be told at a later date.

I'll address this last paragraph after the discussion over this post dies down and it seems that the facts here, if anyone even cares, are digested. There is a difference, but it is not what you think.


02-22-2007, 12:39 PM
Ron, a total novice question: 100% of silver can be used on a too thin of an emulsion. How do you know when you have gone too far in saving silver?

Photo Engineer
02-22-2007, 12:48 PM
The Dmax goes down, or contrast goes down, or both.


Michel Hardy-Vallée
02-22-2007, 01:55 PM
So in essence "silver rich" really means "silver halide rich" not "metallic silver rich," right?

If a modern and a less modern emulsions are matched for dmax and contrast, they will end up with the same amount of metallic silver in the final image, after fixing, for similar exposure (I'm just speaking very broadly here!).

Therefore, with respect to film (looking forward to part 2 about paper), the difference in taste between modern and older films is not caused by a bigger amount of metallic silver, but rather by the qualitative aspects of the emulsion (spectral sensitivity, imperfections, "gestalt" effect, etc).

Guess you'll have to educate a few resellers here....

Photo Engineer
02-22-2007, 02:07 PM

Gestalt would be the right word.

But, silver rich does indeed only end up referring to the amount of silver in the final image. That is all we see. We cannot see the silver from the silver halide that is fixed out.

Now, I must add that the older emulsions were coated thicker, as noted above and this affects sharpness, so todays films are sharper for a number of reasons, including thickness and amount of turbidity.

Maybe this sharpness is part of the gestalt, in that older films were subtly less sharp.


02-22-2007, 02:53 PM
I am thankful you brought one of my pet peeves up in more detail, PE.

Could the Bruce Kahn emulsion recipe be termed an SR emulsion? Or is the BK recipe even further down the rung in terms of crudeness?

Photo Engineer
02-22-2007, 02:55 PM
It is indeed an inefficient SR emulsion, and if you compare it with the one recently posted here from the coffee site, it is close to that. It is a slow contact grade chloride emulsion.


Michel Hardy-Vallée
02-22-2007, 03:33 PM
But, silver rich does indeed only end up referring to the amount of silver in the final image. That is all we see. We cannot see the silver from the silver halide that is fixed out.

Ok, but in reference to your statement "Therefore, in todays modern emulsions, you can coat less silver and still get the same density. At one time, as an example, about 50% of the silver might expose and develop, but today nearly 100% will expose and develop," I thought you meant that in old emulsions, only 50% of the sliver halides reduced to metallic silver, hence the existence of "dead grains," grains of silver halide not being reduced to metallic silver.

Thus what I understood is that for a given density in a piece of processed negative, both modern and old emulsions would have the same amount of g/cm2 of metallic silver, only that to achieve this amount, an older emulsion would need extra halide given its low yield.

(You knew it wouldn't be easy to explain to us hare-brains! :D )

Photo Engineer
02-22-2007, 04:29 PM
Michael, you are correct.

You can coat 50% of the silver, if the emulsion is twice as efficient and get the same density. The metallic silver yield is low in the old emulsions.

I should add that I have taken out of this differences caused by developer and differences in the morphology of the developed silver which both enter into the arguments I presented above in the OP. The argument assumes identical developer and identical silver metal morphology.


Michel Hardy-Vallée
02-22-2007, 05:07 PM
Thanks Ron, I was unsure for a moment of what I understood.

02-22-2007, 05:46 PM
For future reference, here is the site of James (Jim) Browning, http://www.dyetransfer.org/. I had to wrestle a bit with google to find it. Within the Dye Transfer pdf is what I guess is the formula for the RS emulsion mentioned in PEs OP. The Bruce Kahn formula I haven't been able to locate on the web. I know PE has posted it once in a thread here but I am not sure if I should repost it (I have it on my computer)?

Alex Hawley
02-22-2007, 07:31 PM
I will offer up some first-hand observations based upon several prints laying in front of me. This thread has made me realize something. All of these prints are contact printed from 8x10 negatives, and all on fiber base papers. Various developers have been used but I can easily rule out that the developers make no difference in the results I'm seeing. The negatives are made on several different films, some developed in ABC Pyro, some developed in Pyrocat-HD. Some were developed in trays with standard agitation, some were developed in tubes using extreme minimal agitation.

I have printed every one of these negatives on Azo. It is well-known that Azo is an "ancient" silver-chloride (AgCl) emulsion, one that can be considered rudimentary by today's standards. On Azo, each image is very clear and very sharp.

Another group of prints, same negatives, printed on PE's hand-coated paper with his home-made AgCl contact printing emulsion. Again, this could be considered a rudimentary emulsion, with less strict quality control of ingredients and process than what the Kodak-made Azo is subject to. The images are very sharp, very clear. I can see no discernible difference between these prints and the Azo prints in sharpness.

Another group of contact prints printed on on modern, currently available enlarging papers, both graded and variable contrast. Very sharp, very clear images. These papers are not advertised as being silver rich.

The final group of prints, made from the same negatives, printed on papers that are advertised as being silver rich. Again, graded paper and variable contrast. These images are noticeably less sharp. The images are softer. Not softer in contrast, but softer in image clarity.

Why am I seeing this? I'm not sure. It does make a case for concluding that the silver-rich papers produce a somewhat softer image, but I'm not sure its conclusive. I had also noticed this same difference when making enlargements but I think the most objective proof is with the contact prints since it eliminates the enlargement variable.

But why are the prints made on the two rudimentary AgCl contact printing emulsions so much sharper that on the "silver-rich" emulsions? The silver-rich technology is advertised as being that of the 1930s-1950s or so. The AgCl emulsions date back to the 1890s I believe, but they are just as sharp as the emulsions made by two different makers (neither of which is Kodak) with the most modern facilities and technology.

Ron, how does this fit in to the greater scheme of things?

Photo Engineer
02-22-2007, 08:07 PM
Alex, the paper I sent you is 'silver rich'. I will have to address this in another thread I think. It has to do with the paper itself and the emulsion.

Are the silver rich papers on FB or RC? I would assume FB, right?

For starters, today's FB papers seem to have a thinner layer of baryta than they used to and the paper itself is not heavy. The paper I coated on (the non-baryta) is about triple weight. There is about 30% or more silver in the paper than in current B&W papers due to 'dead grains' and inefficiencies in the primitive emulsion.

I'll try to address these all later, but if you could answer as to the type of paper support for the silver rich prints and the support type for my paper, it would help me a bit.



Alex Hawley
02-22-2007, 08:27 PM
Alex, the paper I sent you is 'silver rich'. I will have to address this in another thread I think. It has to do with the paper itself and the emulsion.

Ahh, that's what I thought based on your writing. Is recently-made Azo also silver rich or was Kodak able to modernize it somewhat?

Are the silver rich papers on FB or RC? I would assume FB, right?

Everything is FB. I'm making the comparison to your baryta coatings. The Azo is single weight, everything else is double or triple weight. Everything is on baryta.

I will add that the silver-rich papers (baryta) are comparable in sharpness to your coating on the strathmore base. The strathmore base is inherently softer in appearance to the baryta base, all processing steps being the same, including the same developer.

I'm not sure about the factory origins of the silver-rich papers. Some are certainly from Forte but I'm not sure where the Adox/Nuance brands originate.

Photo Engineer
02-22-2007, 09:04 PM
Well, I'll give a try here Alex. But I may have to start another thread to finish up as it gets pretty broad.

Paper dmax is limited by multiple internal reflections, so no matter what the amount of silver there is in the coating, the dmax will be roughly about 2.2, no higher. Now, there is data in the area above 2.2 if there is enough silver, but you cannot see it. You can, however, see it if you look through the paper using a transmission densitometer for example. The paper scale continues to go on and on.

In the design of paper, you must consider that during exposure, the light is reflected from the back and then scatters. In a silver rich emulsion it scatters a lot, so the trick is to use a low level of gelatin to prevent scatter by having high turbidity. In the case of paper, high turbidity confines the exposure to the top of the emulsion and can actually increase sharpness.

If you add too much gelatin, the back scatter increases lowering sharpness. Therefore, paper design is a balancing act between silver halide level coated and gelatin level coated. The worst is a low silver, high gelatin coating.

Now, add into this a thin baryta layer or a thin paper backing and you make the blacks look less dense in a low silver paper or a high gelatin coating. The back lighting gets through and makes the print look translucent.

My paper is high silver and low gelatin (relative to some) and therefore appears sharper.

So, in my paper, there is enough silver to get to a dmax of about 3.0, but you only see a velvety black of about 2.2 and this is further attenuated during exposure by turbidity to give sharpness. Azo works by a similar balance of silver/gelatin ratio. Other methods to control sharpness include use of absorber dye.

BTW, this all works for film too. That is, the balance of turbidity or scatter as a function of silver to gelatin ratio.

If this seems satisfactory, lets consider this to be an explanation for the paper portion of this myth. And, "gestalt' is the word here. It is your perception of the print that is what works. The engineer has to integrate all of this to make it work in the final product.

Thanks for your comments Alex.


Alex Hawley
02-22-2007, 09:33 PM
Thanks for the answers Ron. I have a package of the Russian Slavich paper here waiting for a try. It will be interesting to see where it lays in all of this. I found their website the other day. I can't say for certain but it would appear that their facility may be pretty modern. Others here on APUG have reported good results with this paper, but I don't remember any comparisons like I just made. We'll see.

Photo Engineer
02-22-2007, 10:00 PM

It does not matter how modern the facility is, if you have the wrong coating formula or have done no R&D. You have to get the film and the paper right.

Even modern coating facilities get the formula wrong sometimes.

Keep up the good work Alex. Keep me posted.


02-23-2007, 06:46 AM
Curious about something. The most recent batch of Azo by Kodak had a quality which ended up needing a change in film development time to yield good prints (i.e. old grade 2 was 1.5, new grade 2 was 1.65). Was this due to a different manufacturing facility, but the same chemistry? I think the paper was produced in a single batch run on an as-needed basis, so there were many stops and starts over the years of production.

Was this variation from batch to batch a lack of QC, initial testing, chemistry or something else? Seems the basic formula should have been exactly the same each time, so what variables caused these types of changes. tim

Photo Engineer
02-23-2007, 09:12 AM
The problem you mention was due either to being made in a new location or to an error in formulation. The results passed Kodak QC though making me believe that the problem, whatever caused it, was one of reciprocity failure.

People who used short exposure times got one result and people who used long exposure times got another in terms of contrast.

Alex may want to comment on that.


02-23-2007, 11:05 AM
Like it or not those emulsions prepared with 'dead grains' in them are going to have a certain set of characteristics peculiar to them. Calling a film with these properties 'silver rich' is a name people hang on them... Some people prefer this look.
We shouldn't get lost in semantics and just admit that films prepared in different ways have a different look.