Gelatin - What, exactly, makes it "active"?

[Robert O'Connell]

First post, I've been lurking here for a bit trying to gain the knowledge required to make my own silver/gelatin emulsions. And what I gather from the experts here seems to make absolutely no sense.

It seems that to prepare a proper emulsion, one must use "active" gelatin. Unfortunately, due to circumstances wherein we do not live one century in the past, the gelatin available is considered to be "inactive." So, why exactly is that? The most common assertion is that the production process is different and in some way strips out sulfur necessary for activity. My question, however, is what is different?

The only answer I can suppose is that historically, gelatin was rendered on an industrial scale from the bones and hooves of cows and horses, while in modern times it is derived from pork skin - Could this be the major difference noted by recreators of the emulsions of yore?

And now for the confusing part - It is easy to find people here and elsewhere that have had luck with Knox gelatin, or swear that photograde "250 bloom" gelatin is the only way to go - They're the same damn thing! Trade secrets aside, I can tell you for a fact that there are no additives, no nothing - some guy with a boxcutter dumped a 50lb sack of (Gelita brand 250-bloom) gelatin down a pipe, two people ran a grossly outdated and horrendously dangerous packaging machine, and another guy/girl boxed up and palletized the cartons for retail. They all bitched about the monotony and were more than likely high as a kite in the process.

The point, if any, is that Knox is in no way any different from what you would get from Adorama or the like, so what, if any, is the difference in gelatins? Unless you're going above and beyond to source it, what makes one better than another?

 

[Photo Engineer]

Robert, active gelatin is NOT desired for making good emulsions because it varies from batch to batch. This is due to sulfur containing compounds in the gelatin put there via metabolism of their feed stock. The main bad ingredient is allyl thiourea, and organic chemical found in all gelatin and which varies due to the sulfur components found in their fodder. One of the main bad sources in old days was mustard grass which is rich in sulfur compounds and which led to the discovery of "sulfur sensitization" and which was published by Sheppard in about 1920. It made emulsions more controllable and led to major speed increases. Among other things it eliminated the need for Hard, Medium and Soft "bloom" gelatins. This "bloom" is not related to the "Bloom" of gelatin related to viscosity and fraction coming from the initial mother liquor at the factory.

Now, on another matter, pig gelatin used to represent a major component of photo products, but was abandoned in favor of bone (ossein) gelatin made from cattle. In either case, no animals are sacrificed for a photo product but rather the parts of the livestock that are not usually used are used here to make something useful.

As far as Bloom goes, that is related to a form of viscosity and 250 bloom appears to be optimum for coating in the home darkroom. I have personally used Bloom index gelatin as low as 75 BI and find it too "sloppy" and that it makes poor emulsions.

Regarding Gelita gelatin and Knox gelatin, what you say may be so, but I have found Gelita to be slightly active when it is supposedly inactive and that Knox gelatin often leaves small particles behind due to inclusion of Sodium Silicate and other ingredients to keep the gelatin from caking or otherwise degrading. I use either Kodak or Rousselot photo grade gelatin and find a big difference compared to Gelita and Gelita appears to be much better than plain Knox gelatin off the grocery store shelves.

It is best to get an inactive gelatin. The more inactive the better. Then, you add the amount of Sulfur in a separate controlled step to get the optimum speed and curve shape.

Hope this helps.

PE

 

[Sirius Glass]

Welcome to APUG

 

[Nodda Duma]

PE, when/where/how do you add the sulfur? I happen to have some and might be tempted to try it when I make my next emulsion batch.

 

[Photo Engineer]

For a very simple formula.

Sequence:

1. Dissolve Gelatin in water and add Halide salts at a temperature and in amounts based on the formula.

2. Slowly add Silver Nitrate in water at a rate determined by the formula.

3. Hold at a temperature and for a time determined by the formula.

4. Wash.

5. Heat to a specified temperature at then add a dilute hypo solution or thiocyanate solution and hold for a time determined by tests.
This is usually 50 - 100 mg of Sulfur / mole of Silver used. < this is done by trial and error for each new formula but is only done one time!

6. Chill set.

7. When ready, heat to 40C, add finals (hardener and surfactant) and coat.

PE

 

[Nodda Duma]

Thanks. Seems fairly straightforward.