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Photo Engineer
09-03-2006, 08:28 PM
Here is a real emulsion formula. At Kodak, we called this an SRAD emulsion (Single Run Ammonia Digest). This is adapted from one by Baker in his 1940s textbook on emulsion making. Comments will follow.


A solution
Potassium Bromide 132 grams
Potassium Iodide 4.5 grams
Gelatin 30 grams
Water 1 liter

B solution
Silver nitrate 130 grams
Water 500 ml

Heat A and B to 45 deg C

Add 28% ammonium hydroxide to B with stirring until a clear solution results.

Add B -> A over 10 minutes

Hold for 30 minutes at 45 deg C.

Let stand for 2 hours or until at room temperature.

Shred into noodles and wash. (make sure all salts and ammonia are removed)

Remelt and adjust gelatin percent to the desired level (5 - 10%)

Add spectral sensitizing dye and hold at 45 deg for 15 mins.

Coat with a hardener and surfactant.

This can achieve up to ISO 40 speed.

Now, here come the caveats. This formula assumed, as they did at the time, that you were using standard (ACTIVE) photo gelatins, and you will be lucky if you get ISO 3 - 6 with it using modern oxidized photo grade gelatins. You cannot get active gelatins that are any good today, for the most part.

The only way to get speed is by chemical sensitization, or finishing. This involves the addition of any one of a variety of ingredients. The original was allyl thiourea, another was thiourea, and then finally they added sodium thiocyanate. Modern emulsions use either sodium thiosulfate or sodium thiosulfate plus a gold salt. It is done after the wash step, as excess halide represses this sensitization. This finishing step varies for every emulsion and sometimes for every batch of every emulsion.

The problem is that the quantity, time and temperature must be determined by trial and error as it is based on the surface area of the emulsion. This is a very complex procedure. This type of emulsion varies from batch to batch quite a bit in speed, contrast and fog.

BTW, this emulsion is polydisperse and the iodide that would otherwise be in the core and not very useful is churned by the action of the ammonia to be more uniformly distributed and therefore increases speed. This gives a rather high speed negative film with a long latitude and an upwardly bowed mid scale. (sound familiar?) Yes, this curve is very similar to some very revered products of the 40s, 50s and 60s.

Enjoy and have fun.

PE

phfitz
09-03-2006, 08:41 PM
HI there,

At what point does this need to be done in total darkness?

Thanks.

Photo Engineer
09-03-2006, 08:47 PM
When B -> A.

Sorry.

PE

Jerevan
09-04-2006, 05:55 AM
Film speed of ISO 3-6, that's about what I need for using my Rodenstock Aplanat with the lenscap... *grins*

BTW, I prefer having all the ingredients of the formula at the top, so I can see what goes in at a glance. Nevertheless, I have nowadays the habit of reading a formula, at least three times, just to be sure...

bart Nadeau
09-04-2006, 09:16 AM
Add spectral sensitizing dye and hold at 45 deg for 15 mins.


PE

So is this where an emulsion starts as blue sensitive and is then made to be orthochromatic or pancromatic?
Sorry if this is an ignorant question but it must happen somewhere in the process.
Also, another question I've wondered about is making silver nitrate. If you disolved as much silver as could be in nitric acid and then air dryed it down to silver nitrate crystals would this give you good enough silver nitrate for an emulsion such as this.

bart

Photo Engineer
09-04-2006, 10:12 AM
So is this where an emulsion starts as blue sensitive and is then made to be orthochromatic or pancromatic?
Sorry if this is an ignorant question but it must happen somewhere in the process.
Also, another question I've wondered about is making silver nitrate. If you disolved as much silver as could be in nitric acid and then air dryed it down to silver nitrate crystals would this give you good enough silver nitrate for an emulsion such as this.

bart

Bart;

Yes, the addition of sensitizing dye can make an emulsion sensitive to any particular portion of the spectrum you wish including infra-red. A raw emulsion is mainly UV and Blue sensitive.

Most silver has impurities in it that will make it unusable for photographic emulsions. It would depend on the silver and on the nitric acid. Reaction of silver metal with nitric acid, IIRC, requires red fuming nitric acid as silver is a noble metal. RFNA, as it is called, is rather hard to get and gives off very toxic fumes.

PE

desertrat
09-04-2006, 12:32 PM
This formula assumed, as they did at the time, that you were using standard (ACTIVE) photo gelatins, and you will be lucky if you get ISO 3 - 6 with it using modern oxidized photo grade gelatins. You cannot get active gelatins that are any good today, for the most part.PE
Can an active gelatin be made from the gelatins we can get today? I'm guessing it would be very difficult, but I'm curious how it would be done.

Photo Engineer
09-04-2006, 12:48 PM
Can an active gelatin be made from the gelatins we can get today? I'm guessing it would be very difficult, but I'm curious how it would be done.

No, it cannot be done.

The active part has been removed (oxidized generally) from the emulsion. It is also a variable quantity from batch to batch of raw gelatin. We used to used what were termed Hard, Medium and Soft grades of active gelatin to achieve the various speeds and contrast grades. That is now achieved in a precise manner by addition of an exact quantity of an active sulfur compound such as was removed from the gelatin.

The original component was allyl thiourea.

PE

Jeanne
09-05-2006, 09:55 PM
No, it cannot be done.

The active part has been removed (oxidized generally) from the emulsion. It is also a variable quantity from batch to batch of raw gelatin. We used to used what were termed Hard, Medium and Soft grades of active gelatin to achieve the various speeds and contrast grades. That is now achieved in a precise manner by addition of an exact quantity of an active sulfur compound such as was removed from the gelatin.

The original component was allyl thiourea.

PE
This -- as well as other postings -- has got me wondering if there is a source for soft grade gelatin here in the US. Seems like most suppliers (B&S, PF, Artcraft) carry 250 bloom only.

Photo Engineer
09-05-2006, 10:26 PM
This -- as well as other postings -- has got me wondering if there is a source for soft grade gelatin here in the US. Seems like most suppliers (B&S, PF, Artcraft) carry 250 bloom only.

Jeanne;

All of their gelatin comes from either one of two suppliers in the US AFAIK.

The only active (non oxidized) gelatin is Knox unflavored food gelatin, but it has so many additives that it is about 30% gelatin and 70% additive. It is virtually unusable IMHO for emulsion work, as it varies from batch to batch.

PE

htmlguru4242
09-11-2006, 01:25 PM
Can't the additives be removed from the Knox gelatine?

David A. Goldfarb
09-11-2006, 01:32 PM
Even if they could, you can get photo grade 250 Bloom gelatin from Artcraft for $19.90/lb.

Photo Engineer
09-18-2006, 11:11 AM
Even if they could, you can get photo grade 250 Bloom gelatin from Artcraft for $19.90/lb.


The additives cannot be removed without specialized equipment.

Inactive (oxidized) photo grade gelatins are available in 3 grades. They are 75 bloom, 175 bloom and 250 bloom.

PE

robopro
11-14-2006, 01:29 PM
PE, here's a copy of a gelatin formula I found an the web as a lab experiment a couple days ago. I'm curious how you would rate it, especially for sensitivity, quality, and longevity.

With the lights on, dissolve 2.4 g of potassium chloride and 1.2 g of potassium bromide in 100.0 ml of distilled water. Place this solution on a stirrer hot plate and bring the temperature to 35C. Add 20.0 g gelatin. Set this aside to thicken. In another beaker dissolve 6.0 g of silver nitrate in 60.0 ml of distilled water. Assemble the constant temperature apparatus shown below. Run the hot water for a while to ensure temperature is about 50C. Heat the potassium chloride-gelatin mixture in the beaker to about 50C.
The remainder of this experiment must be done with a safety light as the photographic emulsion is extremely light sensitive once it is prepared. Add the silver nitrate solution slowly, with continued stirring, to the gelatin-halide mixture. The rate of addition should be no greater than 1 mL per 3 seconds. Ripen at 50C for 2 hours while stirring. At the end of this digestion period, add another 10.0 g of gelatin. Once this has dissolved, you can chill the emulsion with an ice bath. If necessary, the experiment can be stopped at this point. Wrap the emulsion beaker with foil, label and store in the refrigerator.
If the emulsion is shredded and washed, then cold tones will be observed. If it is left unwashed, warm tones will be obtained. The emulsion should now be melted and with continuous stirring, held at approximately 60C for 1 hours. At the end of this second digestion period. 0.125 g of chrome alum (dissolved in a minimum of H2O) can then be added as a hardener, but the alum will make the gelatin very thick.

Photo Engineer
11-14-2006, 02:35 PM
I see a few things to comment on.

The gelatin level is rather high compared to what I make. The silver level is a bit low compared to what I make.

The emulsion will be a normal Cl/Br paper with a rather slow speed, say about 2 - 6 stops slower than current enlarging papers. I can't really tell without testing it, but that would be my guess due to the lack of sulfur sensitization. If the formula assumes active gelatin, then I would not care to predict the contrast, but it could be quite low.

Most published formulas omit telling you that they use active gelatin or that they use a sulfur sensitization step.

I would use 10% glyoxal instead of chrome alum.

PE

OldBikerPete
11-14-2006, 03:55 PM
G'day PE.
Your formula mentions a spectral sensitising dye - can you be more specific?

Photo Engineer
11-14-2006, 05:37 PM
These dyes are hard to get and run about $100 - $200 / gram (US). To save money and effort, I use erythrosine, which is a food dye and is about $20 / gram and can be bought from most chemical supply houses.

It is very very low in toxicity.

It will give you an ortho sensitive emulsion when coated on film.

If you want to know about modern dyes used by Kodak and other companies, there is a good description in Mees, or Mees and James. They include structures and spectra. I have posted some of the spectra here and mentioned erythrosine several times. If you search for it, you will find the wedge spectrograms.

PE

25asa
11-14-2006, 11:11 PM
Gee, I see Pfaltz & Bauer has xenocyanine listed at $180/10mg

How many 120 rolls will 10 mg get us?

Photo Engineer
11-15-2006, 12:51 PM
You use a dye at about 20 - 100 mg / mole of silver.

Go figure.

PE

robopro
11-15-2006, 07:03 PM
PE

Are there any major differences between the formulas for gelatin film emusions and gelatin paper emulsions? I'm guessing, if anything the gelatin percentage and silver content in film emulsions would be higher than in paper.