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Difficulty detarnishing lead-based pewter

August 23, 2008

I am a metal refinisher and plater operating in Indianapolis. I frequently encounter old, lead-based pewter or pot metal antique items which are shiny black (nearly jet black). The only process I have found which removes this very hard, thick tarnish layer is either laborious sanding, or buffing with a medium-coarse steel wire brush wheel. The latter causes ugly grooves in the finish which are unacceptable. Amazingly, a brass wire buffing wheel or nylon-impregnated abrasive wheel will not touch this hard black tarnish. Brown tripoli buffing compound ⇦this on eBay or Amazon [affil links] on a spiral-sewn wheel will completely remove it, but only after causing heating of the work and resulting metal flow, leaving an unsightly indentation with heaped-up rim similar to a golf course bunker.

I have tried many chemical treatments which totally do not work: 10% potassium cyanide, hydrochloric acid, hydrofluoric acid, ammonium oxalate in strong ammonia ⇦this on eBay or Amazon [affil links] water, and strong lye water. I have also tried work-anodic electrostripping in 10% HCL, which really destroys the pewter surface.

HELP! Any responses will be warmly appreciated. The only thing I can think of which I've not tried is sand/bead blasting.

Robert Nelson
Owner, metal refinishing business - Indianapolis, Indiana, USA

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Try electrocleaning in 5% sodium carbonate ⇦this on eBay or Amazon] (object=cathode!).
If alloy contains to much lead try 5% ammonium acetate (immerse object in solution)! Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia
September 5, 2008

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February 7, 2009

I am very grateful to Goran Budija for his advice.

Electropolishing in 5% sodium carbonate with the work cathodic (6 volts, 2 - 16 amps/sq.ft) had excellent results on some pieces but minimal effect on others. Temporarily reversing the polarity, making the work anodic for periods of 30 seconds, improved the results.

However, the second recommendation by Goran is an absolute winner! Soaking any piece of horribly blackened pewter (most=probably lead-based) in 5% ammonium acetate for 24 - 48 hours, followed by very easy removal of black sediment with a brass platers' scratch brush, in **every** instance restored the pewter to shiny base metal.

This recipe by Goran Budija is priceless to anyone who works with pewter or white metal containing lead.

For the ammonium acetate, I made my own from ordinary vinegar ⇦in bulk on eBay or Amazon [affil links] with 14 fluid ounces of the Ace ammonia, then add water to make one gallon (128 fluid ounces). You will have a 5% solution of ammonium acetate. I checked the pH after mixing, and it varies between 6.4 and 7.2.

This 5% ammonium acetate solution does not last indefinitely. With continued use, it will eventually fail to remove pewter tarnish (becomes slightly hazy also). But it is readily made with ordinary household chemicals which are inexpensive.

It probably works by converting insoluble lead oxides to soluble lead acetate and lead subacetate at neutral pH.

My hat is off to Goran Budija from Zagreb, Croatia!

Robert L. Nelson, Maryland
- Carmel, Indiana, USA