View Full Version : Low pAg emulsions

11-03-2011, 02:33 PM
Dear friends, have you any ideas about making low pAg gelatin emulsions (silver excess emulsion)?

All my efforts are unsatisfactory up to date. Best of my results is the emulsion that remained unveiled some hours only. It was making according to this recipe (100 ml):

Solution A (60 deg):
- water 60 ml
- 1M HCl 2 ml
- KBr 2,2 g
- KJ 0,08 g
- Inert Photograde Gelatin Rousselot 8 g

Solution B (room temp):
- water 40 ml
- AgNO3 4,0 g

Synthesis - Sol B pour in Sol A in 4 doses:
- 1/4 at once and 7 min mixing (at 60 deg)
- 1/4 at once and 7 min mixing (at 60 deg)
- 1/4 at once and 7 min mixing (at 60 deg)
- after cooling the emulsion till 46 deg the last quarter of silver nitrate pours in synthesis volume (AgHal microcrystalls charge exchange) and following 5 min mixing (at 46 deg)

After jelling, washing, remelting (at 35 deg), coating on glass plates and two hours drying such emulsion kept unveiled only some hour. I suppose that the main course of such instability is high pH of finish product - after washing ph of emulsion increase upto 4,4. It seem to me that pH = 4,4 is too high for long keeping of low pAg emulsion without to be veiled (even though the very good quality inert gelatin was used). Have you any ideas on this issue?

Photo Engineer
11-03-2011, 02:43 PM
It is more common to measure emulsions in terms of vAg and not pAg. Using that reference, a vAg of much over 100 mv is problematic. At 100 mv, the pAg would be in the range of 8.+ for NaBr at 40 deg C. On that same scale a vAg of -50 mv would be a pAg of about 10.+. Generally, in the range of 100 mv or higher, emulsions tend to fog quickly whereas below that, emulsions keep better. Halide excess is the norm with Silver Halides made in Gelatin.


11-03-2011, 03:08 PM
Thank you, Ron!

Of course halide excess is norm. And such emulsions don't meet any veil troubles. I often overwashed my normal emulsions till pAg=7-8 and all worked good. But now I am especially interesting in low pAg emulsion and read one ancient work by Carroll and Hubbard (1928), who were successful enough in silver excess gelatin emulsions. But they could have the centrifugal separation (of course I can't), that give some advantages. All I can do is to follow they strict limitation they worked out by empirical tests - do not raise pH higher 3,5.

Carroll writes:
"...but we find that if the hydrogen ion concentration of the emulsion is increased well above the isoelectric point (pH preferably 3.5 or less), and deactivated gelatin is used, such emulsions may be kept for at least a month without noticeable increase in fog".

Of course the silver excess is to be not too large. Carroll's emulsion have Ag-excess "the same order as the excess soluble bromide in a normal emulsion." (normal washed emulsion meant).

Photo Engineer
11-03-2011, 04:43 PM
Well, ideally, the pH of an emulsion should be between 5.5 and 6.5. IDK offhand why yours is so low. You might uses some Tetra Aza Indene in the emulsion as described in the formula by Jim Browning. That can work to advantage sometimes, but it does change the pAg.

Burt Carroll changed a lot of his opinions after he began working on emulsions at Kodak. Some of the work you are referring to was done at the NBS before he joined EK, right?


11-04-2011, 08:02 AM
Of course, pH normal unammoniacal emulsion should be 5.5-6.5. But it isn't the case of low pAg emusion. Such pH along with Ag-excess give enough reduction power to gelatin (even inert) and reduce too much silver. I can't use TAI under my task condition.

The only way I see to have Ag-excess and to stop the fog at least some days (without TAI) is probably to hold up pH at very low values. In combination with inert gelatin (the oxidized one is still better, but I haven't it). I hope in this way to attain the acceptable results, comparable with Carroll silver-excess emulsions. At least I'll try it.

Tomorrow I am going to try aforementioned recipe with some modifications. I'll change HCl by H2SO4 for start condition and will be use acid water for emulsion washing - acidifying water by H2SO4. I hope that by using pH=2-3 water for washing I'll keep up finish pH value well below 3,5 of Carroll limitation.

Ron, what is IDK? Unfortunately I can understand this phrase at all - "IDK offhand why yours is so low." My English is limped on both legs...

About Burt Carroll. Unfortunately I don't know the exact date of his joining Kodak. He could change a lot of his opinions under the blessed sky of Kodak. Yes of course. But why do we think about his Ag-excess emulsions that "may be kept for at least a month without noticeable increase in fog" as of the "opinion"? Especially if such emulsions were been using in his experiments on sensitizing primitive emulsions by colloids?

I can think of Carroll as the researcher with his attendant evolution of opinions, but I can't to think of Carroll as a joker.

Ron, who is Jim Browning? And are there among his formulae some interesting for Ag-excess?

Photo Engineer
11-04-2011, 10:09 AM
IDK = I Don't Know!

Search Google for Jim Browning and Dye Transfer. You can then see his emulsion formula for Matrix Film.

Burt was not a joker, but his ideas evolved with time just as mine did as I learned more about emulsions. We all learn more. One thing I learned is that Silver excess emulsions are rarities as far as salable products were concerned and extensive work was done to make them stable on the shelves of any dealers. I know that few products were sold unless they had stabilizers added. One product had a stabilizing interleaving sheet between each sheet of sensitized paper.

I am of the opinion that you should learn how to stabilize this material or abandon it for a different formula that does not involve a low pH and Silver excess.


11-04-2011, 11:36 AM
"Abandon" will be my last remedy. But it is the demand of my special task to do some work with Ag-excess emulsion. That is why I don't stop my trying in this course now.

Ron, I want to accomplish some approaches to this aim, and if there will be any success - I write about it.
If not - "abandon" always have time.

Needless to say that Ag-excess emulsion don't enter into large round a salable products. I know only two exceptions - wet collodion and probably modern Fuji Arcos 100 film. The latter is as I know of Tadaaki Tani handwork and is the really Ag-excess commercial film. This film in all probability is making on especially deep oxidized gelatin and with strong defense by multiple fog-stabilizers.

I know also that 19 century masters often made Ag-excess emulsions for the quick usage. This technique gives large gain in sensitivity for primitive emulsions. And often they use some oxidizers during synthesis (for example, iodine, as it did Abney).

> I am of the opinion that you should learn how to stabilize

That's just the point to make Ag-excess emulsion without stabilizers. My task demand it. And the only extenuating circumstance is that the shooting follows the plate coating in some next days (usually one-two days).

Photo Engineer
11-04-2011, 01:37 PM
If TAI is out of the question, try BTAZ (Benzotriazole) or PMT (Phenyl Mercapto Tetrazole).

Reducing agents were often used such as a small amount of Ascorbic Acid.

See, I have been hitting the textbooks! :)


11-05-2011, 02:04 AM
Ron, BTAZ (pK(Ag-BTAZ) = 13,4) and PMT (pK(Ag-PMT) = 16,2) form the sparingly soluble salts with Ag+. It is very undesirable for me, because I want to keep Ag/Ag+ and AgHal/Ag+ systems to be intact (to the extent of gelatin allows). Of course this is the main root of all my troubles and my only way to escape fog is removing all potential reductons or suppression their reduction power.

Ascorbic Acid (as also other reductons) is a good thing. I don't know about Kodak but Fuji really used AA in his work.
I have used AA for Reduction Sensitization in my syntheses too and have gained good results.
But Red-sensitization is something other than I want to examine now.

By the way, if we speak of Red-sensitization, It is generally accepted to state 1950 year as year of its discovery (pioneer work by Lowe, Jones, Roberts). IDK why so, but I undoubtedly know that there is US Patent 2,487,850 dated 1949 (!), devoted Red-sens by SnCl2 (one more great red-sensitizer). And the author of this patent is none other than Burt! Of course he was at Kodak by the date.

Unfortunately I can't catch the sense of your phrase (again my English limps...) - "See, I have been hitting the textbooks!"

11-05-2011, 06:23 AM
Needless to say that Ag-excess emulsion don't enter into large round a salable products. I know only two exceptions - wet collodion and probably modern Fuji Arcos 100 film.

Lippmann/holographic emulsions might be an other area...

11-05-2011, 08:48 AM
Lippmann/holographic emulsions might be an other area...

Of course. Excuse me, I'm forget completely...

11-05-2011, 09:11 AM
By the way, if we speak of Red-sensitization, It is generally accepted to state 1950 year as year of its discovery (pioneer work by Lowe, Jones, Roberts).
1950? Red sensitive materials were discovered and in general production long before 1950.

Photo Engineer
11-05-2011, 10:14 AM
I have been reading some of my text books to learn more about Ag excess. We never worked with them in my time at EK As Far As I Know (AFAIK).

Sulfite was also used as a preservative.


11-05-2011, 02:10 PM
> Sulfite was also used as a preservative.
It is Red-sensitizer too. Again Burt was the inventor of this role of sulfite (B.H.Carroll, D.Hubbard: The Photographic Emulsion: Sensitization by Sodium Sulphite, 1933).

Ron, 3 hours ago I synthesized new Ag-excess emulsion and poured it on glass plates. This time I washed it by acid water (pH = 2,1) and tomorrow shall test it in shooting and developing.

To Hexavalent:
In this case "Red-sensitization" is not spectral sensitization at all but shortening of "Reduction Sensitization" (excise me for unhappy contraction). I mean the chemical sensitization ("after-ripening" in term of english reseachers).
Reduction Sensitization was the main companion of Sulfur Sensitization before 1930-s (when Koslovsky from Agfa discovered Gold Sensitization) - that was the time of active gelatins.

Photo Engineer
11-05-2011, 02:55 PM
Yes, spelling out the "reduction" would help some.

I have been looking up "R" typing in my notes. Stannous Chloride has been mentioned. It can cause fog.


11-07-2011, 10:03 AM
Yea, temporary abandon...

My yesterday's shooting was commencing with the great news - fog was overcame.
Finish emulsion (before coating) had pH=2,7. After one day storage I hadn't any fog after 5 min developing in metol-hydroquinone mixture at 23 degree (Density of fog+glass was 0,08).

I was preparing to celebrate the victory... But the very first snap showed that something was wrong.
I'll not enter in all detail, but only short sum.

1. Strong acid emulsion has ve-e-e-ry weak adhesion to the glass. After developing and fixing I had emulsion floating at one corner and glass resting at its corner... Such acid emulsions need strict tunned sub-layer.

2. Strong acid emulsion quick damages basic developer (and almost every developer is basic).

3. Developing is strongly impeded by acid reaction of emulsion. And what is more strange - I had 20 sec "induction" time when with all my past Ag-excess emulsions I observed only immediate developing. The whole 20 sec! It pushed me to deep thinking and ... next

4. The most bad news is ... I most likely hadn't Ag-excess at all. How it could turned out if I put Ag in excess to bromine. The answer is probably consist in acid washing (I don't see any other possibility). I acidified water by H2SO4. But as I've learned recently Ag is build the non-dissociated complexes with ion SO4- (AgSO4- etc.) But these complexes have good solubility. I indeed washed out in this way all my silver excess which I had on surface of microcrystals AgHal.

Yea, I am setting (at least for time) abandon. And I see now only one really working possibility of shooting with Ag-excess gelatin glass plate - It is bathing of normal Br-excess plate before shooting in silver-ammoniac solution (well described elsewhere).