Coin Cleaning: How to Clean Coin Collections?

When we start collecting coins, the question of how to clean old coins and preserve them without damaging them over time immediately arises.

In this article we aim to give you as much information as possible so that you can make your own decisions, but you should bear in mind that you can damage the coin or lose its value, so we always recommend that you consult with your numismatist of trust.

When you start collecting coins, the question arises of how to clean old coins and preserve them without damaging them over time.

In this article we aim to give you as much information as possible so that you can make your own decisions.

What is the patina on a coin?

The patina in numismatics is a film that covers the coin and that forms on coins naturally with the passage of time and contact with the environment. When the coin goes further and suffers damage, we speak of corrosion.

Metals, with the exception of gold, undergo a chemical reaction when in contact with environmental agents such as air or humidity. This corrosion, which is nothing more than an oxidation of the metal, is produced by the loss of electrons from the atoms in favour of the amount of oxygen in the metal.

There are metals that are more prone to the formation of patinas such as bronze, copper or silver. In numismatics some coins with patina are highly valued if this patina is considered of good quality, this is because there are types of patina:

Stable patina: protects the coin as it has formed naturally, if it is removed it will re-form and lead to the loss of material.
Unstable or destructive patinas or bronze cancer: the presence of chlorine salts, atacamite and paratacamite on the surface of mainly copper coins. These are greenish stains on the coin.
Lack of patina: this is due to coins that have been in the absence of oxygen and slightly acidic environments for a long time.
Incrustation: These are deposits of foreign matter such as dirt or sand which cannot be removed by simple washing. It is related to the patina as it is generated in the process of patina formation.

Conservation of coins

For the conservation of coins we must take into account which are the main pollutants as we can prevent the deterioration of the pieces of our collection. There are products for handling, storing and preserving coins.

Air

It is important to store coins in containers that are as airtight as possible and to keep them away from contact with air. The main pollutant contained in the air is hydrogen sulphide and it reacts with the metals producing their oxidation.

Although some patinas are well considered in numismatics it is possible that if the process is not stopped the coin will deteriorate even more. The process is:

Start: thin golden patina
Intermediate: blue patina
Final: thick black patina (cleaning does not give good results)

Humidity

Storing coins at a humidity of more than 40% causes deterioration. To preserve them properly, it is recommended to have an environment with a lower % of humidity than indicated and a moderate temperature.

The main cause of bronze cancer (unstable patina) is high humidity.

To control the humidity of collections, silica gel is used, which acts as a regulator of the environment.

Footprints

Our skin contains grease and elements that produce that if we handle the coins without gloves, especially the freshly cradled ones, these are marked with our fingerprints. These fingerprints over time will tend to produce a stain on the coin.

The use of cotton or latex gloves is recommended for handling particularly delicate coins. If they have already been marked by a fingerprint, you can try to remove the stain with an eraser and alcohol.

Cleaning coins - yes or no?

Now that you know what patina is and the main causes of deterioration of coins, can old coins be cleaned? You will have already noticed the importance of patina, coins age and over time their surface creates a protective film due to the natural oxidation of the metal, if it is beautiful it has a great numismatic value. Good collectors appreciate the original appearance of a coin more than the artificial shine of a cleaned coin, that is why only buy coins whose appearance guarantees that they have not been cleaned before.

They will only buy coins whose appearance guarantees that they have not been cleaned before.

If you are still determined to clean them, here are some tips:

Once the product has been used, wash the coin with distilled water so that there is no residue that could start an irreparable chemical process. Do not use tap water which may contain high amounts of chemicals such as chlorine.
There is no product that will return the coin to its original state, although there are specific products for numismatics with instructions for use.
If you rub the coins with a cloth it may cause some micro-rubbing. If you want to dry them with cotton or toilet paper without rubbing, another option is to apply hot air.
If you clean with acetone it must be 100% pure, never nail polish remover.
Do not touch a PROOF coin ever.

The best option for cleaning coins is to use a non-abrasive method, that is, one that preserves 100% of the coin material.

To remove dirt, grease or grime, it is recommended to soak the coin in distilled, warm soapy water for hours or days and rub the coin with your hands or a toothbrush in a circular motion.
For PVC residues, the best option is pharmacy alcohol or acetone.

Cleaning old coins

Ancient coins such as Roman or Greek coins, even some more recent ones, are found underground and this usually requires cleaning in order to make their characteristics visible and to be catalogued afterwards. In many cases these remains of soil, clay and mineral aggregates are very difficult to remove without the use of mechanical methods such as brushes and points or even electrolysis.

The first thing to do if you want to clean an old coin is to wash it with hot water and soap, rubbing it delicately with your fingers in circles.

Then, for coins made of materials such as bronze or silver, it is recommended to give them baths of olive oil or distilled water. It consists of leaving the coins submerged for 15 days, after which time they should be cleaned with soap and water and then submerged again. This process does not carry a very high risk.

Finally, there are different ways of finishing the cleaning process depending on each case and the condition of the coin, always taking extreme care not to damage the coins:

Mechanical cleaning using tools: sticks, brushes, etc.
Chemical cleaning, there are formulations that are used by various collectors. This process generally removes the patina.

Electrical cleaning, electrolysis. It is the most invasive and consists of electric discharges under water with salts for minutes where the ions from the negative electrode are transferred to the positive electrode. It destroys the patina.

Cleaning gold coins

Gold coins, being a relatively inert metal, suffer almost no oxidation, although some do turn orange. They accept a mild cleaning with alcohol, or a warm bath of distilled soapy water. Never use cleaning products on gold.

Cleaning silver coins

Silver is also very reactive and will form gold, brown and finally black patinas.

When the silver environment is plastic and not earth, green carbonates and blue sulphates or air will produce reddish oxides. Silver in the open usually produces black patinas (silver sulphates).

Ammonia can be used to clean silver with a fineness of over 900 thousandths. Simply immerse the coin in a glass container with distilled water and a splash of ammonia and wait for the oxidation to be removed, and the water may take on a bluish colour from the copper that precipitates from the silver (this should theoretically be avoided). The speed of cleaning depends on the purity of the ammonia. After cleaning, neutralise with distilled water for a few hours.

Cleaning euro coins

Many people wonder if euros can be cleaned, the answer is yes. Centimos and two-euro coins sometimes present problems due to oxidation of the metals they are made of. Storing them in low quality plastic or subjecting them to high humidity causes the patina to turn green or acquire a pale or greyish appearance. To clean coins you can follow these tips or use these products:

Dirty euro coins: Use an anti-limescale such as Cillit bang. Soak for 5 minutes and rinse with water. Dry with cotton wool.
Euro coins (common in centimos) with black stains: do the previous step and then clean with a mixture of 50% metal cleaner (for example: Sidol) and 50% alcohol 90º. Rinse with water and dry with cotton.

These operations should always be done with gloves as one of the problems when it comes to damaging a euro coin is derived from its handling, the sweat and dirt that we have on our hands accelerate the oxidation of metals.

Summary

contaminants from the mint, recent fingerprints	alcohol
fingerprints	cream eraser
green PVC residues	acetone
dirt	distilled water with soap
non-abrasive copper abrasive	olive oil / distilled water and soap
non-abrasive silver	distilled water and soap
gold non-abrasive	alcohol
copper abrasive	coin cleaner
silver law >= 925 , abrasive	ammonia
buried bronze	olive oil / electrolysis / distilled water and soap rubbing with toothbrush.
buried silver	distilled water and soap by scrubbing with toothbrush.