Cleaning bronze after olive oil dip

[VickySilver]

Wow, not sure if I like the responses. Olive oil is acidic, and can not say if this property deteriorates with time but know that it will continue to react with coin surface - I have had some strip to an unnatural color (20th C. pennies & farthings). I have seen some go to a pinkish colour after some while, stripping down to the copper base.

Maybe depends on the oil...I remember living on Cyprus and visiting Crete where they had some of the mildest and finest olive oils - I wonder if they were as acid as the base Italian stuff?

I remove excess oil with dish detergent and LOTS of water, tamp & not rub dry with a high nap CLEAN white towel.

PS - A gentleman in the States, Weimar White wrote I believe two books on coin chemistry and will have a look around and see if I can find at least one title.

PPS - Once when I was brave I used a celly charge cord as the power behind an electolyte source to reduce the oxidation at coin surface; believe it or not I found out by this method that one 1936 penny was actually a proof!

Edited January 26, 2013 by VickySilver

[Coinery]

I guess, there is also another factor, and that is...

What does the acid component of Olive Oil become once any micro-smear on the coin has reacted with the coin's surface, and thus attained a new chemical property?

I'm tending towards more towards NO oil for Big-Lustred coins, maybe only using it to 'cheer up' already toned coins, though the jury is still out for me on that one. Although, if VS has reported changes in Olive-Oiled 20th century bronzes, then maybe there's nothing left to debate on the matter!

[Peckris]

I guess, there is also another factor, and that is...

What does the acid component of Olive Oil become once any micro-smear on the coin has reacted with the coin's surface, and thus attained a new chemical property?

I'm tending towards more towards NO oil for Big-Lustred coins, maybe only using it to 'cheer up' already toned coins, though the jury is still out for me on that one. Although, if VS has reported changes in Olive-Oiled 20th century bronzes, then maybe there's nothing left to debate on the matter!

The acid is oleic acid, and very mild. It doesn't combine with other acids, which is why oil and vinegar always separate (or maybe the acid is bonded into the oil in such a way that while an oil, it doesn't behave like an acid?)

[Coinery]

I guess, there is also another factor, and that is...

What does the acid component of Olive Oil become once any micro-smear on the coin has reacted with the coin's surface, and thus attained a new chemical property?

I'm tending towards more towards NO oil for Big-Lustred coins, maybe only using it to 'cheer up' already toned coins, though the jury is still out for me on that one. Although, if VS has reported changes in Olive-Oiled 20th century bronzes, then maybe there's nothing left to debate on the matter!

The acid is oleic acid, and very mild. It doesn't combine with other acids, which is why oil and vinegar always separate (or maybe the acid is bonded into the oil in such a way that while an oil, it doesn't behave like an acid?)

So, perhaps it gets a chance to work as an acid, at the point when the oil goes off and starts to break down into its more basic chemical forms?

[VickySilver]

Uhhh, Peck I think my Organic chem Prof would have been horrified at that explanation. And since its been 35 years I've forgotten some of the "bits and pieces".

However, I do remember polar and non-polar & hydrophilic/-phobic, etc. Olive oil is a complex mixture of substances with most of them being hydrophobic (archaeic terminology to be sure), lipophilic substance properties that even with shifts in polarity tendency such as with the oleic acid would not be so readily miscible in water & not particularly that the acid would not be "friendly" to other acids (i.e. vinegar)....

OK, sorry for some sorry pseudo-science....

Anyway, IMO, I would not like to keep a mint red penny of lated milled vintage to bathe in olive oil for any long period of time...

In a related vein, I recall reading somewhere about changes in acetone and its degradation after longer exposure to copper surfaces. As far as it goes I believe I have seen

some slight colour changes with copper exposure to acetone for even shorter periods.

I am quite leery of some of those substances occasionally available such as "Blue Ribbon" or the more worrisome "MS70". One of them causes copper to turn blue and

I have seen one of the 1860 proof copper farthings with this bluish lustre! Yikes!!

[Rob]

Uhhh, Peck I think my Organic chem Prof would have been horrified at that explanation. And since its been 35 years I've forgotten some of the "bits and pieces".

However, I do remember polar and non-polar & hydrophilic/-phobic, etc. Olive oil is a complex mixture of substances with most of them being hydrophobic (archaeic terminology to be sure), lipophilic substance properties that even with shifts in polarity tendency such as with the oleic acid would not be so readily miscible in water & not particularly that the acid would not be "friendly" to other acids (i.e. vinegar)....

OK, sorry for some sorry pseudo-science....

Anyway, IMO, I would not like to keep a mint red penny of lated milled vintage to bathe in olive oil for any long period of time...

In a related vein, I recall reading somewhere about changes in acetone and its degradation after longer exposure to copper surfaces. As far as it goes I believe I have seen

some slight colour changes with copper exposure to acetone for even shorter periods.

I am quite leery of some of those substances occasionally available such as "Blue Ribbon" or the more worrisome "MS70". One of them causes copper to turn blue and

I have seen one of the 1860 proof copper farthings with this bluish lustre! Yikes!!

I presume you mean it looks something like this? It made it into an MS63 slab though.

[Mr T]

That will be inevitable I'm afraid, where the verdigris has actually eaten away the affected surface of the coin. The only alternative is to convert the verdigris itself into another, non-corrosive substance - it may be that diluted vinegar will do that? Not sure - it will certainly lighten the rest of the coin and leave the verdigris as a dark patch, but not green.

That depends how deep the coorosion went through the material. EDTA do not desolve the elementary metal atoms, but the charged metal ions within the green copper carbonate.

So it shouldn't noticeably ruin the rest of the coin? I've tried cleaning some worthless coppers before but have ended up with bright orange coins with entirely pitted surfaces.

What does the acid component of Olive Oil become once any micro-smear on the coin has reacted with the coin's surface, and thus attained a new chemical property?

With olive oil, all sorts of things given that olive oil seems to be made up of all sorts of acids.

If you were take the simpler example of vinegar which is acetic acid, acetic acid will react with the verdigris (copper carbonate) to give water, carbon dioxide and copper acetate. The copper acetate is soluble (dissolves in water) so if you give your coin a rinse afterwards it should be fine. I don't know if the copper acetate would affect a coin or not but you should probably rinse it off to be sure.

If you generalise the above reaction to the acids in olive oil, the same thing should happen anyway i.e. the acid will react with the verdigris to give water, carbon dioxide and a soluble copper compound.

[ChKy]

Well... that depends on concentration, time scale and initial condition of the coin. The treatment with EDTA can cause an orange coloured and pitted surface.

Surely I can bet, that you used an highly concentrated EDTA-solution, incubating for several hours a strongly corroded coin...

I am talking about a diluted EDTA-solution and a treatment of minutes, not hours or days. And the coin was not "holed" by corosion. Such a coin tends to by worthless for a collector (in terms of financial value) anyhow...

Edited January 27, 2013 by ChKy

[Coinery]

In a related vein, I recall reading somewhere about changes in acetone and its degradation after longer exposure to copper surfaces. As far as it goes I believe I have seen

some slight colour changes with copper exposure to acetone for even shorter periods.

Sorry to press on details, but are we talking about there being a damaging acetone 'residue' left behind on the coin (if acetone does leave residue, that is) after evaporation, or are we talking about the long term soaks in acetone that one occasionally reads about?

[Mr T]

Well... that depends on concentration, time scale and initial condition of the coin. The treatment with EDTA can cause an orange coloured and pitted surface.

Surely I can bet, that you used an highly concentrated EDTA-solution, incubating for several hours a strongly corroded coin...

I am talking about a diluted EDTA-solution and a treatment of minutes, not hours or days. And the coin was not "holed" by corosion. Such a coin tends to by worthless for a collector (in terms of financial value) anyhow...

It was a pretty badly corroded coin and I didn't dilute the stuff (some descaler) so that could have been the issue.

[VickySilver]

Acetone - no damage just colour change usu. very subtle. I am an absolute fan of acetone esp. on silver (can you imagine?)...

I have no experience with EDTA although ChKy's example shows a bit of redness - I think I see it - so perhaps a danger to otherwise mint red coins with good lustre.

Vinegar is a much stronger acid source in my experience, so be careful as it doesn't just work on the desired verd. spot...

Rob, I think I am seeing that effect but have seen it even stronger. There is supposedly some solvent that takes care of the blueness as well???

FYI:

The book, maybe obtainable from Amazon or the like is: Weimar W. White "Coin Chemistry Including Preservation and Cleaning"

Edited January 27, 2013 by VickySilver

[ChKy]

I have no experience with EDTA although ChKy's example shows a bit of redness - I think I see it - so perhaps a danger to otherwise mint red coins with good lustre.

It is a coin with original lustre. It was red before, the EDTA removed a bit of the initial brown colour at the surface... The problem is, that I can not allow the green verdigris further growth on the coin´s surface...

Edited January 27, 2013 by ChKy