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    Activating Gelatin

    In a previous post by PE, he mentioned that the modern gelatins available are unactivated due to oxidation. As an avid student of chemistry, I wonder if a reduction of a gelatin with a substance of high reduction potential, IE NaBH4. However, you could probably reduce your gelatin with small amounts of metol, hydroquinone, and similar reducing agents. I would have to find some reaction mechanisms to be sure, and know exactly what is oxidized in the gelatin as compared to an active gelatin, however, I think that the older activated gelatins may still be availible with a little work.

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    Firstoff, all gelatin after the initial preparation is active. The active gelatin contains the Amino Acids which contain sulfur such as Methionine and etc. During the preparation, these form Allyl Thiourea and its derivatives. The oxidation removes the Sulfur amino acids and the Allyl Thiourea.

    Since Thiourea, Thiocyanate and Thiosulfate can be substituted for the Sulfur compounds in gelatin which vary wildly from batch to batch, there is no real point in trying to use active gelatins or to "reactivate" gelatin. It is a very qualitative way to solve what is actually a non problem that is gotten around today very well in a quantitative, easily repeated and measurable fashion.

    Sorry, but that is my opinion.

    PE

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    The speed of holographic emulsions can be increased greatly (at least 5 fold) by immersing it in a 1% solution of Ascorbic Acid at a PH of 5 and drying before exposure. Holographers often do this as it can increase the speed of an emulsion without causing grain growth. I have never heard of this clever idea of activating the gelatin before precipitation is done. I think I will give it a try.

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    Acid was often used for increasing contrast of emulsions. Ascorbic acid will "R" type an emulsion adding reduction sensitization. This is similar to work using Tin Salts and other reductants.

    It is not new, but useful in Holography I guess, or at least new to the holographers out there.

    PE

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    Thank you, PE. I thought there may have been other benefits of an active gelatin. As I understand, electrons move from the silver halides to shallow electron trap sites, these trap sites are more prevalent in elements which may have an expanded octet, such as sulfur. I was thinking that the electron traps formed by amino acids in active gelatin may be more suited to forming smaller grain clusters due to dissipation of a charge over an organic molecule, however, if you say that there's no difference, then obviously, that must not happen. What's the exact process of sulfur sensitization with Na2S2O3?

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    Simply put, you apply a heat treatment to the AgX crystal in gelatin + the Sulfur compound. This reaction produces materials which increase sensitivity to light or radiation. With Hypo, the reaction is AgX + S -> Ag2S ... etc.

    How and where the Ag2S is formed is critical and how much is critical. Usually 20,000 atoms / grain are preferred and are on the surface of the grain. Each grain shape, size and halide type react differently.

    Usually, about 50-70 deg C is used for up to 1.5 hours.

    Hope this helps.

    PE

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    I suppose then, knowing your grain and halide type (using a double run method, when making emulsions of a 2 halide types) you could ascertain the amount of S needed, IE, if you knew your RMSG was near whatever amount, you need 3.32x10^-20 moles of S per grain, and if you know approximately your grains per area, and area per amount of liquid emulsion, you could then derive the exact amount of Na2S2O3 needed, or am I off somewhere?

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    Well, I've only done this empirically, kinda knowing how to do it.

    Basically, a coarse grain with lower surface area requires less Hypo than a finer grain and so a 1 micron crystal may use 3 mg/mole of Hypo whereas a 0.1 micron grain may require 100 mg/mole. My emulsion is about 1 micron, but needs 100 mg/mole, so you see that I had to adjust for 'windage' in making the emulsion. This is due to grain type, Iodide content and placement, and the vAg that I am working at. These are all significant.

    For example, the higher the salt present, the more hypo needed as a starting guesstimate. Surface Iodide acts like higher salt in solution due to equillibrium effects. I try to devise fool proof formulas that don't need much experimentation, but a lot of people love to do it. Regardless, the Sulfur emulsions are harder for me to make repeatably due to the lack of equipment to measure and control final vAg. That is on a high priority TO DO list, but I have made no headway so far.

    PE

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    I think that you could measure vAg pretty easily, as I understand it's the same as pAg, correct? Find something that silver complexes easily with within the visual spectrum, and if you have a color densiometer, you can measure the 'density' of a few known pAg concentrations, and then you can interpolate the pAg of your emulsion by interpolating it verses your densities from known pAgs.

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    vAg and pAg are similar but use different scales for reasons of accuracy. As far as doing it your way, I have never heard of it being done and it would continually sacrifice a portion of the emulsion if it could be done. I am working on a non-destructive method!

    Thanks though.

    PE

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