Treating Iron Coins With A Tannic Acid Coating

For one thing be careful of what you read on the internet as to chemical reactions with anything. Many articles are posted by people with either no knowledge of what they are saying or just making stuff up for what they think is fun.
Treating any iron or steel coin with almost anything could either ruin it or worse, if that is possible.
First of all you should try to figure out EXACTLY what your coins are made of. Iron is usually mixed with other metals or materials to give different results. Using an Acid, any Acid, may react differently due to what the coin is made of. An example is the word STEEL. It is Iron mixed with different materials for different reasons. One Acid may do little and others may react horribly.
My only suggestion is if you really want to mess around attempting to clean them up, start with distilled water. And too, remember that even water could react with meals. Also, try the old favorite Acetone. Some collectors may even say try Olive Oils. Regardless of what you try, just remember to be careful.

Ah, something I can speak to. In the first place, throw out any idea you have of "oh, don't alter it, do as little as possible!" Iron, no matter what it may be alloyed with is still going to rust so long as it has active, red rust and oxygen.
Airtight holders would work if you could remove the air from them first but they are going to have air in them when you close them up. Rust does not take a huge amount of oxygen to continue.
Here is what you are going to have to do. First, you need to remove the red, active rust. Yes, you are going to have to alter the surface of the coin. Considering the coin is currently self-destructing, this is a small price to pay, sot of like how Ancients collectors remove Bronze Disease to preserve the coin itself. Careful work with small implements will be required,as well as copious amounts of patience, but you have to do this. Once you've gotten the red/orange active rust gone, you have to seal off the coin from oxygen. There is a product called Renaissance Wax, it's for conserving fine old items. Apply a coat to your coin, and you will note no color change but your coin will be sealed from air and moisture.

True, but it would only continue oxidizing until all the oxygen is used up. Similar to landfills, decomposition only continues until all water and oxygen are gone. Then it's basically mummified. Permanently. (Mini-lesson: be mindful of what you throw out, just because it's biodegradable doesn't guarantee it will) Sure, air-tites aren't as air tight as trash buried two hundred feet down, but slabs aren't truly air tight, but they work well enough.

My only concern with the coins that aren't too rusty is potentially damaging them while trying to get the rust off. The OP needs to make a determination as to what coins he'd be willing to risk damage to -vs- which he'd want want to do his best to prevent further oxidation.

Thanks to everyone for the input.
A few points. The solution is made acidic to allow the ferric oxide "bond" better with the tannic acid.(2ml acid per liter of water).See article. Renaissance Wax or other similar products or lacquer will help IF ALL of the corrosion if gone. The reason for the tannic acid treatment. With a couple of the museums I talked to, using the wax is 50/50 if the coins are going to be kept in a low relative humidity environment after treatment. I am not sure how many people read the article but the treatment changes the ferrous oxide to ferric tannate (Fe2+ to Fe3+). Most of the tannate gets brushed away leaving a thin film of the tannate on the iron. This film is supposed to be a blue-black color similar, I was told, to wrought iron.

A few of my irons have survived being buried over a 1000 years and I would like them to survive not only for my children but also for collectors in another 1000 years.

I am going to test this procedure and others and will post the processes and results with photos. I am thinking of getting iron "slugs" with various levels of corrosion to be guinea pigs however eventually I will have to test the actual coins.

What would be a good source of coins to experiment with?

Interesting, I've never heard of using specifically using tannic acid. Personally, due to the acidic nature of it and the fact that it leaves an ugly coin, I would not use it. It may provide a layer of protection but there are much better choices these days for corrosion inhibitors.....there are literally thousands.

The biggest thing as far as Notgeld (iron or zinc) is making sure it is stored in a DRY low humidity environment. Each double-wide box I have of Notgeld is stored in my safe with a pack of silica gel, which I do change regularly...

I have seen people take more drastic measures to remove rust on Notgeld (such as wire brushes!), which I personally think looks worse than the rust!