Gold toner going black

I am dipping my toes into Kallitype printing. Very interesting. Would like to try gold toning the prints, and based on a description I read on this forum (from @revdoc posted January 23) I settled on Clerc´s formula, and mixed up the Na-thiourea, tartaric acid and NaCl in demineralised water. Used 20 ml of this fluid, and added 20 drops 1% gold chloride. The mixture turned black. The black stuff turned to particulates when I poured it on the print. What is going on? The salt is sea salt (maldon sea salt). It states on the box that it is pure NaCl (no additives).
I found a gold toner recipe on the unblinking eye site, from Sandy King, that doesn't contain salt. For the chemists - what is the purpose of the salt in the original recipe?
Feedback is very much appreciated.

Torleif

I cannot comment on your specific problem, but from a lot of cooking experience I can comment on Maldon (and other) salt. "Pure MaCL" means no additives. It does not mean 100% or chemically pure. For culinary salt, "pure" generally means tha that there is no iodine or anti-caking ingredients addded. So it's natural and closer to chemical purity. Maldon is great salt and is made by precipiatating seawater; it has trace minerals in it. I would not consider it the best option of use in chemistry. Here is an analysis of Maldon salt, which I find a lot of reading but quite fascinating:



If you can use it for photographic chemistry, that's great. If not, use it generously as a finishing salt... sprinkled on top of your favorite meal. Maldon is particularly good as a topping on meat.

Hi Brian, thank you - I clearly see that I have to find another source of salt. Based on the link I see that Maldon salt is a mixed breed - I fully agree that it is a nice topping in the culinary sector

Here is another article worth reading. If I were using culinary salt for chemistry, I'd consider using plain (non-idodized) table salt. Not sure what brands are available to you; in the US the most common brand is Morton, which is said to be more chemically pure than all others. https://www.mortonsalt.com/home-product/morton-table-salt/ Table salt is highly refined. I think 99% pure is an oft-cited claim but I couldn't verify on the packaging because even the non-iodized version has anti-caking ingredients. Maybe there's another on that our markets don't stock.

BrianShaw said:

. Not sure what brands are available to you; in the US the most common brand is Morton, which is said to be more chemically pure than all others. https://www.mortonsalt.com/home-product/morton-table-salt/

Click to expand...

Probably Mortensen's in Norway

pentaxuser

I generally just use culinary salt for purposes like these and I don't think that the trace amount of sodium iodide will affect this toner formulation any. Nor do I expect problems from the even smaller amount of ferrocyanide or ferricyanide anti-caking agent.

torleif said:

The mixture turned black. The black stuff turned to particulates when I poured it on the print

Click to expand...

That's really not supposed to happen. Can you show some clear pictures of the ingredients, especially the thiourea and the gold chloride, as well as a receipt or label of both that shows what it is? I suspect you may have something other than what you think you have.

torleif said:

... mixed up the Na-thiourea, tartaric acid and NaCl in demineralised water...

Click to expand...

"Na-thiourea"? I would guess that's a sodium salt of thiourea. Any chemists reading this?

You need just plain thiourea. I think if you are using a sodium salt, the resulting pH will bee too high and it won't work as a toner. You'll probably also get colloidal gold precipitating out, which might be the black solid you saw.

Hi all, thank you for the feedback. I am in Europe and the chemicals have CAS numbers. They are: Natriumtiosulfat, or sodium thiosulfat (CAS # 7772-98-7)- Na₂S₂O₃, Tartaric acid (CAS # 87-69-4) - C4H6O6 and Gold Chloride (CAS #16903-35-8) - HAuCl4, in 1%solution.

The first one, sodium thiosulfate, is causing the issue. You need thiourea, CAS 62-56-6.

Just buy actual sodium chloride from a chemical supplier for photographic purposes. It's cheap enough. Second guessing this or that evaporated culinary salt is always a potential risk, even if there aren't conspicuous additives like with ordinary Morton table salt, which not only contains iodine, but also a fair amount of titanium dioxide whitener. But my advice is probably elliptical in this particular case. Others have already spelled out the more probable issue.

You might want to fact-check yourself on titanium dioxide in Morton table salt…

See page 11 and continuing list:

Other Substances During the production process of salt, Morton Salt, Inc. and Windsor Salt Ltd. do not use or intentionally add any of the following substances:
• Titanium Dioxide

Click to expand...

It's right there fine print on our container here. That's been the case for as long as I can remember. Otherwise, it wouldn't even look pure white. Same goes for most toothpaste. Titanium dioxide as a whitener is in all kinds of things, including white paint, although most house paints contain cheaper less effective substitutes like silica "sand" (industry jargon) or calcium carbonate, which weathers poorly.

Fact checking? I've done way more than my own fair amount of homework. But that's another story. I do practice what I preach, and you'll only find pure sodium chloride on my darkroom shelf, not any kind of table salt. People unknowingly eat and drink a lot of sodium sulfite too, unless they're allergic to it, and then they sure know fast!

As an electrical engineer, chemistry is in the black arts, and my age is probably compromising my memory. Off course the recipe states thiourea (CAS 62-56-6), and I have used hypo. Sorry - my mistake. Thank you for the guidance. Could I ask what the purpose of NaCl is in Clerc´s formula - since Sandy King has stated one without salt on the Unblinking Eye web site.

No need to apologise. I'm just gad you're back on the right track.

Regarding the salt, it's a bit of a mystery. I haven't found an explanation. Clerc himself doesn't explain it anywhere. However, it shows up in old formulae for platinum and palladium toners as well, so it its purpose is probably not specific to Clerc's toner.

I think it might function as a stabiliser or preservative for the metal salt. The only evidence for this is a comment Clerc made in La Photographie Practique, 1902:

"The exceptional stability of this bath, which is unbreakable by the reducing agents that precipitate the gold in its usual combinations, is invaluable because it ensures that gold is deposited on the prints only where they contain silver..."

So he promotes its stability of the toner. Could be due to the salt... or not. The text is here and the quote above is on page 254:

"Gold toner was the most widely used toning bath. A wide range of tonalities, from sepia to purplish black, could be obtained with a gold toning bath. The action of gold chloride on the silver image can be expressed by the following equation: Ag + Au Cl → Ag Cl + Au. The size and, thereafter, the color of finely divided gold depends very much upon the rate at which gold is deposited, that is, the speed of toning. This rate depends on the alkalinity of the bath. In an alkaline medium, gold (III) chloride (AuCl3) changes progressively into gold (I) chloride (AuCl), which deposits three times as much gold per unit amount of silver as the auric salt (Clerc 1937, 350). However, if the bath is too alkaline, the gold (I) salts change to the inactive aurite state, and the toning does not take place (Clerc 1937, 350). Neutral and slightly alkaline baths tone very quickly because the reduction takes place rapidly. The gold particles appear blue because they are somewhat large. In an acidic bath, reduction takes place very slowly. The gold is deposited in a dispersed form and reddish images are produced because of the extremely fine state of division of the gold."

Sodium Chloride provides the neutral bath needed by the toner for maxium efficiency and blue tone.

Yeah, thiosulfate is a reducing agent and most likely it caused gold chloride to instantly precipitate gold particles.

Excess chloride probably keeps the gold chloride from spontaneous decomposition to metallic gold (shifting equilibrium to the left) thereby extending the shelf life of the toner. It might also help in completing the toning action by more readily providing chloride to Ag to form AgCl that precipitates out (similar mechanism to adding KBr to a re-halogenating bleach.)

I guess it is complicated....

:Niranjan.