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"Pewter oxidizer mishap: is there a chemist in the house?" Topic


6 Posts

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1,844 hits since 27 Nov 2018
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Comments or corrections?

MikeMonaco27 Nov 2018 12:50 p.m. PST

So I've tried a pewter oxidizer which I believe is a solution of HCl and Copper sulfate (it's a knockoff of JAX Pewter Black, or possibly just the same product relabeled, by a seller of miniature molds in the US).
Anyway I've found that about half of my castings don't completely blacken, but actually end up plated in a thin coating copper. This is usually just over part of the model, not the whole tihng. I'm not adding anything to the solution and certainly not running a charge in it. This happens whether I dip the minis or just brush it on. What could be happening and how can I stop it?
Additional background: I'm using 50/50 and 60/40 lead/tin solders and graphite or talc as the mold release release agent.
TIA

Winston Smith27 Nov 2018 2:42 p.m. PST

Why are you oxidizing pewter?

DyeHard27 Nov 2018 4:13 p.m. PST

By chemical means, if you Oxidize one thing you must reduce another.

It has been many years but:

If you start with Lead (Pb0) and tin (Sn0) and copper sulfate (Cu+2) you will oxidize Pb+2 and Sn+2 and Cu0 (metallic copper)

These things can be looked up with the term electro chemical half reactions. Or electo-negativity:
link

This is a randomly found explanation of terms and such:
PDF link

or this:
link

Nice Wiki page:
link

Cu2+ + 2 e− ⇌ Cu(s) +0.337
Pb2+ + 2 e− ⇌ Pb(s) −0.126
Sn2+ + 2 e− ⇌ Sn(s) −0.13

The more positive the voltage the more the reaction goes as written. The story if you expose a lead tin mix to copper (+2) you will end up with metallic copper and oxidized lead and tin.

Thresher0127 Nov 2018 10:07 p.m. PST

So, for those basing metal minis on pennies, and/or other copper coins, will their feet/ankles rot off in time?

If so, and it sounds like that will happen, how quickly will it occur?

DyeHard28 Nov 2018 9:32 a.m. PST

If you electrically bond a tin or lead object to a copper coin with oxidized copper, then you can expect the copper to reduce back to the metal and the tin and or lead to oxidize. But only to the extent of the original quality of oxidized copper (a limiting reagent).
I would suggest this is not really a problem.

picture

Tin is Sn, lead is Pb, Copper is Cu

Now the lead-rot, miniature-rot, coin-rot effect is a bit different. For very high lead content items, a reaction with acetate ion can occur that is catalytic (just keeps on going). This can happen from gases given off from wood or wood product.

Some info:
link

TMP link

link

TMP link

TPPnotts15 Dec 2018 2:21 a.m. PST

Try using a pickle agent "for pewter" before applying the patina . And maybe try a different product, novacan black works well.
The reaction will only work well on good clean non oxadised metal.

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