Cleaning and Treating Ancient Coins with Chemicals

Cleaning with chemicals

Coins made of precious metals (gold, pure silver) are quite suitable for chemical cleaning, because acids do not affect them as much as they do the baser metals.
For base metals, one possibility is to clean the coin right down to the base bronze and then to re-patinate it. Collectors do not think much of this idea. There are, however, other ways of cleaning non-precious metals with chemicals. Precious metals such as gold and silver can easily be cleaned using chemicals and can even be recommended.

Cleaning Bronze and Copper Coins with Chemicals

METHOD I: The following method is rapidly gaining popularity especially in the USA and was discovered and developed by a group of amateur enthusiasts. This is an excellent method for cleaning non-precious metals, especially for badly encrusted coins ("crusties") which so many buyers often simply give up on and throw away. Note: Do not use this method for silvered coins such as Antoniniani !

What you need is a mixture of sodium hydroxide (NaOH, also called lye), and water. The mixture loosens the hard, encrusted minerals (Sand, stones etc) yet leaves the metal of the coin fully intact. Lye is usually used for cleaning drains and for this reason it does not attack the metal of the coin itself. (or it wouldn't be much good for old copper pipes, would it ?)

NaOH can be bought at the chemist shop, in Switzerland (probably in other countries too) in globule form, in US hardware stores under the name "Red Devil", and in Germany under the name "LAXYL" (made by BAUFAN: Bauchemie Leipzig, Ludwig-Hupfeld-Straße 19, D-04178 Leipzig / Tel.0341-446550.

NaOH is poisonous and so you should always follow these few, basic, simple rules:

1. Wear rubber gloves
2. NEVER mix it with hot water ! When lye is mixed with cool or cold water, it heats up on its own, thus helping the lye dissolve. Mixing with hot water results in dangerous fumes!
3. Keep away from children - if possible store it in a container with a child-safe lid. If you don't have such a container, explain to your kids (if they're old enough to understand) that it is dangerous. There's nothing more dangerous than not telling your kids what is dangerous!)
4. Mark the container with the name of the contents.
5. Do not use the mixture in a container made of thin glass - use a thick jam jar or a firm, plastic container with a water-tight lid). Loosen the lid after the lye has dissolved to avoid a build up of pressure.

Procedure: Mix 1 part (or less) NaOH-powder or globules with 3 parts of cold or cool water. If you use a jam jar: Place the coins in the empty jam jar, sprinkle the NaOH over them and add the cold or cool water. Close the lid tightly and carefully rotate the jar to mix the lye with the water. The coins will help it mix well. Remove the lid and check the coins 1 to 3 hours later. Give them a brush with an old toothbrush (in very "bad" cases with a brass brush). Be careful when opening the lid. Turn your face away in case of splashing. The fumes sometimes smell pretty foul as well..
Really bad coins can be treated in this way for up to 5 days.

After cleaning, put the coins into distilled water (e.g. from your tumble dryer or from the DIY) for 3 days to be neutralised. Change the water every day.

METHOD II: Bronze coins can also be cleaned by solvents containing Freon (trichlorotrifluoroethane). Basically, all gases in the group of fluorinated hydrocarbons (FHC) are "freons" (Freon is a commercial name) and were formerly used as coolants, for example in refrigerators. These days Freon is forbidden because it is said to damage the ozone layer around the earth, so it is pretty hard to get hold of. Mind you, in view of the fact that Freon used to be distributed by the millions of tons all over the world and we only need a couple of gramms, I don't think we should have a bad conscience if we were to get hold of any. But here again you must be extremely careful because the fumes are poisonous. Apart from that, being a solvent, it is very volatile, disappearing into the air in a fraction of a second, so you'd need to use it in a vacuum. Apparently though, Freon only removes the dirt and not the patina. Whether that is correct or not I have no idea, as I've never managed to get hold of any.

METHOD III: Franz-Bernd Karbach was kind enough to send me this suggestion which he has used with success. Many thanks!

For copper or bronze coins with a white or grey layer, he uses the following "soup": I mix together 1 litre of Aquadest (distilled water), 84 g of Sodium bicarbonate (baking soda) and 106 g of Sodium carbonate (washing soda or 19% sodium sesqui carbonate). This is your basic, concentrated mixture. To clean your coins, water it down to a 1 to 3% solution, again using Aquadest. You can let the coins rest in the mixture for as long as required if you change the solution regularly. Finally rinse the coins in distilled water.

Chemical cleaning of gold and silver coins

Not even the most aggressive acids in the earth can do much damage to gold coins and a quick rinse in soap and water is often enough to clean gold coins. But if the coin really does need a clean, place it for a short while in citric acid (lemon acid).

Silver coins tend to damage more easily. A certain dark, antique looking colour (oxidisation layer) can make the coin more valuable and looks attractive. If it is really strong, or some dirt is still remaining, you can place the coin in a mixture of distilled water and citric acid (lemon acid) (ratio 4 parts water to 1 part citric acid), and leave it in for anything from a couple of hours to several days depending on the state of the coin. (Do not use this method for silvered coins such as antoniniani, it will destroy the layer of silver). Change the solution regularly.

Instead of citric acid, you can also use ascorbic acid (= pure vitamin C, available at the chemists). Just take a knife-point full and put it into 100 ml of water, add the coin, leave it in for 5-10 minutes, take the coin out, dip it into natron powder and rub it between your thumb and first finger before rinsing it under cold running water. Repeat if necessary.
It works wonders ! And the coin will not darken again because the alkali of natron neutrilises the ascorbic acid

Another method for cleaning silver coins and artifacts is to place the item in a concentrated solution of washing soda (Sodium Carbonate). Spread a piece of silver foil onto the bottom of a plastic container and fill the container with very hot water. Then add 4 soup spoons of washing soda, dissolve it by stirring and then add the coins, making sure that they are all covered by the water. You'll little bubbles appearing - the sign that the chemical process has begun. As soon as the bubbles stop, the chemical process has ended. Remove the coins and rince under running water. Then buff them with a soft cloth to make them shine (although some collectors don't like that !)

Chemical cleaning of iron articles (e.g. Roman artifacts)

Really dirty articles (e.g. artifacts) can best be cleaned using the citric acid solution mentioned above (four parts of distilled water to one part citric acid). After a couple of days the solution has turned into a thick brown soup. You can repeat the process with fresh solution until the solution stops changing colour then remove the article and dry it with a hair dryer. Do not touch the artifact with your bare hands. Wear plastic gloves or hold it with a cloth. The artifact is then sandblasted using the smallest size sand particles possible and, if required, painted using clear laquer. There are small hand-held sand blasters available on the market, e.g. from "Boehler-Minitool" company. The tool is pretty expensive (about $250) but can be ordered online from Boehler Germany. The sandblaster tool should only used for iron articles, because it will destroy the patina of other metals (iron does not form a patina). Sandblasting is expensive so is usually limited to the professionals ;-)

Repatinating

There are several ways to repatinate coins, although it must be noted that specialists can always tell the difference between a fake patina and a genuine one. I will just list a couple of methods here. It needs a lot of practise so use some grotty coins first.

Bronze and copper coins can be given a brown patina using potassium sulphide. You should be able to buy potassium sulphide at the chemists. It comes in the form of little yellow crystals which stink like rotten eggs. You only need a couple of gramms (and a nose peg). Dissolve the crystals in water, add the coin and watch the patina form from light brown to dark brown depending on the time you leave the coin in the liquid. As soon as the desired colour has been reached, remove the coin and rinse it under cold running water.

If you want a turquoise patina you first need to remove any patina on the coin and clean it right down to the bare copper. Then place the coin for about 24 hours in a mixture of copper sulphate and ammonium chloride. The longer you leave the coin in the solution, the thicker the patina will be.
A black patina can be obtained by cleaning the coin down to the copper and leaving it in an oven (temperature about 200° C or 400° F) for a while. This will make the metal oxidise.
Other methods:
- Boil the coin for a couple of hours in distilled water
- (I've not tried this method myself so I don't know how long it takes or how effective it is:) Allow a weak solution of sulphuric acid to stand in the air for a few hours then add the coins and let stand until the desired colour is attained.

I sell a special, tested mixture for the price of 7 Euros per bag, with which you can repatinate coins very simply. If you're interested in buying a bag, drop me a line !

Follow-up treatment

After using chemicals on the coins, all traces of the chemicals must be totally removed in order to ensure that these will not continue to work after you put the coin into your collection. The best way is to put the coins into distilled water for two weeks and replace the water every day. You can shorten the time by warming the water. The coin must then be totally dried by placing them in a warm, dry place for several days. If you want to improve the drying procedure, you can dunk the coins in chemists alcohol first (a glass of Jacky does not count!) .
The crowning gift you can give your coins is a dab of "Renaissance Wax" - one can goes a long long way and is worth the high price because you need so little. Ren Wax, as it is popularly known seals the coins against external influences, avoids finger prints and smells nice, too. In the US you can buy Renaissance Wax from Knifeandgun.com and in the UK direct from the very friendly and helpful manufacturers, Conservation By Design Ltd. (on their website under "Sundries")

Some collectors use a special silicon spray (available in stationers shops and DIY stores) to spray their coins, thus sealing them from external influences.

Bronze Disease

If your coins are stored in a damp environment or were stored before they were completely dry, they can get bronze disease, a blue or greenish, soft powdery substance which begins in one area and spreads. Over time the coin will be destroyed because the rot releases tiny amounts of muriatic or hydrochloric acid which attacks more areas of the coin. Even if you manage to remove it, without treatment it will certainly re-appear after a while.
Treatment: Manually remove as much rot as you can and then place the coin in distilled water for 10 to 30 days depending on the size of the afflicted area, changing the water daily. Then you can place the coin for two hours in an oven at 200° C or 400° F although this may not be necessary (don't forget that this will darken the coin). Then use Ren Wax (mentioned above) to seal the coin and prevent the rot from forming again.

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