PDA

View Full Version : Activating Gelatin



Pages : [1] 2

drazak
01-11-2009, 08:22 PM
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.

Photo Engineer
01-11-2009, 08:45 PM
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

Hans2008
01-11-2009, 09:36 PM
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. :)

Photo Engineer
01-11-2009, 10:04 PM
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

drazak
01-12-2009, 05:07 PM
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?

Photo Engineer
01-12-2009, 06:08 PM
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

drazak
01-12-2009, 07:34 PM
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?

Photo Engineer
01-12-2009, 07:45 PM
Well, I've only done this empirically, kinda knowing how to do it. :D

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

drazak
01-12-2009, 07:57 PM
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.

Photo Engineer
01-12-2009, 08:07 PM
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

drazak
01-12-2009, 08:11 PM
Assuming you have decent graduated equipment(or volumetric equipment), you could probably do it with 1mL of emulsion dilution in a solution of the complexing agent, basically you're using the color densiometer as a primitive spectroscopy machine, the theory would be that you set it to the color channel opposite that which the complex is, IE. blue if it's orange, green if it's red, and so on and so forth. I can see that it is fairly destructive, however if you only measure your emulsion a few times during production, it's probably not destructive enough to worry about.

Ben

Photo Engineer
01-12-2009, 08:24 PM
Ben;

Out of a 400 gram make, I could use 40 grams or 10% on the test. Using a meter with appropriate electrodes, there is no loss whatsoever.

PE

drazak
01-12-2009, 08:27 PM
Ah, well that's a much easier method, I didn't know there was such an easy method! I'm just up in buffalo, so let me know if you need anything to make your meter.

Ben

P.S.: I wonder how much it would be to try one of your paper emulsions, before I start making my own, it'd be nice to someone elses made emulsion and compare it to commercial ones, I'd be willing to pay, of course.

Photo Engineer
01-12-2009, 08:45 PM
I can make you some. It costs me for Silver, heat, water and gelatin mainly. Lets tall off-line.

As for the meter, I need two pieces of silver wire right now, about 99% fine and about 6" long each.

I've been thinking of a jewelry hobby store or something.

Thanks.

PE

Kirk Keyes
01-12-2009, 10:56 PM
Ben, I've got a meter, I just need an electrode. Actually, I just need a good reference electrode. As PE says, one can use a billet of silver for one half-cell.

PE - try this: 99.99% round wire -
http://www.ccsilver.com/silver/superfines.html#four
12 or 14 gauge should be good. You want to split an order?

Kirk Keyes
01-12-2009, 11:19 PM
Here's some really useful info on Ag-AgCl reference electrodes:
http://www.consultrsr.com/resources/ref/agcl.htm

Ray Rogers
01-13-2009, 07:22 AM
My emulsion is about 1 micron...
PE

Which one are you referring to?
The pure chloride paper one or the "ISO 40" one ?
:confused:

Photo Engineer
01-13-2009, 09:22 AM
Which one are you referring to?
The pure chloride paper one or the "ISO 40" one ?
:confused:


ISO 40 SRAD. Picture posted here in another thread.

PE

Photo Engineer
01-13-2009, 09:23 AM
Here's some really useful info on Ag-AgCl reference electrodes:
http://www.consultrsr.com/resources/ref/agcl.htm

Which will not work with anything but AgCl emulsions I would think.

PE

Ray Rogers
01-13-2009, 09:58 AM
As for the meter, I need two pieces of silver wire right now, about 99% fine and about 6" long each.
PE

Why do you need 2 ?