[Eroica]

The father of mineralogy Georgius Agricola mentions a reaction in his De Natura Fossilium that gives silver a brilliant whiteness:

... the silver object is heated in a charcoal fire, and quenched in water in which argol and salt have been boiled in order to make it white - Georgius Agricola, De Natura Fossilium, Book IX

Argol is potassium tartrate; salt, I presume, is ordinary sodium chloride.

Could someone please explain the chemistry of this process? What is produced when argol and salt are boiled together in water? How does this react with silver, if at all? Is the charcoal a vital ingredient, or would it be sufficient to simply heat the silver?

Thanks.

[renge ishyo]

My best guess would be that silver chloride is formed in the process since it is a white solid that precipitates readily out of aqueous solutions. I would have more confidence in this if Agricola mentioned afterwards that the white solid quickly darkens to a violet color upon exposure to light. Yes, I believe that the charcoal in this example is simply being used as the source of heat and can be replaced by another heat source not available in Agricola's time. The confusing part is the presence of potassium tartrate. Potassium tartrate is an acid and a saturated solution of this acid can be used to create a buffer solution at a low pH. Perhaps having a low PH helps the reaction go forward by keeping Ag⁺ in solution, but other than that I have no idea why it would be in there.

[Eroica]

Thanks for the reply, renge. I don't think the white coating can be silver chloride if that is not stable in light. Galileo (in his Dialogue Concerning the Two Chief World Systems, First Day) mentions silver vessels whitened by boiling them in a solution of argol and salt, which is basically the same reaction, as though the purpose is to produce permanently whitened vessels.

[renge ishyo]

I looked into this further, and I think I have a better explanation than I had before. What threw me off was considering the reaction with NaCl present....chloride would not be desirable in this reaction because it would likely form AgCl and precipitate out of solution. If a salt such as sodium nitrate is added then the reaction makes more sense because silver nitrate won't precipitate and compete with the reaction in question.

The reaction forming the white precipitate appears to be the following:

K(Na)C₄H₄O₆ + 2Ag⁺ + 2NO₃^-  Ag₂C₄H₄O₆ + K⁺ + K⁺ + 2NO₃^-

The ions are included for clarity. Ag₂C₄H₄O₆ is indeed a semi-permanent white substrate.

Hope this helped.

[Eroica]

Thank you, again, renge. Agricola just says "salt", so he could mean sodium nitrate.