Letter 37098

How far do I have to oxidate the silver layer?  

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Hello,

We have a small plant here, putting a layer of silver on decorative parts (on Ni after Cu acid, Cu alc on zamac pieces).

We have a liquid oxidant to oxidize the silver (don´t know the substances).
In the beginning we used 50% solucion and the pieces turned dark, almost black.

We diluted the solucion and now the oxidant is at 15 vol % in water. The oxidation process is slower. Problem is I don't know where to stop it to have fine pieces at the end. We immerse the wet (just washed) silver pieces and get the following process:

- First (after some sec.) the pieces turn like yellow-gold.
- Then (idem) they get a bright blue colour, but it is still not a uniform colour.
- Then (idem) they turn a bit red, gray.
- Finally (idem) they get a dark blue and uniform colour.

I wonder at which colour I need to stop the oxidation process. Now we stop at the end but it seems to me we are losing some of the bright silver refexion (compared to other pieces from other suppliers).

(Silver bath is new and checked by provider.)
(there is a post treatment, so the pieces won't oxidate further.)

Thanks,

Marc
- Mexico

Since there are dozens of ways to oxidize silver, it would be helpful if you could tell us which one you are using. But, in general, simple dilution will not usually give you the desired finish. There is usually an optimum concentration for a particular antiqueing or aging, or oxidizing solution, and there is often more than one component so dilution will throw off the reaction in ways you won't be able to anticipate. For instance, if the original solution contains 10 g/l sodium chloride, and 50 g/l of another chemical, diluting to 50% may put the chloride ion at a level where it does nothing, rather than the effect desired at 10 g/l.

I would try other oxidizing solutions, proprietary or not, and follow the directions on the technical bulletin.

Tom Pullizzi
- Levittown, PA

What we do here is dipping the pieces that come out of the Ag-bath (and after washing them) in a water (not distiled) solution with 10 to 30 vol% of the oxidizing liquid. We do not have the tecnical bulletin of that oxidant and that is indeed a pitty (don't ask me why).

You wrote : there are a dozen of ways. Maybe we need to try another method here. Can you give us more information about the diferent ways that exist to oxidize silver? How can I obtain more information about that topic?

I want the silver pieces to have a slight blue, more or less bright oxidation layer.

Thanks,

Marc
- Mexico

Since your "silver pieces" are plated with silver, the silver blackening immersion time you would like know is related to your silver plated thickness, its thickness is equivalent to the plating time.
In order to keep the brightness of the silver pieces after blackening, I think that by increasing a little more the plated silver thickness and using undiluted blackening solution will help. Heat up the blackening solution at 60 *C or keep the blackening time as short as possible, you will have a brighter surface with a thicker silver plated layer.

Yung-Han Kuo
- Omnoi, Samutsakorn, Thailand