ABOUT THE AUTHOR
I am a numismatist and chemist in the metalworking fluid industry with over 25 years of experience. I am in the final stages of finishing a book on the topic of coin verdigris. As an avid Lincoln cent collector, the subject of coin verdigris removal has been of interest to me for many years. This guide has been written to help my fellow numismatists/conservationists address the problem of coin verdigris.
WHAT IS VERDIGRIS?
Verdigris is a term used to describe the corrosion of metallic copper. Copper is a very reactive metal, it will readily combine with many elements to form verdigris. Verdigris is NOT a living organism! This is a common misconception or folklore tale. It is not a virus, bacteria, fungus or mold. Coin verdigris is a complex mixture of many chemical compounds. For this reason, the removal of verdigris from coins can be somewhat complicated. Here is the definition of verdigris that I posted to Wikipedia:
Verdigris is a variable chemical mixture of compounds, complexes and water. The primary components are copper salts of acetate, carbonate, chloride, formate, hydroxide and sulfate. The secondary components are other metallic salts, acids (organic and mineral), atmospheric gases and water. All the components are in an ever-changing and extremely sophisticated electrochemical reaction equilibrium that is dependent on the ambient environment.
HOW TO REMOVE VERDIGRIS
A simple internet search will produce many methods of verdigris removal. People have suggested methods including: lemon juice, catsup, acetone, mineral spirits, etc. The problems with these methods are many fold and most of them result in damaging the coin. The methods that don't damage the coin will turn out to be ineffective. Perhaps the best way to address verdigris for most collectors is to neutralize it and stop it's growth. This is something to seriously consider before you end up damaging your coin by using an improper cleaning process. If the verdigris bothers you, I generally suggest buying a coin to replace the problem coin.
THE FIRST STEP TO CONSERVATION
First you need to make sure you have verdigris. Often people mistake a green residue for verdigris. Always verify the substance is verdigris by following what I call the SOLVENT POLARITY LADDER. Start at the top of the list and work down one solvent at a time. DO NOT MIX ALL OF THESE SOLVENTS TOGETHER! Soak the coin in the following solvents for 10-20 minutes each to see if the residue is removed. These are all coin safe chemicals.
- Distilled water
- Acetone (do NOT use fingernail polish remover)
If the residue was removed be happy! You didn't have verdigris!
If you choose to neutralize and halt the verdigris, I suggest the following steps:
- Add just a few, tiny grains of baking soda to about 20-mL (about 2 tablespoons) of distilled water and dissolve.
- Soak the coin in the solution for about an hour. Make sure all of the verdigris is exposed to the solution.
- Remove the coin, rinse with fresh distilled water and pat dry with a coin-safe towel (do not rub).
- Soak the coin in a high-grade acetone (do NOT use fingernail polish remover) for about 5 minutes.
- Remove the coin and allow it to air dry for about a minute. Acetone evaporates QUICKLY.
- Immediately store the coin in an Air-Tite brand capsule (or equivalent). Water and air are the feed materials for verdigris formation. If they are kept away from the coin, the verdigris cannot grow.
Alright, you decided that the verdigris has got to go so you'll still reading. Unfortunately, ALL of the methods outlined on the internet will either damage your coin or they will be ineffective. You are more than welcome to figure this out for yourself, but I suggest practicing on some non-valuable coins first. Don't dive in and start trying the internet suggestions on your fine coin or you'll make a mistake you will always regret. There's a fine line between CONSERVING and CLEANING a coin! Once you cross the line, there is no turning back. Your coin will be ruined forever if you clean it AND the value will be substantially less than from when you started. If you do try these methods, I suggest diluting the chemicals with distilled water to start. Use a very weak solution first, then increase the concentration later if the weak stuff isn't working.
I'll outline some of the methods recommended on the internet and the problems you'll have with them:
- OLIVE OIL - This is the most commonly suggested method. While it can sometimes be effective, it will usually alter the patina due to the acids present. It can also take months to work and the collector risks the oil turning bad during the process. Most brown, copper patinas are soluble in the oil so a layer will be removed and the coin will look different. Also, olive oil will only remove the lightest of verdigris from the surface, it will have no effect on the moderate to heavy stuff. If you soak for months, be sure to change out the oil every week or so. Olive oil is very inconsistent and the acid levels vary from batch to batch. It will also often leave behind reddish patches where the acids have reacted with the corrosion (verdigris).
- LEMON JUICE (ORGANIC ACIDS) - Sometimes used with tin foil, lemon juice is highly acidic (citric acid). It will be effective at verdigris removal, but the acid WILL damage the coin surface resulting in a cleaned coin. This applies to ALL juices including catsup, they are all acidic cleaners and will damage the coin surface quickly and permanently.
- VINEGAR (ORGANIC ACIDS) - Effectively the same as using lemon juice but the acid is a little weaker. Vinegar is also very acidic (acetic acid). While it may be effective at removing verdigris, it's also effective at stripping the surface resulting in a cleaned coin. Be sure to dilute it with water to reduce the strength.
- ELECTROLYSIS - Very effective at removing verdigris, but it will completely strip the coin! When you're done you'll have a bright and cleaned coin that most collectors will reject. The only time electrolysis is very useful is with heavily encrusted coins. GREAT care must be taken with this method because it will EAT your coin until it's no more. It easy to lose ALL of the coin detail doing this so use extreme caution.
- LIGHTER FLUID/ORGANIC SOLVENTS - Verdigris is mainly composed of inorganic salts that are insoluble in organic solvents. Solvents such as acetone, xylene, mineral spirits generally have NO EFFECT on verdgirs. Using these solvents is a complete waste of time. The only thing they are useful for is to dehydrate and inactive the verdigris because they will not remove it.
- COMMERCIAL CLEANERS - There's a few products out there (usually powders) that contain trisodium phosphate or other chelating agents. Just as the name implies, you will end up with an obviously cleaned coin should you decide to use them. They will remove verdigris, but they will also remove a layer of metal from the surface. Use at VERY low concentrations to start. If the directions say use a tablespoon, use 1/10 of a tablespoon. ALWAYS use weak solutions to start!
- AMMONIA - Horrible idea! It will remove verdigris but just like the commercial cleaners, it will remove metal from the surface resulting in a cleaned coin. If you try it, use a very weak dilution in distilled water (i.e. one drop in a cup of distilled water). Do not use straight ammonia on the coin!
- HYDROGEN PEROXIDE - Again, we have another harsh chemical that will remove verdigris, but it will also clean the coin and remove the patina. Another chemical that should be diluted before use. Some people recommend boiling the hydrogen peroxide and placing the coin into the solution. I recommend using tongs to dip the coin into the boiling solution so you can constantly check the progress and not over-cook the coin.
- DRY RUBBING - Many people suggest using a toothpick or a rose thorn to remove verdigris. This is actually one of the better techniques out there since it only involves physical removal and there are no chemicals that may damage the coin. However, should you decide to use this method, DO NOT DRY RUB. Dry rubbing may leave micro-scratches on the surface. Submerge the coin in distilled water, allow it to soak for about an hour, then try rubbing off the verdigris (while the coin is submerged). At first, try an up and down tapping motion to avoid the chances of making scratches on the surface. If it's not working, then start gently rubbing. Be careful! It's easy to rub off the patina.
I'm sure there's many more methods you will find on the web. For example, collectors of ancient coins use many extreme methods on heavily encrusted coins (my guide is not really intended for them). While the classic removal methods may work to various degrees, they will probably damage your coin in the process. Remember, once you've gone too far and cleaned your coin - there is no turning back! So be very, very careful! Always be conservative with your chosen method and practice on non-valuable coins first!
As a chemist, verdigris has always interested and frustrated me. I am a fairly hardcore collector of Lincoln cents and verdigris is something us copper collectors cannot avoid. When I started collecting I had no idea of how to address the problem so I turned to the internet and asked many questions. Unfortunately, I received a lot of bad advice along the way. Recommendations from using catsup to mineral spirits were given to me. Keeping an open mind, I tested each and every method suggested only to be disappointed in the outcomes. At this point I began to seriously study verdigris and I became so obsessed and knowledgeable about the topic I decided to write a book about it. After nearly two years I'm almost finished with it. In the process of writing the book I greatly increased my knowledge and decide to develop my own chemical to remove verdigris. Many hours of laboratory research and testing resulted in my first commercial product VERDI-GONE™. It was able to remove light to moderate verdigris with minimal effects to the coin surfaces. However, VERDI-GONE™ could not be used on brilliant uncirculated coins because it was shown to dull the surfaces so I had to keep pushing on.
I was still not satisfied, I wanted a product that would not only work faster, but would enhance the surfaces and have less of a chance of altering the patina. My latest verdigris removal and conservation fluid is called VERDI-CARE™. After many hours in the lab I've finally developed a product that I use reguarly on problem coins with verdigris. The highlights of VERDI-CARE™:
- Fast! - Light to moderate verdigris is typically removed within seconds.
- Coin Safe - Preserves and enhances the patina instead of stripping it. Safe for use on BU coins.
- Health Safe - No harsh chemicals, safe on skin and eyes, non-corrosive, non-carcinogenic, non-toxic, non-flammable, non-volatile.
- Three Steps - APPLY > DRY >DONE. No need for post treatment neutralization or rinsing.
If you're interested in purchasing my product, please ebay or google search VERDI-CARE™.
Thanks for reading my guide and I hope you found it helpful! :)