Help with Verdigris issue

I think I would stay away from ammonia for awhile.

 

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Your results look good, but ammonia still worries me. You say 10%, but a quick search says that commercial household ammonia varies from 2% to 10%, so exactly what do you mean by 10%?

 

Calling @BadThad

 

Let's clarify few things:
1) If it is verdigris, you don't need help. Verdigris is not harmful but a protective patina. If it was harmful, you wouldn't have the statue of liberty still erected.

2) If it is bronze disease, first mechanically get rid of the green powder (copper chloride) as much as you can. Then, you can treat it with a 20% sodium sesquicarbonate solution. You need to change the solution every week, before dipping in fresh solution you can brush it gently with a soft brush. When it is done, you can bake it for a few hours at 200 degrees celcius. And try to keep it in a dry environment.

3) Do not use harsh chemicals like ammonia, sodium hydroxide etc. Bronze disease is an autocatalytic reaction between copper and the chlorides. When there is humidity involved chlorides form hydrochloric acid which further reacts with the copper and forms copper chloride (CuCl2) which is powdery. As long as there is water (humidity) and the chlorides, it will continue to "eat" the copper. Strong chemicals destroys the patina but sodium sesquicarbonate treatment (also there is a benzotriazole treatment but the SSC is better) works best.


4) I am a total noob on Roman coins. What is the silver content in Roman coinage? The picture you posted doesn't look like it is bronze disease but a copper deposit. I have such silver coins and they are totaly OK. See below. Are you sure that they were not there but grew over time?

 

Thank you Altaycoins, Indeed, I checked a few old pictures and noticed that green deposits were already there...- specifically on the coin that i posted. But few other ones (Alexandrian Tet- which was black- had a green deposit on it which is something new - was not there when I purchased it). I will try to share few more pictures of the coins in question - so we can get a better idea of what's going on- will also measure humidity level in my room and report back to see if this is an issue.)

 

using sodium sesquicarbonate always risks messing up the patina, sometimes darkening or even stripping it. I haven't seen a coin in this thread with BD, but if you are treating for BD, I only use sodium carbonate (Arm & Hammer Super Washing Soda Detergent) as it is much gentler on patina, but takes longer for treatment.

 

The earliest Roman silver coin I have seen with BD was a Julia Domna of an Eastern mint that looked a bit more base than some and I would estimate at under 50%. That is not a report of scientific study so it may be possible to get hit at a higher silver content.

Sodium sesquicarbonate is not easy to find everywhere but can be made by mixing sodium carbonate (washing soda) and sodium bicarbonate (baking soda). I am away from home now and do not have the proportions but will point out that Victor pointed out the possibility of using straight sodium carbonate so you could just add a little bicarb. This is not quantitative chemistry and close will count. Whatever you do, the important thing is to watch the dried coin closely and regularly for a long time after you think it is cured. Daily for a week, weekly for a month, monthly for at least a year and occasionally for several years. If you see new green fluff, the metal involved is lost so you want to catch it when it first shows up and not after it covers half the coin. New green means starting over. The Domna below had BD on the reverse upper left but has been clear for several years.


This Volusian Alexandian tet was well involved a year ago and was ruined well before I got it. It still is the only known to me of its type. Show me better.

 

No. Exactly opposite. It can of course darken it but it is gentler than the sodium carbonate alone. Sodium sesquicarbonate is a a double salt of sodium bicarbonate and sodium carbonate. The problem when you just mix the two salts (bicarbonate and carbonate) it doesn't form sesquicarbonate. However there are some products that are just prepared by physically mixing them. They can mess up the patina. But sesquicarbonate doesn't. It can be extremely slow process which can take up years. You can refer to Plenderleith and Werner 1971.

 

Unfortunately it is not true. Please see the attached patent for the production of sesquicarbonate.

 

AFAIK verdigris is not a term used to refer to a protective patina.

 

Quote from Wikipedia:

"Natural or artificially created coatings of verdigris are commonly used as a patina to protect copper or bronze objects, especially in architecture."

 

Here is an example of the bronze disease and the sesquicarbonate treatment on a rare Ilkhan fals.

First image is taken just after I purchased the coin. Some green spots are visible.

Second image is taken about 2 months after. I saw BD growing heavily especially on the bottom of the reverse.

Third image is after sesquicarbonate treatment for 3 months. It is baked and Renaissance Wax applied. No problems up to now (after 1.5 years).

 

However, as commonly used in numismatics, verdigris (at least in things I have read) is used to refer to the form of copper corrosion that is also called bronze disease and is a destructive form of corrosion instead of a protective form. I think it is usually copper carbonate instead of copper chloride. That is why I referenced @BadThad who is a chemist who has worked in the metal treatments fluids industry for many years. I know you are a chemist, so am I and so is Thad. Treatments used by people not experienced in doing so are best left at an elementary level to prevent harm. Washing the coins in water and brushing them with a toothbrush would remove the looser material without damaging the coins or the protective patina. Treatment with VerdiCare will reduce any residual verdigris and protect the coin. Do a Google search if you are not familiar with VerdiCare.

 

Very nice job. I am just saying that treatment with VerdiCare could have done the same thing and would have been easier.

 

I am afraid there are many wrong information in your post.

Verdigris is copper carbonate or acetate and bronze disease is copper chloride. Here is some chemistry for BD.



I don't trust any commercial product that doesn't explicitly gives exact chemical formulation to put on my coins.

 

Since you seem to like Wikipedia, here is how their "Verdigris" entry starts...

Verdigris is the common name for a greenpigment obtained through the application of acetic acid to copper plates[2] or the natural patina formed when copper, brass or bronze is weathered and exposed to air or seawater over a period of time. It is usually a basic copper carbonate, but near the sea will be a basic copper chloride.[3] If acetic acid is present at the time of weathering, it may consist of copper(II) acetate.

 

Yes that is true near the sea. But the mechanism of formation is totally different.

 

I guess what I was trying to say is that terminology is not as exact as we would like and trying to help someone and allay their concerns needs to be made relatively easy. Many people here on CoinTalk have used VerdiCare and it is not a harmful material.

 

The Wikipedia paragraph is poorly written because it conflates too many phenomena into a single paragraph. They mention the formation of a green pigment on copper by the addition of acetic acid, and give the impression that the copper salt formed is a carbonate. ??? Not so. It is a copper acetate. But they jumped the thought, and blew it. Altay is quite correct that Bronze Disease concerns the product produced when copper is acted upon by chloride ions. Verdigris is a common name for a whole class of substances chemically distinct from the two valence state copper chlorides. It is physically hard and only operates on the outer surface of a coin, forming a shell that will not progress deeper unless physically disturbed in a way that exposes the bare copper again. The chloride that is BD on the other hand, when exposed to humidity, produces an acid that will act on metal it contacts. The destruction is self sustaining, and will continue until all the copper is chemically used up, or it is arrested by external chemical alteration.

@Kentucky, I urge you to help set the record straight on the difference between bronze disease and verdigris by abandoning the all too common tendency among uninformed collectors to conflate the two. Coins are being mishandled.

 

You guys are working WAY to hard at this. I never have a problem getting that Bronze Disease off...



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For those that worry that newb's are going to believe this, yes, I am JOKING.
(but for @TIF , yeah, it is a TRUE STORY. )