Silver Gelatin Emulsion Stabilisers

[Lee Lira]

Mind boggler question re-stabilisers/antifoggants,
I’ve read that stabilisers for example KBr, PMT, TAI or Benzotriazole, to name but a few can retard or even prevent ripening. So I figure that it would be best to add them near the end of digestion (2nd ripening)
Thoughts please.

 

[iandvaag]

That is a really fantastic question. Unfortunately Photo Engineer is no longer with us, and I don't know if there are any current contributors to this forum who are able to answer the question. I am certainly not qualified to speak on such things, but I'll take a swing at it.

It's my understanding that stabilizers can be used at different points in the process for different purposes (1).

During precipitation, growth and ripening
Some stabilizers adsorb very strongly onto the surfaces, preventing access to the crystal surface. This causes them to be growth restrainers, as you mention (2, 3). But I think that this can sometimes be a desirable effect if used in a controlled fashion. If the stabilizer preferentially adsorbs to a particular crystallographic facet, this can prevent growth in that direction, causing it to be a shape-directing agent. So in addition to controlling shape with vAg, it can also be controlled with shape-directing agents.

Before and during chemical sensitization
Apparently, stabilizers can also be used to control the specific location on the grain where sensitization occurs (4). My guess is that in order to achieve this effect, stabilizers would be added at various points before and during chemical sensitization. I believe that chemical sensitization (aka finishing) is the biggest gap in the knowledge of home emulsion makers wanting to get to high speed. This is a total black box from what I can tell and AFAIK has not been published anywhere in detail. It seems like it's an incredibly complicated process with dozens of steps and precise additions, temperatures, and hold times. Unless some insider comes forward, I think we'll be stuck with trial and error if we want to try and venture beyond ISO ~80. But I'm guessing that stabilizers are involved in some of the steps.

After sensitization
I think stabilizers are also added at the end of the make to prevent fog in the stored, coated material.

Certain stabilizers may also have secondary effects: PMT sharpens the toe and 5-methyl benzotriazole affects the image tone to become more black (5).

So what to make of this? I dunno, but if you are doing a digestion, it seems reasonable to add stabilizer(s) after that step and before chemical sensitization. Before or after washing? I dunno. I would guess that for strongly adsorbed stabilizers, it wouldn't matter that much if you have added the "correct" amount. I suspect that more weakly adsorbed stabilizers might be able to be displaced by the wash somewhat. This is just speculation, I really don't know at this point. My guess is that if you are not overwashing, you may as well add the stabilizer(s) after washing, since I don't think there should be any fog formation occurring during the wash step.

TAI is used at a fairly high amount (0.1 - 5 g per mole Ag) whereas most other organic stabilizers are used at lower concentrations (0.001 - 0.05 g per mole Ag), and the amount is dependent on the surface area (grain size) of the emulsion (6, 7).

I'm very keen to hear any details on the emulsion you're making. I hope this post was helpful and did not contain false or misleading information.

_______________________
(1) https://www.photrio.com/forum/threads/photographic-emulsion-stabliizers.37914/page-2#post-2088673

(2) https://www.photrio.com/forum/threads/photographic-emulsion-stabliizers.37914/#post-544767

(3) I believe this is discussed in Particle Growth in Suspension proceedings, 1973, SCI monograph #38, but I don't have a copy yet.

(4) https://www.photrio.com/forum/threads/how-was-tech-pan-emulsion-different.27617/page-2#post-386089

(5) Emulsion Making, Coating and Testing, Ron Mowrey. Pages 34 and 56.

(6) https://www.photrio.com/forum/threads/photographic-emulsion-stabliizers.37914/#post-544760

(7) https://www.photrio.com/forum/threa...and-sulfur-gold-treatment.31363/#post-2074735

 

[grainyvision]

I've seen references that BZT doesn't truly stabilize an emulsion, it instead only modifies development so that less emulsion fog develops. However, not sure how BZT would affect chemical/physical sensitization though. I'm still waiting on Photographer's Formulary to get a new batch of PMT in before I can do much with emulsion stabilization though

 

[grainyvision]

Also I assume sensitizing beyond ISO 80 involves panchromatic sensitization with dyes. There's likely a very good reason Ilford Ortho is stuck at 80 ISO (and is in my experience very weakly pan sensitized) while other ortho films are 25 ISO or significantly slower. Messing with dyes at that kind of speed involves a lot of steps to avoid significant amounts of fog, not to mention coating in complete darkness is typically out of reach for the home emulsion maker

 

[Nodda Duma]

You’re combining anti-foggants and stabilizers in your list when they serve two separate purposes.

PMT, for example, acts as an anti-foggant that is added before or at the start of 2nd ripening to suppress fog while still allowing sensitivity specks to form. It can be added even as early as 1st ripening, depending on what the emulsion wants. A key point to remember is fog and sensitivity specks are similar but not the same. In any case, anti-foggant allows longer ripening before fog kicks in or, said differently, you can ripen the emulsion to a given speed with less fog. In the case of PMT, you even get a speed increase. KBr does the same but reduces speed slightly.

A stabilizer reduces (Benzotriazole) or stops (TAI) ripening activity and is added at the end. A very small amount of TAI helps reduce fog growth and can be added at the beginning, but stabilizing concentrations of TAI should/can be used to end the ripening stage. In the correct amounts, TAI also provides a slight speed boost.

In all cases, ripening time isn’t static: Adding anti-foggants inhibit fog build-up, but will change ripening time needed to get to a desired speed. The difference is that when that speed is achieved, the base fog is lower. Conversely, if you’re adding chemicals and not adjusting your ripening time to compensate, then you may think the anti-foggant inhibits ripening.

-Jason

 

[Lee Lira]

Wow, thank you people for these super informative responses! It’s going to take me awhile to read through & digest (excuse then pun.) Agreed & alas PE is no longer with us, the mention of his name means that he will be forever in our thoughts.

 

[Lee Lira]

I've seen references that BZT doesn't truly stabilize an emulsion, it instead only modifies development so that less emulsion fog develops. However, not sure how BZT would affect chemical/physical sensitization though. I'm still waiting on Photographer's Formulary to get a new batch of PMT in before I can do much with emulsion stabilization though

I think I purchased their last 40ml of 1% solution last year. Was hoping they’d supply some TAI for me to play with.

 

[Lee Lira]

That is a really fantastic question. Unfortunately Photo Engineer is no longer with us, and I don't know if there are any current contributors to this forum who are able to answer the question. I am certainly not qualified to speak on such things, but I'll take a swing at it.

It's my understanding that stabilizers can be used at different points in the process for different purposes (1).

During precipitation, growth and ripening
Some stabilizers adsorb very strongly onto the surfaces, preventing access to the crystal surface. This causes them to be growth restrainers, as you mention (2, 3). But I think that this can sometimes be a desirable effect if used in a controlled fashion. If the stabilizer preferentially adsorbs to a particular crystallographic facet, this can prevent growth in that direction, causing it to be a shape-directing agent. So in addition to controlling shape with vAg, it can also be controlled with shape-directing agents.

Before and during chemical sensitization
Apparently, stabilizers can also be used to control the specific location on the grain where sensitization occurs (4). My guess is that in order to achieve this effect, stabilizers would be added at various points before and during chemical sensitization. I believe that chemical sensitization (aka finishing) is the biggest gap in the knowledge of home emulsion makers wanting to get to high speed. This is a total black box from what I can tell and AFAIK has not been published anywhere in detail. It seems like it's an incredibly complicated process with dozens of steps and precise additions, temperatures, and hold times. Unless some insider comes forward, I think we'll be stuck with trial and error if we want to try and venture beyond ISO ~80. But I'm guessing that stabilizers are involved in some of the steps.

After sensitization
I think stabilizers are also added at the end of the make to prevent fog in the stored, coated material.

Certain stabilizers may also have secondary effects: PMT sharpens the toe and 5-methyl benzotriazole affects the image tone to become more black (5).

So what to make of this? I dunno, but if you are doing a digestion, it seems reasonable to add stabilizer(s) after that step and before chemical sensitization. Before or after washing? I dunno. I would guess that for strongly adsorbed stabilizers, it wouldn't matter that much if you have added the "correct" amount. I suspect that more weakly adsorbed stabilizers might be able to be displaced by the wash somewhat. This is just speculation, I really don't know at this point. My guess is that if you are not overwashing, you may as well add the stabilizer(s) after washing, since I don't think there should be any fog formation occurring during the wash step.

TAI is used at a fairly high amount (0.1 - 5 g per mole Ag) whereas most other organic stabilizers are used at lower concentrations (0.001 - 0.05 g per mole Ag), and the amount is dependent on the surface area (grain size) of the emulsion (6, 7).

I'm very keen to hear any details on the emulsion you're making. I hope this post was helpful and did not contain false or misleading information.

_______________________
(1) https://www.photrio.com/forum/threads/photographic-emulsion-stabliizers.37914/page-2#post-2088673

(2) https://www.photrio.com/forum/threads/photographic-emulsion-stabliizers.37914/#post-544767

(3) I believe this is discussed in Particle Growth in Suspension proceedings, 1973, SCI monograph #38, but I don't have a copy yet.

(4) https://www.photrio.com/forum/threads/how-was-tech-pan-emulsion-different.27617/page-2#post-386089

(5) Emulsion Making, Coating and Testing, Ron Mowrey. Pages 34 and 56.

(6) https://www.photrio.com/forum/threads/photographic-emulsion-stabliizers.37914/#post-544760

(7) https://www.photrio.com/forum/threa...and-sulfur-gold-treatment.31363/#post-2074735

Thanks heaps for this info, very interesting & helpful. I’ve been making emulsion since April of last year & I’m up to my tenth batch. I use Denise Ross’s AmBr Ortho base recipe that I’ve since tweaked & added to as part of a fun learning journey. So far I’ve added 90mg p/mole Ag of hypo, 1:3 Au in the form of Steigmann’s solution, 153mg p/mole of Ag of Erythrosine & Tartrazine, 5mg p/mole of PMT & 30mg p/mole of Benzotriazole. First temperature hold after precipitation is 14mins at 55° & digestion is 80mins at 55°. The results can be seen on my Instagram account, My user name is, Lee Lira. Or on https://ello.co/leelira Thanks again.

 

[Bokaj14]

PMT, for example, acts as an anti-foggant that is added before or at the start of 2nd ripening to suppress fog while still allowing sensitivity specks to form. It can be added even as early as 1st ripening, depending on what the emulsion wants. A key point to remember is fog and sensitivity specks are similar but not the same. In any case, anti-foggant allows longer ripening before fog kicks in or, said differently, you can ripen the emulsion to a given speed with less fog. In the case of PMT, you even get a speed increase. KBr does the same but reduces speed slightly.

A stabilizer reduces (Benzotriazole) or stops (TAI) ripening activity and is added at the end. A very small amount of TAI helps reduce fog growth and can be added at the beginning, but stabilizing concentrations of TAI should/can be used to end the ripening stage. In the correct amounts, TAI also provides a slight speed boost.

Do you have any actual numbers how muct PMT would be used as antifogant and how much TAI to stop reaction at the end. So the coating can be done slowly and not to worry to fog emulsion by heating it to coat plates?