Bronze Disease and Sodium Sesquicarbonate

[Obryzum]

Last year I treated a few bronze-diseased coins with a 5% solution of sodium sesquicarbonate.  The process worked after just one treatment.    The coins I was experimenting on were long-neglected coins I did not really care about -- I just wanted to figure out how the process worked. 

Now I have an coin with a nice patina that has come down with bronze disease.  I want to be a little more careful this time.  I do not want to use a hatchet when I need to use a scalpel.

I have read from some sources that sodium sesquicarbonate will remove patinas.  I have also read that boiling a solution in de-ionized water is the most effective treatment.

Interestingly, this link seems to say exactly the opposite:

http://nautarch.tamu.edu/class/anth605/File12.htm

It says that the best way to start is with a solution of sodium sesquicarbonate in TAP water.  Later it suggests switching to distilled water.  It also claims that a 5% solution of sodium sesquicarbonate will NOT remove green patinas.  Because this material is from a class in Conservation of Cupreous Metals, and because the technical explanations are so precise, I am inclined to believe the source.

Has anyone else had a different experience? 

[larry c]

Do a search on bronze disease or sodium sesquicarbonate, there are
many threads on this in the uncleaned section.
Lots of input from khowledgeable users.

[slokind]

Here is one that I realized is probably quite rare.  It had BD wherever now you see pits, most plainly in the pit at 5 o'clock on the obv., at the tip of Hadrian's neck.  First I washed it, thenjust soaked it in DW, which made the rusty stuff come up, too.  Then I used the 1 tsp of Sodium Sesquicarbonate in two cups of distilled water, boiling for an hour (and watching).  Then neutralized in DW.  This preserved all the patina that it had in the first place (it had had some touch-up and/or wax on it when I began).
It was very worn to begin with, but the sodium sesqui. worked fine and it was possible to photograph all the identifying letters and the style, though with difficulty, since it has to be turned under bright light under a magnifying glass just to see it in hand.

It is important to dry it very well.  I have some old gas heaters, and I put a coin for drying on a piece of paper on a pie tin, which I set on top of the heater.  Watching (and of course not running the heater too high) I dry it that way for a day.  You could put it in an electric cooking stove oven on Low (like 200°) and use that instead.
Pat L.

[bruce61813]

The answer is yes, but it is a mixed answer. If the patina is solid, it will not be harmed, as the sodium compound would only form Copper carbonate, which is malachite. However, and this is important, if the patina has an underlay of BD or is mixed with BD [CuCl] it will dissolve and be removed/dissolved. These are not simple black & white issues, and rarely is a single answer available. But the carbonate/bicarbonate mix will not produce a shiny metal surface, and does not seem to bother ordinary copper oxide. I tried it on an old english penny from the 1900 era , that had a spot od corrosion, and after two weeks of soak and scrubbing the spot, it was dissolved and the penny looked as brown as it had to start with.

Bruce

[moonmoth]

I have had similar experiences.  Most patinas are not damaged by an hour's gentle simmering in the solution.  Some, though, disappear completely.  Here is an example.  But it is worth bearing in mind that some of these coins have an artificially applied modern surface, and one of those might be more vulnerable.

On the second photo, I think the yellowish-orange patches are areas which had bronze disease.  The blobs by the chin might be pieces of original surface coating or corrosion, covered later by an artificial one after a crude stripping.  I have seen this on other coins. 

This coin looked very odd when I got it.  Some of the greenish areas did not look like bd, but some did, so I felt I had no option but to treat it.

[bruce61813]

A note to everyone. I was testing a slug with BD and had forgotten about it for a couple of weeks. One edge had develpoed what looked like a pale green cauliflower mineral growth. This was fairly hard and about the consistancy of one of those white after dinner mints. the areas seen in moonmoth's coin - the before pictures - have filled in areas withthat material. He is correct in the supposition that the 'yellow' areas were old BD. That is one of the probles, it tend to 'run' and spread in the softer areas of the coins flan, and somethimes will run under a nice green malachite patina. That is how I began studying the BD problem, I had a beautiful Probus with a deep green patina, suddenly errupt.  That was several years ago. and not something i want to see again.

Bruce

[moonmoth]

Some of that whitish stuff is still there, in cracks.

I think the history of my coin is quite possibly this: 
- Dug up with lumpy or irregular patina, or maybe obvious bd. 
- Stripped crudely, perhaps with acid, which left some dark green lumps such as those visible around the chin in the "after" shot.  This did not kill the bd. 
- Repatinated artificially or even painted, covering up those lumps and whatever bd remained but still allowing some colouration to show through. 
- bd continues its evil work under the paint while we wait.  White stuff develops in the cracks.
- Then bought by me, and boiled in sodium sesquicarbonate.

If the bd does not come back after a few months, I will put it outside in a lump of horse dung for a year or two and let it develop a bit of a coating.  This will be thin and fragile, but will look a lot more natural than Jax. If still no bd, wash, dry and flip!

Bill

[bruce61813]

I doubt if it was stripped with acid, unless it was a very fast job, possible. But bronze or any of the copper based alloys react poorly to acids, and leave you with 'pink' raw copper, and it is really hard to repatinate. So some of your thoughts might be correct.

I have found that when treating BD, mild heat to the solution helps. I use an old coffee maker, it keeps the solution at about 150 deg. F, so it helps dissolve crystallized salts and other things embedded in the crevasses of the coin.

Bruce

[Danny S. Jones]

Sodium sesquicarbonate (NaSC) is a Trona ore derivate that possesses the potential to neutralize acids. This neutralizing property is what makes it effective in combating bronze disease. Bronze disease occurs when copper chlorides mix with water vapor in the air and create a chemical reaction.

Stage one: 2 CuCl + H2O = 2 HCl + Cu2O
Stage two: 2 HCl + 2 Cu = 2 CuCl + H2

Basically, this chemical reation means that exposure to chlorides and water cause the formation of hydrochloric acid, which eats into the metal of the coin. Sodium sesquicarbonate (NaSC) work by neutralizing the hydrochloric acid.

I found an interesting and insightful article about bronze disease and its treatment on coins here:
Crescent City Coin Club Bronze Disease pdf

Another interesting and related use of Sodium sesquicarbonate is neutralizing the in dairy cows. It is used as an antacid to help in digestion and increase lactation in cows. You can read all about it in this report: Sodium Sesquicarbonate pdf

Sodium sesquicarbonate should prove very effective in stopping the decay of bronze disease in coins, although the benefits of increased lactation in the field of numismatics may require further study.

[moonmoth]

For the benefit of anyone looking at that "NaSC": NaSC is not a chemical formula, despite looking like one.  Sodium sesquicarbonate is either a salt with the formula Na3H(CO3)2, or an equimolar mixture of sodium carbonate, Na2CO3, and sodium bicarbonate, NaHCO3. It contains no sulphur.

[Danny S. Jones]

You are correct! I should have cleared that up in the post above. I used that designation, as it is commonly abbreviated... but the "SC simply is short for "sesquicarbonate." As moonmoth mentioned, you can make sodium sesquicarbonate with equal molar amounts of sodium carbonate Na2CO3 (also called soda ash) and sodium bicarbonate NaHCO3 (also known as baking soda). So in layman's terms, one could consider  sodium sesquicarbonate an antacid for your coins' heartburn.

[moonmoth]

And sodium carbonate can be bought in the UK as "washing soda,"  which is how I got mine.

[Danny S. Jones]

Jérôme,
I've got several small late Roman bronze coins that were inadvertently stripped of patina overnight in a freak olive oil accident last week. You can read about it here:
https://www.forumancientcoins.com/board/index.php?topic=56037.0
Some of them are already starting to show signs of bronze disease. I might use this as an opportunity to test out your sodium bicarbonate theory, since your Nero turned out so beautifully toned. I'll let you know the results.

Nice coin, by the way.

[bruce61813]

Here are several points to consider:

1. The Cl in tap water is of a different for and will not remain in water more than 24 hours at room temperature. Tap water causing BD is an old myth.

2. Generally the sodium carbonate/sodium bi-carbonate mix with not damage copper-carbonate patinas, it will remove the ones containing soft copper-chloride mixed patinas.

3. The use of sodium carbonate/sodium bi-carbonate has been in use for at least 90 years, and was heavily discussed and articles publish as far back as the 1930's.[/font] There is a recent document published by the University of Texas [year 2000], and re-written and modified by me, and is here on Forum, with references, it has been here for 5 years or so. I would also recommend this article:  
JAIC 1994, Volume 33, Number 2, Article 6 (pp. 141 to 152)    
A PERSPECTIVE ON THE HISTORY OF THE CONSERVATION OF ARCHAEOLOGICAL COPPER ALLOYS IN THE UNITED STATES by TERRY DRAYMAN-WEISSER, this can be found on-line and is worth reading. There are several other books, recently published that also talk about the carbonate/bi-carbonate method of removing chlorides.


Bruce

[CzarMike]

1. The Cl in tap water is of a different for and will not remain in water more than 24 hours at room temperature. Tap water causing BD is an old myth.

Chlorine ions are chlorine ions the ones in the tap are the same as the ones that cause BD.

[areich]

If you disagree with a chemist on chemistry you should perhaps give reasons?

[Robert_Brenchley]

I've never believed water did cause BD, but it's good to hear it from Bruce. It's been a long time since I did any chemistry, but it's hard to see how a brief period of exposure to gaseous chlorine could affect copper.