Author Topic: Zinc pest or oxidation?  (Read 3853 times)

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Offline Figleaf

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Zinc pest or oxidation?
« on: November 13, 2014, 12:02:50 PM »
These arrived among others in a small lot of Danish coins kept in a small plastic bag since the fifties or so. There was crumbly dust all over them. Some had the kind of growth on them that schoolboys use to yuck out their female classmates. They look worse in hand than on the scan. If it's oxidation, I suppose I can just wipe it off. What should I do if it's zinc pest? Can it be neutralised?

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline Arminius

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Re: Zinc pest or oxidation?
« Reply #1 on: November 13, 2014, 01:27:13 PM »
I think (and hope) it´s just oxidation as Danish zinc of those days should be pure enough. Zinc pest is caused by too much lead impurity.

According http://en.wikipedia.org/wiki/Zinc_pest: The irreversible process (of zinc pest) will eventually destroy the object.

Try to brush the dry oxides off with a soft brass brush til you get a better looking surface. But stop before you excavate the corrosion pits too much.
Then treat the surface with some alkaline wax or oil afterwards (e.g. weapons oil or machine oils - but NO olive oil or other oils from plants with unknown acid and water content).

regards

Offline Figleaf

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Re: Zinc pest or oxidation?
« Reply #2 on: November 13, 2014, 05:31:34 PM »
I should have thought of Wikipedia. Thanks Arminius. Your link led me to the answer to my question: wet storage stain. I guess the plastic bag was not closed and condensation formed inside.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline Figleaf

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Re: Zinc pest or oxidation?
« Reply #3 on: May 13, 2015, 11:51:37 AM »
The story has a sad end. I could not clean the coins satisfactorily. Either much of the white stuff remained or the coin got severely damaged. I had to throw out all but a few unaffected coins. The moral of the story is clear. Do not keep your alu coins in a plastic bag or on top of each other and beware of humidity and condensation.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline bruce61813

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Re: Zinc pest or oxidation?
« Reply #4 on: May 14, 2015, 05:33:04 AM »
Since I have not tried to work with this problem, this is only a recommendation. I would treat it as I would Bronze Disease [technically BD should be termed copper disease, but that is another story].  Try soaking the coins in a mix of washing soda and baking soda, mix them 1:1.  A teaspoon of each in a pint of water is fine.  You may want to use a nylon brush , like a toothbrush, to scrub the surface.  Allow this to soak for a few days, hopefully surface acids will be neutralized.

 Bruce

Offline Figleaf

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Re: Zinc pest or oxidation?
« Reply #5 on: May 14, 2015, 10:40:31 AM »
I live in France. You can buy baking soda in tiny portions - they are meant for people making their own cakes. You cannot find washing soda. Big supermarkets rule supply.

Peter
« Last Edit: May 14, 2015, 03:28:43 PM by Figleaf »
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline malj1

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Re: Zinc pest or oxidation?
« Reply #6 on: May 14, 2015, 12:03:22 PM »
Look for bath salts that is very similar to washing soda with a bit of perfume added.
Malcolm
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Offline Globetrotter

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Re: Zinc pest or oxidation?
« Reply #7 on: May 14, 2015, 06:05:32 PM »
I have an ultrasound cleaner, like the jewelers use, which works quite well on that type of problems. On Amazon.fr you find such a thing for around 35.00 Euros and the cleaning liquid, which I think is mostly soap, is around 7.50.

Mine was a xmas present some years ago from my wife, and it works fairly well, just don't expect wonders, but it's really worth the investment.

Ole
Ole

If you're interested in coin variants please find some English documentation here:
https://sites.google.com/site/coinvarietiescollection/home
and in French on Michel's site (the presentations are not the same):
http://monnaiesetvarietes.esy.es/

Offline bruce61813

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Re: Zinc pest or oxidation?
« Reply #8 on: May 15, 2015, 11:02:28 PM »
Washing soda , sodium carbonate, is also used for swimming pools to raise the pH level, you might check those sources. You really don't need a lot of of either chemical, so a small box of baking soda goes a long way.  17 grams of a 50/50 mix in 350 ml of water is fine.  I can send some mix, but it is the same price for up to 20 pounds shipping weight, or you can order Gringgotts #1 or 2 from Noble Roman Coins  [Kevin Sandes]. These are backed in 8 ounce bags.

Bruce