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Diy black hexavalant chromate recipe for making watch dials

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chrome72:
I am a watch maker and want to replicate an old method that the likes of rolex used.

They would plate brass with zinc then apply a hexavalant black chromate to turn the zinc black. Over time some of these dials when exposed to UV turned various shades of brown.

My understanding is the chromate contained silver which is photosensitive.

I want to make some watches that mimic this and could either be black or over time turn brown with UV exposure. I know I could tinker with the silver content in a black chromate solution.

I was looking through old parents and came across a recipe for hexavalent chromate :

50 grams chromic acid and 3 ml sulfuric acid per 1 liter of water.  I would then add silver ions to the mixture in varying %s.

Is making my own black chromate as simple as buying the ingredient and simply mixing them?  I fully understand that hexavalent chromate is hazardous and would be limiting exposure and having appropriate disposal.

For silver ions would silver nitrate be a good option or finding “silver ions” from a lab supply a better option ?

Thanks for any advice!

Borek:

--- Quote from: chrome72 on June 28, 2024, 08:09:52 PM ---apply a hexavalant black chromate
--- End quote ---

Hexavalent chromate compounds - at least these soluble - are yellow.


--- Quote ---My understanding is the chromate contained silver
--- End quote ---

Chromate per se doesn't contain silver. Perhaps what you means is silver chromate. If you add silver salt (like silver nitrate) to sodium chromate solution (technically any soluble chromate will do, just sodium and potassium are most common in the lab practice), silver chromate will precipitate. It not black though, more like red-brown.


--- Quote ---which is photosensitive.
--- End quote ---

Silver is not photosensitive. Some silver compounds are (halogens and pseudohalogens for sure, no idea about chromate).


--- Quote ---50 grams chromic acid and 3 ml sulfuric acid per 1 liter of water.
--- End quote ---

That's probably more like a recipe for dichromate solution - chromate converts to dichromate in low pH, and adding sulfuric acid makes the solution acidic. Dichromate solutions are orange (compared to the yellow chromate).


--- Quote ---Is making my own black chromate as simple as buying the ingredient and simply mixing them?
--- End quote ---

Assuming "black chromate" is just a slurry of silver chromate - mostly yes. It won't be easy though, you need to control three parameters - pH (if too low chromate converts to dichromate) and concentrations of chromate and silver. A bit like guessing PIN to the dial padlock found lying on the ground ;)


--- Quote ---For silver ions would silver nitrate be a good option or finding “silver ions” from a lab supply a better option ?
--- End quote ---

No such thing as "silver ions" separated from counterions, you can only make/buy salts. Yes, silver nitrate is the most common and the most useful source of silver in inorganic synthesis.

Corribus:

--- Quote from: Borek on June 29, 2024, 03:55:21 AM ---
--- Quote ---which is photosensitive.
--- End quote ---

Silver is not photosensitive. Some silver compounds are (halogens and pseudohalogens for sure, no idea about chromate).

--- End quote ---

Complex topic. Silver ions are light sensitive and photoreduction is a route to make silver nanoparticles. Generally a sensitizer is needed to make it efficient enough for synthesis.

See, e.g., this paper that used light to generate metallic silver particles from Ag chromate.

Harada et al. Langmuir 2010, 26, 23, 17896–17905 https://pubs.acs.org/doi/10.1021/la102705h

Silver is a complicated metal :D

chrome72:
Got any advice then how I should proceed with this then?

Corribus:
I am no expert on metal plating, sorry.

But silver chromate seems fairly simple to make provided you are willing to hazard working with chromate solutions.

I found this report from 1927(!): https://pubs.acs.org/doi/10.1021/j150276a012

The color is described as brick red and/or lustrous depending on how the reactants were added. I guess you can find more modern procedures.

Ag chromate is a semiconductor, like AgCl, so I guess it will have some photoactivity. Probably shorter wavelength (UV) light will be better, but visible light seems to be effective as well.

https://www.tandfonline.com/doi/full/10.1080/17458080.2015.1110624

Couldn't say what the result will be in your application.

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