A plated denarius with corrosion/BD

[Skolot]

Hello ! This is a fourree silver-plated denarius. The corroded stuff in the hole is soft. What should I do to this coin to save it ?

[Joe Sermarini]

Wax it and keep it in a very dry place.  It will not last forever, but if it is kept dry, it will probably outlast you and me.

[Meepzorp]

Hi Skolot,

I have a plated Octavian/Augustus denarius from the same time period (scroll down, fourth coin):

https://www.forumancientcoins.com/meepzorp/ri_aug_ar_pt02.htm

My fifth coin in that page looks like your coin. My example isn't plated.

Meepzorp

[Meepzorp]

Hi folks,

I wonder what the protocol is for treating plated coins for potential BD. Is it the same as copper/bronze coins?

Should Skolot treat his coin before waxing it?

I treated about 20-30 coins for BD a few years ago, but none of them were plated. Some people claimed that I "over-treated" them, but I was generally pleased with the results.

Meepzorp

[SC]

Pretty advanced BD on a very nice coin.  Also the bi-metallic aspect complicates cleaning options.  Might be worth seeking professional help.

If you decide to treat alone the basic steps are:

1) Get rid of as much of the light green powder as possible.

For this you can use toothbrushes, including ones with cut-down short bristles to make them tougher. Toothpicks.  Bamboo skewers.  Even sharp steel pins if you pick carefully.  I think it is best done dry as much as you can but you could use water if you need to.  The goal is to get rid of all the soft green stuff.  This may expose shiny copper.

2) Dry.

This is a case where I would bake the coin to drive out all moisture.  You should search through this cleaning thread for tips on duration and heat as I can't remember what I have used in the past or what others have done.   I think I have put them on baking sheets at fairly low temperatures (150 F) for 30 minutes or so but can't remember for sure.  Haven't done it for years.

3) Store or seal.

Once the clean and dry stages are done you can either leave it as is and monitor it to see if the BD reappears, or try to seal it.  Sealing reduces the chance of the BD continuing as it needs contact with the air to really advance - though even sealed coins can have the BD advance due to some chemical reactions already under way, or improper/incomplete sealing.  But on the other hand sealing makes it more difficult to treat if the BD does advance as you have to remove the sealant before re-treating it.  Depends in part on the humidity level where you live. Personally I would clean it and watch it.  Might not need sealing. 

If you seal you can use renaissance wax, though in cases like this it might be worth trying to have someone seal it with benzotriazole.  Benzotriazole is definitely a "don't try it at home" thing - not until you know what you are doing at least.  It is a listed carcinogenic chemical.  You can't even buy it in many places.  In effect is a fancy clear lacquer that stops the shemical reaction and seals it from the atmosphere. 

SC

[Joe Sermarini]

I do not recommend trying to remove the corrosion. It clearly extends under the silver.  Much of the core may be impacted. Attempting to remove any of the corrosion will likely have a bad result, perhaps even with the coin breaking apart into pieces.

[Skolot]

Thank you very much for you opinions !
I have Paraloid B-72, may I use it ?
Recently I have made experiments with sodium sequicarbonate on a bronze coin affected by BD but I was not satisfied with the result.

[SC]

Paraloid is another lacquer.  But I don't know how it reacts on a bi-metallic object.  Worth checking before you use it.

SC