1. Photo Engineer

    Photo Engineer Subscriber

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    There are a many gelatin types for making silver gelatin emulsions and most current textbooks don't discriminate between them. Here is some terminology to help you out.

    1. Food gelatin: These are active gelatins with lots of additives. At my current estimates as little as 30% of the weight is really gelatin in some brands. Using these can lead to foggy emulsions.

    2. Active gelatins: These are active gelatins that interact with an emulsion during making and can cause increases in speed and contrast. They are loaded with thoureas and allyl thioureas. For those concerned about the toxicity of thiouread, be forewarned that it is a natural product in gelatin due to the cows and pigs eating mustards. They can lead to foggy emulsons.

    3. Inactive gelatins: These have had all sulfur compounds removed. They are preferred.

    4. Pig Gelatins: Not commonly used in photo materials nowdays. They have their maximum swell at pH ~9.

    5 Bone Gelatins: Made from cow cartialge. Common consituent of current photo products. Minium swell at about pH 4.5.

    6. Gelatin Bloom: This is a measure of gelation strenth AFAIK. We never used this at EK. In my personal experience, the higher the bloom, the higher the apparent viscosity and the less pepper grain and aggregation in an emulsion. It also gets harder with higher bloom. Lower bloom gelatin hardens more rapidly though in my experience. This is very subjective and difficult to assess.

    7. PA gelatin: Gelatin treated with phthalic acid. Used in coagulation washing of emulsions (PA washing). This gelatin is softer, hardens more slowly and adheres less well unless regular unmodified gelatin is added.

    Using low gelatin when making an emulsion results in larger grains and higher speed. Higher gelatin results in lower speed and finer grain. This is a generalization.

    Heating gelatin for too long or at too high a temperature decreases viscosity and hardneing capability.

    PE
     
  2. Jerevan

    Jerevan Subscriber

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    PE,

    Are bone gelatins the same thing as Ossein?

    (maybe a stupid question, but I had to ask!)
     
  3. Photo Engineer

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    Yes, they are.

    Pig gel is made mostly from pig skin with some bone and cartilage, but bone gel is made mostly from cattle bones, with some cartilage and skin.

    Bone gelatin is made by treating the materials with lime and is sometimes called limed gelatin. To be used in photography it must be alkali and calcium free.

    PE
     
  4. juan

    juan Subscriber

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    The only thing I can add to this is that I saw a great improvement in the smoothness of my coating (salt prints - Richeson brush) when I switched from Knox gelatin (grocery store) to 250-bloom photo gelatin from PF.
    juan
     
  5. Photo Engineer

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    This is quite the case Juan.

    The store bought Knox gelatin has a lot of extra stuff in it.

    A lower bloom gelatin, about 75, might be very good to try as well.

    PE
     
  6. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    I was curious to know what exactly the Bloom value is, and found this useful description--

    Source-- http://www.milligan1868.com/gelatins.html
     
  7. sanking

    sanking Restricted Access

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    This may be true for making photographic emulsions based on silver salts. However, carbon is photography, right? If right, I have used both porcine source gelatins, including plain Knox from the grocery store, and ossein source gelatin intended for photographic applications. Both gave very good results for me in making carbon tissue.

    Sandy
     
  8. Photo Engineer

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    David;

    I searched and searched for a postable definition, but could not find one suitable. Thanks for this one.

    Higher Bloom Index (BI) means greater strength and greater hardness and greater viscosity. They are all subtly related, but there is sometimes no exact correlation. We did not use BI at Kodak. Even so, Eastman Gelatin produces gelatins with listed BI values from 75 to 250. These generally relate to fractions obtained from the purification process of the gelatin. Using either 75 or 250 is best. I use 250.

    Poise or centipoise is the metric measure of viscosity. We did not use it at EK. Even so, viscosity is an important measure for photographic coating, so we had an even more accurate measure called RBT (Rolling Ball Time). This measurement was a very accurate metric for judging suitability for coating.

    PE
     
  9. Photo Engineer

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    Sandy;

    Both bone and pig gelatin work just fine in making photographic emulsions and coating them. Kodak has changed to bone gelatins due to the greater availability, purity and the isoelectric point. Processing seems to be better in pig gelatin.

    As for edible gelatins, the added ingredients tend to interfere with precipitation and subsequent operations as well as being rather low in gelatin content, so they will work as gelatins per-se, but have drawbacks in that you are not sure what extras are there or how much gelatin you really have.

    In such a case, the emulsion content is critical to achieve a given grain size and you can never tell the exact gelatin content of food gel.

    PE
     
  10. htmlguru4242

    htmlguru4242 Member

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    Very interesting, PE!

    Two questions, though:

    1.) Are pig gelatins considered active or inactive gelatins?
    2.) What kinds of things are added to food gelatins that cause such an issue with the photographic processes?
     
  11. Photo Engineer

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    Pig gelatins currently sold are inactive photo grade.

    Food gelatins contain silicates to prevent caking and carboxy methyl cellulose to thicken the gelatin. In addition I have seen preservatives such as antioxidants added. Basically, the lowest gelatin I have seen is only 35% gelatin with the rest as additives and the highest I've seen is about 75% gelatin. That is a big range.

    PE
     
  12. Vaughn

    Vaughn Member

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    Like Sandy, I have been using Knox Unflavored Gelatin for carbon prints for years with success. Grain (as in grains of silver) are not an issue since there is usually no silver or other metals in a carbon print.

    The ingredient list on the box of KUG says only gelatin...but I note that it does not say 100% Gelatin...which probably leaves the door open for undeclared additives. From what I understand, it has a bloom of around 100. I know where I can buy 250 bloom, but where can one buy other blooms in reasonable amounts?

    Vayghn
     
  13. Photo Engineer

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    My KUG listed several ingredients on the side of the box. I don't have it here. Rather than use it in emulsions, we flavored it and ate it.

    PE
     
  14. Vaughn

    Vaughn Member

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    Odd -- the box in front of me says only gelatin (no sodium, no fat, no carbohydrates on the nutritional part of the label). Wonder if they have different manufacturing plants/labels. I know Knox also makes an edible gelatin product for strengthening fingernails -- I believe that it does have additives labeled, but I'll have to check up on that.

    The clarity of the KUG when melted is pretty good -- no sign of particulate matter suspended in it, or sitting on the bottom of the jar, from any additives. After I melt it, I toss in sugar (cane) and lampblack watercolor pigment (which has honey and gum arabic as the carrier for the carbon).

    I would like to switch to bulk gelatin because of price -- but prefer not to change the bloom significantly as I have my carbon process geared towards the lower bloom of the KUG. There has been some differences in highlight retention noted by other carbon printers during developing of carbon prints of different gelatin blooms.

    I do call my carbons prints "Jell-o prints" sometimes!:tongue:

    Vaughn
     
  15. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    If you had gelatin of a higher Bloom index, couldn't you just use a more dilute gelatin solution?

    Taking another tack--I can make a nice clear aspic, so say I wanted to make my own photographic gelatin (not that photo grade gelatin isn't readily available, but because I like to do such things). What additional chemical processing would I have to do to make my rendered, clarified gelatin suitable for photographic use?
     
  16. Vaughn

    Vaughn Member

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    It doesn't work that way...however, from what I understand, if you mix equal parts of 100 bloom and 200 bloom, you would have 150 bloom. So if I could find the 75 bloom PE talks about, I could possibly mix it with 250 bloom to get blooms in between the two.

    Vaughn
     
  17. Photo Engineer

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    Well, I just looked at a packet of regular Jello and the contents have changed a bit (for the same flavor) as last time. They have left out the silicate and the CMC. So, all bets are off as to what KUG contains. I won't go either way on this. I'll let you all make up your own minds on it.

    As for photograde gelatins, well in fact all gelatins, when made have a rather high content of staph and strep bacteria in them that have to be neutralized before sale. In fact, each batch of photograde gelatin that comes from a place like Kodak or elsewhere has come to me with a certified count of staph, strep and tuberculin bacilli (the latter of which must be zero). In addition, use of gelatin requires antibacterials in all batches that you mix up or it will spoil merely by letting your breath touch the gelatin or letting your hands touch it.

    Lowering gelatin content will lower viscosity, but the set gelatin will be at the same BI. BI is not a property of the wet gel but rather the dry gel in the sense of your reference David.

    Photo grade gelatin is probably as pure regarding bacterial content as food grade, or we would have a lot of sick photo engineers, but IDK what to do about removing other things. I know that photo grade is calcium free and oxidized. I would assume that was some ion exchange coupled with treatment by peroxide oxidation, but then that leaves the question of what to do with the leftover peroxide.

    Bottom line is this. I used to get my gelatin as 22% bricks (about 2" x 2" x 1 foot) or 12.5% flakes from a stockroom with all of that stuff done. I was pampered and now I would have to learn all of that. I never thought to question what my friend Jack could have told me when he was at Eastman Gelatin. He moved from our labs to Mass. I used to communicate with him often, but these questions never came up.

    Being pampered (or isolated) kept the secrets very well, but makes it difficult answering some of these questions.

    PE
     
  18. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    I'm willing to believe you, but do you have any idea why that would be the case? If the goal is to produce a gelatin solution of a certain stiffness, I know from the kitchen that you can make a stiffer gelatin mold by adding more gelatin powder to the solution, or that beef stock gets thicker and more gelatinous when you boil off the water. Isn't adding water just like adding 0 Bloom to the solution? Shouldn't, say, a 15% solution of 100 Bloom gelatin as stiff as some smaller % solution of 200 Bloom gelatin? Otherwise, why would the standard specify a 6-2/3% solution for testing, if it were not a relevant variable?
     
  19. Vaughn

    Vaughn Member

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    Wish I knew why, David -- and probably so do those who study the stuff! Gelatin is mysterious stuff. It does all sorts of strange things. Add water to it and you can melt it, but dry it out, and it will burn before it melts. I think I heard something about if you make a certain concentration of gelatin, dry it out, then toss it into water again, it will "remember" its former concentration (it will absorb only enough water to return to its original concentration), but I'm a little fuzzy on that. One "swells" the gelatin in cold water before putting it in a water bath to melt it. But unlike adding water to rice, the total volume does not change as the gelatin absorbs the water.

    (One of the most dangerous cargos for a ship to carry way back when was rice -- it they lost a hatch in a storm and sea water got to the rice -- the ship was doomed. The rice would swell and literally split the seams of the ship apart.)

    But in the end, bloom is not tied to concentration, but to some other physical trait of the stuff. I add 85 grams of Knox Unflavored Gelatin (110 bloom) to 750 ml of water to make my carbon tissue...so I have carbon tissue made of gelatin with a bloom of 100 and all the properties of a tissue made with a bloom of 100.. If I added 100 grams of KUG to 750mm water, I still have carbon tissue made with a bloom of 100, and all the properties of a tissue made with a bloom of 100. Properties such as hardness, how quickly it sets up, how fast it dissolves, how quickly it would release pigment, etc.

    Vaughn

    PS -- but take anything I say about gelatin with a grain of salt...I am really no expert on the stuff.

    Another mystery is why chromates cause gelatin to "tan" and raise its melting point (which is why we have carbon prints, etc) --- oh, they know what happens...but the why is slippery!
     
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  20. Photo Engineer

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    Gelatin melts in water at 68 degrees F or about 20 deg C. This is why old unhardened emulsions were processed at 68 degrees and we have carried that over until today.

    Gelatin + water does swell. Sorry, but I've had it swell up to nearly overflowing beakers. It behaves just like rice in the early stages. The difference is that it gradually melts and subsides back to close to the original volume unlike rice which swells and stays swollen.

    Bloom Index refers to the strength of gelatin which is related to the length of the molecule. The shorter molecules have a lower BI and lower dry strength and the higher BI have higher strength. Even if the solutions are made up to the same viscosity, the dry strengths differ. Low BI gelatin melts and dissolves more quickly and makes a softer film even at identical gelatin weights per unit area. High BI gelatin is more difficult to dissolve. Hardening goes faster with low BI gelatins but yields a more swellable coating. The use of aldehydes as hardeners with gelatin give different results depending on chain length of both the aldehyde and the gelatin based on BI.

    Chromates (and other light sensitive materials) tan by releasing a hardening agent. In the case of alum used in hardening fixes, the Al+3 ion gloms onto 3 gelatin molecules and ties them down in a 3 way salt, and a similar reaction takes place imagewise due to exposure when you use chromates. Tanning developers do the same as their oxidation products react with gelatin and cross link it just like formalin.

    Most all of these reactions can be reversed, so you can unharden your film with the wrong treatment.

    PE
     
  21. Vaughn

    Vaughn Member

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    Thanks for the info PE!

    Vaughn
     
  22. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    Thanks, that explains a few things!