Can Copper Patina be Applied to Lead-Free Solder in Stained Glass Work?

[CWatters]

I've just starting out doing some stained glass work which involves applying copper tape to the edges of glass then soft soldering the parts together.

You can buy a Copper Patina solution that is intended to be brushed onto the solder to give it a nice shiny copper patina. However reports on various stain glass forums say this can be problematic. Some people say they get a black/grey or mottled coating instead of a nice uniform copper finish.

Most of the solutions I've found suggest the importance of cleaning the solder but I'd like to get a rough idea of the likely reaction to see if the type of solder also makes a difference.

The patina I have says it contains "Water, Selenium Compound, Nitric Acid and Copper Sulphate"

There are two main types of solder:

1) Lead Based Solder contains a mix of Tin and Lead in the ratio 60:40 (although other ratios are available).
2) Lead Free Solder contains mostly Tin (95%), silver (3-4%) and copper (<1%)

So presumably the Nitric Acid is used as a cleaner and the Copper Sulphate is the main active ingredient.

Some forum posts suggests you can't or can't reliably apply copper patina to Lead Free solder. That suggests the reaction is between the lead and sulphur leaving the copper behind? Does that sound right?

In which case would it make sense to try a solder with a higher lead content such as 40:60 instead of 60:40?

Thanks.

Edit: I found a post that suggests copper sulphate will react with tin as well as lead so perhaps the ratio doesn't matter?

 

[256bits]

https://en.wikipedia.org/wiki/Tin(II)_sulfate
a displacement reaction between metallic tin and copper(II) sulfate:[4]
Sn (s) + CuSO4 (aq) → Cu (s) + SnSO4 (aq)
Lead, zinc should do a similar metal displacement.
Try an iron ( steel ) member to see if that form a copper surface, just for fun.

Though I don't know the concentrations of the aqueous solutions for all those reactions to be carried out.
Your patina should be somewhat dilute I would presume blue.

In preparation of the patina solution,
It could be that the nitric acid was used to dissolve copper, and sulfuric acid was added to change the 'species' so the metallic displacement would occur when applied to another metal. And they call it nitric acid, copper sulfate solution - just guessing.

see here also,
http://dwb5.unl.edu/CHEM/SmallScale/SmallScale-016.html
could explain the black color for some applications.

Not a chemist, just trying to give you some avenues for exploration.

 

[Borek]

Definitely displacement is what drives the deposition. Tin and lead have very similar redox potential for the X → X²⁺ oxidation, so they should both behave similarly.

Trick is, devil is often in details. Basic ideas are easily covered based on GenChem101 but then at some point it becomes just a black magic. Minute amounts of other elements can change the behavior of the surface and make the deposition difficult. If something goes wrong I would try solder from a completely different source.

 

[CWatters]

Thanks folks.