This is one of the questions that rise more controversy among numismatists. There are firm advocates of the coin cleaning process, however the purists think the value of a coin increases with its own history and the patina that times produces on the coins is the most clear example of that history. One of the most common concerns among the profane collectors is to have a coin collection the most brilliant the better in order to make the coins look like they are new, giving more significance to the bright of the coin than the wear of the details in the coin.
The patine generated in the coins depends on the material which they are made of, usually its a dark tone of oxid or aging. However some of the silver coins show a golden, blue or even iridescent (rainbow) patina very appreciated between numismatists.
As most of the questions that rise controversy, the prudent move here from our part is to provide you with all the information we can and let you be the one deciding in which cases a cleaning is worth it or not. Now, of course, taking into account that the cleaning process has to be made with patience and a lot of care in order to only eliminate the dirt and to not alter the composition or finish of the coins. As it happens with all kinds of restorations, experience is important, so if you finally decide to clean your coins and make them look better you should start trying the different techniques on coins with similar characteristics but with very low or even no value. Then, apply the same techniques to those coins than the ones you will use in the final more valued coin in order to assure that the results are similar.
There are several coin cleaning techniques: depending on the quantity of dirt, the material of the coin, quality of the details, the antiquity, etc.
Fine, I got it, so Should I clean it or not?
As a quick sum up we leave you here a list of questions that might help you out when deciding if we should clean the coin or not:
Do you have enough time?
The coin cleaning process, as any other restoration task calls for a lot of precision, being in a hurry never helps and would only lead you to ruin your coin. If you do not have in mind to spend the needed amount of time maybe it is better for you and the coin to postpone the task for some other time.
Will you be able to fully concentrate in the task?
As we said, coin cleaning is a process that requires lots of attention and focus, the techniques might not be extremely complicated, precision when executing them is key factor.
Forgetting a coin under chemical treatment because of a phone call could end up ruining your coin forever.
Do you know all the features of the coin you plan to clean?
The techniques you could apply vary depending on the coin you are going to clean as well as its composition. Make sure you know the materials used in the alloy as well as the resistance of them. This would help you avoiding applying a well executed technique over the incorrect alloy that could produce unexpected corrosion, abrasion or signs of wear in the final result.
Is it going to be more attractive to buyers if I clean it?
Many collectors appreciate better the original appearance of a coin than the bright obtained when it has been cleaned, so they only buy coins with an aspect suggesting that they have never been cleaned. Using chemical products, containing acids, make the first layers of metals go away, replacing the original patina of the coin with a non genuine bright. On top of that, copper and brass coins will go back to a dark aspect really fast obtaining a unequal, artificial and sometimes not attractive color.
Will improve the coin preservation after being cleaned?
The patina from a coin is not bad, but also gives it a natural protection. A good patina should not be eliminated, when moved away the brilliant surface is again exposed and will end up oxidizing once more. This could end up in an infinite loop provoking minor metal loss with each iteration.
Will the coin value increase once cleaned?
We can state that, in general, the value of a coin will not be increased with its cleaning, but decreased instead. Elimination of the patina produces an artificial bright in the coin that could make the mistrusts from potential buyers. Also, you can create scratches and damages on the coin surfaces highly reducing the coin value.
Finally, it should be taking into account that:
Collectors and sellers clean coins in very rare occasions. If the coins have value as collection items, nothing can reduce the price of it than a incorrect cleaning. An attractive patina could improve the value of the coin and getting rid of it could damage that unique color developed along many years.
In general terms, the cleaning process is usually only used on very old coins which are hard to identify due to the high amount of dirt stick on its surface or on low value coins in which their final aspect is more important than the loss that a cleaning process could cause.
Techniques for coin cleaning
Here you have the main coin cleaning techniques along with their main advantages and disadvantages.
It consists in a chemical product based cleaning process with a certain kind of acid in its formulation that removes the dirt and the patina through the chemical reaction. The main problem is that every time we submerge a coin in the chemical product, even for just a few seconds, a thin layer of metal is erased. Repeating this process several times could end up deleting all the details in the coin, or even worse and making the final coin totally wear away and removing all its glaze.
Also, generally an anti natural bright is produced. Chemical products have different results on different metals, so it is very important to take care on applying the correct product. On the other hand, this cleaners with acid activate the metal after the cleaning, speeding up the process of a new patina formation. This new patine will be specially unequal in copper coins and very uniform on silver ones. This process is the opposite to the natural oxidation and aging process of this metals, this is why the final results would have a very artificial look.
After the coin immersion in the cleaner, the chemical products needs to be neutralized, clearing the coin with distilled water using some cotton. It’s important to never use tap water since it contains minerals that could cause the coin oxidation. Distilled water can be bought at pharmacies or any big supermarket. It is also good to control the drying process of the coin using a soft cloth or a soft source of heat.
As an example of usage of these kind of abrasive products it is possible to use ammonia in the cleaning process of sterling silver coins with a density over 925 thousandth. You only need to submerge the coin in a crystal recipient and wait for the oxidation to go away. Note that the liquid can get a blue color. After the ammonia cleaning it needs to be neutralized using acetone or distilled water.
There is a huge variety of coin cleaners in the market, however, it is very important to pay special attention the metals they are meant for. The usage of a generic cleaner can produce good results in some coins and, at the same time, end up being a disaster in some others.
Contrary to the chemical cleaners, dirt elimination is based in a physical process, basically a friction one, instead of a chemical reaction. A good example is the bicarb application on silver coins. For that a wet plaster with bicarb and water (preferably distilled water) is created, and it is rub against the coin. This process should not be applied to coins with a very fine finish (proof, brilliant uncirculated or uncirculated), since the application zone cannot be determined precisely and a stain elimination could produce scratches on the surface.
In the same way as using chemical abrasive products, is easy to detect if a coin has been cleaning when this process has been applied. Once this cleaning process ends all remainings of the product should be cleaned using acetone or distilled water.
Non abrasive products
The ideal cleaning would be that one not damaging a single atom of the coin, preserving the original patina. In order to eliminate stick on dirt, grease or grime, the coin is submerged in warm water with soap along several hours or even days, without rubbing the coin with brushes or hands, only leaving water and soap soft and remove the dirt. Any recipient but metal is good for the water-soap mix in order to avoid oxidization processes.
Another possible product that can be used is acetone. It can eliminate PVC remainings that produce a fine green layer in the coin (usually due to the usage of not proper storage and conservation systems), grease remainings, residue of sticky tape or glue.
In the same way that abrasive products, depending on the coin material the usage of different products is recommended. I.e., gold being inert develops no oxidation, in order to clean it only alcohol is used, no other cleaning products should be used.
Olive oil is also recommended for copper and bronze coins in order to eliminate dirt and other substances stick on the coin. A copper coin can be submerged in olive oil for days or weeks in a closed recipient, changing the oil monthly. No chemical products with acid on copper coins should be applied since an artificial unequal patina will be developed right after the cleaning.
Other used technique in the coin cleaning process is the mechanical cleaning. It is based in the usage of different tools: toothpicks, made of bamboo, brass or some other flexible materials brushes… This technique is usually applied in the early phases of a cleaning process, in order to remove soil or crust, or in order to eliminate specific residues hardly stick on the coin. It consists on a soft rub with the tool, always wet and clean, over the coin trying to avoid scratches to the patina.
For the tough or highly stick on precision or corrosion points tools with different metal ends (with different level of harness) as aluminium, brass, titanium, steel, even also fiberglass scrapers, steel burnishers, or even scalpels.
Also there are rubber tools usually used by collectors to eliminate small dirts or fingerprints. This tools are very soft so they don’t affect much to the details of the coin.
Summing up, this technique should be used with an extreme care, given the high risk of deteriorating the coin. Its application should be restricted to the initial phases of the cleaning process, specially for those old coins found in the ground that are specially dirty and oxidized. These kind of coins should be cleaned in order to make its imprints visible and allow its classification.
Electronic cleaning (Electrolysis)
The electrolysis is the most invasive cleaning and consists in cleaning a copper or silver coin that have been buried and contains such a hard crust that you cannot get rid of, not even with oil or rubbing with toothpicks or other sharp end tools. It is also a solution to the bronze’s cancer, since using mechanical methods for this will make it come back in just weeks.
This cleaning process is done in a brasive way, using an electric underwater shock with added salts during minutes or hours. The ions of the positive electrode (anode) are transferred to the negative electrode (cathode). The problem in this process is that the patina of the coin is usually destroyed.
Using electrolysis over a silver coin, the current separates copper from silver. In high level of copper coins like the vellones, an external surface of pure silver will be created that can be easily removed. Because of that you have to be very careful with electrolysis on silver coins, specially if they are low quality silver coins. This technique should not be applied in copper coins with a green/brown homogeneous patina. This patina has probably 2000 years of history and protects the coin from air and humidity.
Electrolysis kits can be acquired in specialized shops, although building one at home is relatively easy using an AC/DC current transformer between 6 and 12 volt, i.e. reusing the one from a laptop. The more volts the less time for the cleaning is required.
Curiosity. Bees based cleaning system:
As an anecdote we will point out a special cleaning system used in Korea, in which they use acacia pollen in order to attract bees. This bees suck or sip the surface of the coin, coming up with a final coin clean surface, brilliant and shiny. Here is a pic of the process: