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Techniques for CollectorsCleaning Zincite

30th Jul 2019 20:24 BSTDavid Dantes

I have a Polish zincite specimen which has collected a dull, white patina, due to humidity in Hawaii. A zincite expert in New Jersey opined that the patina is hydroxyzincite. Cleaning with regular vinegar was of negligible benefit, so I'd like to know if it would be safe to use 30% vinegar.

31st Jul 2019 23:58 BSTKeith Compton Manager

While not addressing cleaning, this former thread may be of interest:

I would try a small section/piece with iron out.

1st Aug 2019 00:21 BSTDavid Dantes

Thank you!

1st Aug 2019 01:28 BSTSteve Hardinger Expert

I would not expect Iron Out to do anything because (probably) there was no change in the zinc oxidation state has occurred as the patina formed. If the patina is some sort of zinc oxide or zinc hydroxide, I would try (on two separate scarp pieces) mild acid (dilute HCl) and mild alkali (dilute NaOH). Either or both might work because these might be amphoteric(?).

Regardless, let us known how it turns out.

1st Aug 2019 01:57 BSTDavid Dantes

Thank you Steve. I have some 30% vinegar (household vinegar is 5%). Is it reasonable to test that? Household vinegar did not make a noticeable difference.

1st Aug 2019 17:36 BSTSteve Hardinger Expert

When performing tests like this, I don't rely only on my senses. I weight (two decimal places) before and after treatment, and (in many cases) also photograph before and after as well. I don't trust my eyes or brain to differentiate or remember small changes in luster. The weight change has surprised me in some cases where I saw to obvious visual change before vs after but there was enough weight loss to cause concern about using a certain treatment especially for a prolonged period.

Assuming 'Polish smokestack' zincite has the same chemical composition as natural zincite (ZnO) my reference materials say it will 'be soluble' (amount unspecified) in acids and alkalis (which acids or alkalis, and what concentration and temperature, unspecified).

Also, I don't understand your reference to 'hydroxyzincite'. Perhaps you mean hydrozincite? If so removing the patina without dissolving the zincite itself could be difficult.

And if if you can find a procedure to remove the patina and leave the zincite, the resultant surface might be dull and rough.

So maybe this is a case where no cleaning should be performed, but instead, curation efforts made to stop further damage.

1st Aug 2019 17:54 BSTDavid Dantes

Thank you Steve. I did mean to say hyrdrozincite -- thank you for noticing that. I found some photos of zincite, online, which illustrate the issue. At if you look at crystals near the center of the first photo, whitish speckles are seen on some facets. When these become confluent, they dull the surface. I certainly agree that doing no harm is the first priority.

1st Aug 2019 21:12 BSTSteve Hardinger Expert

It's also possible that zincite will always absorb CO2 and water from the air, even after cleaned, unless it is covered with a protective layer such as varnish. If this is so, your choices are (a) Accept the patina as inevitable; (b) Remove the patina and start over every now and then, or (c) Store the specimen in a sealed, inert atmosphere (such as nitrogen or better yet argon).

1st Aug 2019 22:13 BSTDavid Dantes

Yes. If infrequent, periodic maintenance was required, that might be acceptable. Otherwise, I'd opt to accept the patina. It's still beautiful.

2nd Aug 2019 00:56 BSTKyle Bayliff

I do not know how reactive or soluble ZnO may be with acids, but hydrozincite is almost certainly more reactive. Therefore with the right concentration of the right acid you should be able to preferentially remove the carbonate coating. Start with a low concentration (as you did with the vinegar) and gradually work your way higher until you're satisfied that it is working and not damaging the crystal underneath too much (working out the right dilution with a test piece rather than your display specimen would be ideal.). To be safe, I would make a few intermediate dilutions instead of jumping from 5% to 30%. If weak acids don't work at high concentrations, start from a dilute solution of a stronger acid such as HCl and work your way up again.

2nd Aug 2019 01:15 BSTDavid Dantes

Thank you for your explanation and recommendation.

2nd Aug 2019 05:50 BSTSteve Hardinger Expert

Why do you say that hydrozincite is almost certainly more reactive? Carbonates are not always more reactive than oxides.

2nd Aug 2019 21:45 BSTThomas Lühr Expert

>Carbonates are not always more reactive than oxides.

This is true. For example CaO is more reactive than CaCO3.

But from intuition i tend to agree with Kyle and would give the same advice.

Also, hydrozincite is not a 'pure' carbonate but also contains the OH-group which causes usualy an easier solubility.

Finally, a crust/patina is often less dense/compact than the underlaying crystals.

An agent that solves haydrozincite will most probably also solve zincite, but probably not that fast.
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