View Full Version : Gold sensitisation?

Martin Reed
11-20-2007, 04:35 AM
When we were doing the 'Silver Gelatin' book there were a few avenues that looked promising but there was not enough time to follow them up. One was the idea of using a ready-made commercial gold toner as an emulsion additive for hyper sensitivity, containing as it does a source of both gold salts and sulphur.

An initial test was made by Phil Robbins, who added a small diluted volume of Tetenal Gold Toner to the mix in the early stages of emulsification, which used an inert photographic emulsion. There was an amazing increase in activity of the final emulsion; unfortunately the fog went up to match it. We didn't take the idea any further but always meant to come back to it - maybe Photo Engineer could give a comment on this?

Kirk Keyes
11-20-2007, 03:15 PM
Martin - can you tell us how much toner was added in relation to the amoung of silver?

I'm sure PE will have some numbers, but off hand, I seem to remember 100mg of hypo per mole of silver being a good target amount of sulfer to add to you emulsion.

And the cooking time after addition is important. Can you give us an outline of your test formulation?


Photo Engineer
11-20-2007, 07:48 PM
Martin, there is a straightforward nomograph to show the amount of sulfur to add / mole of silver and is related to grain size. This basic number can only be determined by trial and error if you don't know the grain size.

You also have to pick the optimum temperature and time. This is critical to avoid fog.

Last but not least, you must have the correct ratio of gold to sulfur. This is usually 1:3 wt/wt but depends on the salts used.

If you go outside of any of these, the emulsion will fog. Basically, addition of sulfur increases contrast, speed (by as much as 5 stops) and fog (unless controlled somehow). Under the right conditions, fog is not increased.

Addition of gold with the sulfur adds another 1 - 3 stops, but decreases contrast and increases fog substantially. Therefore, it is imperative that you have the right salt content in the washed emulsion and the right restrainers added to the emulsion to retard fog.

The matrix emulsion is a good generic example of this. It is on Jim Browning's web site and also posted here.

The value Kirk mentions is good for one of my emulsions and was selected based on grain size. A coarser grained film emulsion might use as little as 15 mg/mole.


Martin Reed
11-21-2007, 04:01 AM
I don't have a record of the dilution or quantity, this was more than 10 years ago, all I remember is we did make a reasonable estimate, and the result was interesting enough to make nailing it down worth investigating further. If an off-the-peg product could be used successfully as a sensitiser it could get a few more emulsion makers up and running. Looks like it's back to the red light & the magnetic stirrer.....

Photo Engineer
11-21-2007, 09:06 AM
Well, one of the problems is that the sodium thiosulfate is unstable just like hypo and does not keep well. Therefore, it must be made up fresh ever few days and must be refrigerated when used in this context. See Jim Browning's note elsewhere on this.

Sodium thiocyanate can be used, but is far less effective because the decomposition rate of the sulfur salt is what causes the sensitivity to increase and hypo decomposes more readily than thiocyanate salts. Thiocyanate has not been used for ages.

You should use some of the lavish profits from your book (I laugh knowing how much authors actually make) and take my workshop.

Basically though, I suggest taking drops of the emulsion at different times during the sulfur-gold treatment (this is called "finishing" or "chemical sensitization" at Kodak) and placing them on a blotter. You then saturate the blotter with developer. The last drop in this time sequence that does not fog is the optimum time at that level of finish. Or, the last drop in a concentration sequence that does not fog at a given time is the optimum level of finish.


Kirk Keyes
11-21-2007, 01:12 PM
Martin -

Looking at the Formulary's "Nelson Gold Toner" formula, it has 611g anhydrous sodium thiosulfate and 0.5g gold chloride in about 4.2 liters. That works out to 1.844 moles/L sulphur (3.87 moles/L thiosulfate) and 0.00061 moles/L gold. That puts the ratio of gold to sulfur at about 1:3000 - way outside 1:3 that PE suggested above.

If Tetenal's formula is similar to the Nelson one, I'd suggest that all the speed gained was merely from the sulfur/thiosulfate and not any effect from any gold that happened to be present...

Looking at other fomulas, both Kodak GP-1 and GP-2 have a ratio of 1:50. Still a ways off, but they don't use thiosulfate - GP-1 uses sodium thiocyanate and GP-2 uses thiourea. I bet that thiourea in the GP-2 breaks down fairly fast to react with the silver and form sulfide like it does in toners, but it has toxicity issues and is anticipated to be a carcinogen so maybe not a choice most people would make to use.

But perhaps the slower decomposition rate of the thiocyanate would work to our advantage when used at the gold to sulfer ratio found in the GP-1 solution... But then you are back to mixing your own solutions and not using a commerially available one as you were hoping to do.

And for PE - you wrote, "This is usually 1:3 wt/wt but depends on the salts used." And would those salts be gold chloride (AuCl3*ClH*3H2O) and sodium thiosulfate, anhydrous?


PS - I'm going to mix up my first emulsion at home on this weekend!

Photo Engineer
11-21-2007, 02:09 PM

Good luck on the emulsion.

I use sodium thiosulfate pentahydrate as it is the most stable in weight and most stable to formation of sulfides on the shelf when dry.

The gold is Potassium Chloro Aurate. I think Jim Browning uses a different salt though so you might want to check his formula. Either will work.

The first sulfur sensitizer was allyl thiourea. This is the ingredient that is refined out of all gelatins today. It was even present in food gelatins, and AFAIK, is now removed from them.

I do not do gold sensitizations presently, as they are too difficult for the student to master during a short workshop and a lot more expensive both for the student and for me to develop. The ISO 40 emulsion I teach would be more like ISO 100 with gold, but would be lower in contrast and foggier unless I made other changes to the formulation to adjust for the gold.

You can see the ISO 40 result here, and in the workshop you attended we got a good ISO 80, but with low contrast and high fog. Much of the fog at the workshop was from everyone using red flashlights looking into the can as I carried out the wash though, and also from all of the 'safe' light used during the cutting operations.

In fact, to date the plates have looked best wrt fog level because they get much less exposure to light as they need no cutting step. A note of caution though is that plates need more curing time after they are dry, and they need to have a chrome alum hardening step somewhere in either the coating or processing operation.

Oh, more information.... Unwashed emulsions finish more slowly and have less fog, but over washed emulsions will fog rapidly when finished. The wash must be precise enough to keep just a slight salt excess. This keeps fog down. The pH must be acidic, and ammonia must NOT be present.


Kirk Keyes
11-21-2007, 03:18 PM
"Safe" lights - yes, I think for next year, (fingers crossed), I'll bring some of those red LED lights with standard screw in bases and replace those bug lights. I also have a little red LED flashlight that was in my pocket that we could have used at the workshop that might have been better than the filtered flashlight.

PE -You mentioned overwashing: My el'cheapo condictivity stick-style meter that I got new from eBay for about $30 arrived yesterday and I compared the readings/calibration to some actual conductivity standards. The readings were very bouncy at 1412 uS/cm so that made checking the single calibration point kind of diffucult, but it was rock solid and accurate when reading at 147 uS/cm and 75 uS/cm.

When talking about potassium chloride solutions, 147 uS/cm equates to 0.001 moles/L and 75 uS/cm is about 0.0005 moles/L, or about 75 and 38 mg/L of KCl respectively. My Portland tap water gave readings of around 30 uS/cm which is a very reasonable number for our water. If it can read down to 15, that should equate to about 7 mg/L of solids in solution. So it looks like the meter should be pretty good for low level readings.

Washing excess bromide and ammonia from the emulsion should give approximately similar conductivity readings as KCl solutions when figuring mg solids amounts. I'll take note of the wash readings for future reference.

Can one buffer the wash water for pH to get it to 6? Would that eliminate any issues with residual ammonia? Say even just a very, very, very dilute HCl or H2SO4 wash solution?

Martin - sorry for the divergence from your original question for a bit.

Photo Engineer
11-21-2007, 03:28 PM

Never ever use halide acids to adjust the pH of emulsions. Use only sulfuric or citric. Nitric is also a No-No!

I have to do some calculations to get the values right for you, but it would be about a vAg of 50 - 100 mv compared to KBr or NaBr. The ionic effects of K and Na would not be important at that concentration. I think vAg or pAg not in conductivity units and I can't convert, so I need to go back to Debye Huckle and Nernst to ask them their opinions. :D Bear with me.

Make sure that one electrode is silver though and have no permeable liquid junction. That is, use glass and metal only!


Kirk Keyes
11-21-2007, 06:49 PM
I need to go back to Debye Huckle and Nernst to ask them their opinions. :D Bear with me.

Please do! The last I saw those guys was back in college more than 20 years and they just never called since so I don't talk to them anymore...

Nowadays, I'm mostly a Beer and Lambert guy, and more lately, a Van Deemter disciple. Oh, how I long for more theoretical plates.

Photo Engineer
11-21-2007, 07:13 PM
I almost forgot this.

Here are the values you need. At 40C a 0.001 molar NaBr solution (0.1029 g/l) will give a vAg of about 67.5 mv. The pKsp is about 11.61, the pAg is about 8.595, and the pX (or pBr) is about 3.0152. Using KBr will offset this a bit due to the Activity Coefficient change and the Ionic Strength change. The answer will be very close though.

The Activity Coefficient of the NaBr solution is about 0.96 and the Junction Potential is about 0.04. This all assumes a water solution with no silver present. If you want that it will cost extra and take a lot longer.

This is what those three guys said. And, this is where you should be roughly at the end of a wash, at about 0.001 molar +/- a tad to be between 50 and 100 mv for optimum finish and keeping.

Does this help? Please say yes! I hate lots of math.


Kirk Keyes
11-24-2007, 02:32 PM
Thanks - that gives me something to shoot for.

Kirk Keyes
11-24-2007, 02:37 PM
So that looks like around the 100-150 uS/cm level I mentioned above - that will be easy to measure with the meter I bought.

Would there be any benefit from adding some KBr to the emulsion after the wash or we have enough there already provided we did not overwash?

(And it did it - I made an emulsion at home. I've got it in the fridge digesting right now. I'll pull it tomorrow and wash it.)

Photo Engineer
11-24-2007, 03:45 PM
Ahhh, the film emulsion? No, if you wash to rid it of the ammonia smell, that is about the right vAg value. At least it finishes well and keeps well for about a week or so. If you add KBr, do it after the finish. And remember that metal electrodes other than silver, and other salts released by porous electrodes affect the vAg and can change the emulsion.