Olive oil has been used as a softening agent for ages to aid the removal of built-up verdigris or dirt accumulations on coins that are up to centuries old.

Some collectors swear by it. Other collectors, and some conservators, swear at it, claiming the fatty acids that the olive oil contains can actually damage coins, particularly copper.

Ironically, copper or copper alloy coins are usually the subject of the olive oil immersion process.

Susan L. Maltby, a conservator at the University of Toronto in Canada and author of the Coin World column "Preserving Collectibles," commented on the practice in her Nov. 24, 2003, column.

She said that during a collectibles preservation seminar in which she participated, one participant sought her opinion on the practice of using olive oil in the treatment of ancient bronze coins.

"I certainly have heard of this technique, but it is not one that I can recommend," Maltby said in her column. "Cleaning a coin that is heavily encrusted is a time-consuming process requiring skill and patience. When I clean these coins, I do it under a microscope using a sharp scalpel. It is important to know when to stop cleaning. My main concern with soaking a copper alloy coin in olive oil is the fatty acids inherent in the oil. Fatty acids are corrosive to copper alloys.

"Readers who wear glasses are likely familiar with the green waxy corrosion products that form by the bridge of the glasses and by the arms – basically where they come in contact with your skin.

"I would expect that the fatty acids in the olive oil would have the same effect on a coin. Secondly, I am not certain how one would be able to remove the oil from the coin once the treatment was completed," she added.

A number of printed and online sources devoted to the subject of cleaning coins, including the employment of olive oil, propose two rules – the first, never clean your coins, and the second, always refer to Rule No. 1.

The methods to remove dirt and similar buildups of surface debris on a coin often read like a surgery textbook.

Coin World neither advocates nor condemns the use of olive oil as suggested in this article.

Specialists strongly advocate that new collectors practice cleaning techniques on nonvaluable coins before using the same techniques on coins in their collections.

Acceptable for ancients

Some specialists say some of the harshest methods are acceptable for ancient coins, since most, if not all, of the ancient coins known today came from sites that had to be excavated, which means the coins were often encrusted with dirt when they were found.

Some ancient coin hoards, especially bronzes, are sold in multiple lots with dirt still encrusted on the coins, to be removed by the purchaser before attribution can be made.

Addressing this point in a presentation before the Classical Numismatic Society during the 1997 New York International Numismatic Convention, coin expert Philip A.J. de Vicci explained that ancient coins excavated from archaeological sites often exhibit one of several different types of patina dependant on how long a coin was buried and the type of soil in which it was found. He said that the use of olive oil can aid in preserving the natural patina for coins.

Patina is a chemical compound formed on the surface of metal and can protect a coin or object from further corrosion.

Distilled water or olive oil will dissolve chemical bonds, according to de Vicci.

He suggested brushing the coin afterwards, but warned corrosion may cover pitting in a coin that becomes visible when the encrusted materials are removed from the coin's surfaces.

De Vicci said that a coating of a very high quality olive oil can protect coin surfaces once they are cleaned.

De Vicci suggested cleaning inexpensive coins until either collectors understand how to clean coins properly or have decided to hire a professional as needed.

The fall 2004 issue of The C4 Newsletter, the official publication of the Colonial Coin Collectors Club, chronicled online discussions between collectors concerning the positive and negative consequences of using olive oil or mineral oil as solvents to remove dirt and similar substances from copper coins.

It was pointed out that while olive oil will remove dirt and other contaminants, its acidity – depending on the oil's crudeness – will also remove desirable patina, darken the copper coin's surfaces and expose it to further contamination.

Discussions also noted that mineral oil contains no acids and will remove dirt, grease and wax.

It does little or no good in removing verdigris (oxides that can form on copper over time as the copper reacts with substances like carbon dioxide and oxygen).

What to do

In 2002, on his Enchanted-Treasures.com Web site, coin specialist Robert Beauford contributed comments concerning cleaning and preservation of ancient coins and artifacts, similar information that is also mentioned at a number of other Web sites as well as in some numismatic references.

In all cases, it is recommended that the methods be tried on inexpensive items first.

The use of olive oil may not necessarily work on all coin compositions.

The compositions used for U.S. circulating coinage include copper, copper-zinc, copper-nickel, copper-silver-manganese, silver and gold alloys, with different percentages of each element in a specific alloy.

There are also a number of short-lived compositions (zinc-coated steel, for example).

Similar compositions have been used for foreign coins. Ancient coins were also produced in a variety of alloys, including bronze, and their production often depended upon what metals were available at the time and what level of refinement could be obtained.

Why clean coins, especially when the mantra recited by collectors for decades has been, "Never clean your coins"?

For U.S. coins retrieved while metal detecting, the reasons for removing the dirt may be simply to use the coins in circulation, if they're current circulating issues, or to be able to place them into rolls for redemption at the local bank.

Or such dirt removal may be necessary to identify the date and Mint mark.

Beauford notes that repeatedly soaking and washing a coin in plain or distilled water is generally safe and appropriate for the removal of simple dirt and clay accumulations.

Some numismatists suggest avoiding tap water, which may be mineralized or have other contaminants that could be harmful to a coin.

It is also necessary to use a soft absorbent cloth to remove any residual water. The cloth should be made of a fabric with fibers that will not scratch the coin.

If a toothbrush is used to scrub simple surface contaminants, it is suggested a collector use a soft-bristled style.

However, such brushing still may be harmful to a coin and impart scratches.

Using sharp implements to pick dirt away from a coin's surface is also a risky practice, but one that can achieve the desired results if the person doing it possesses ample manual dexterity.

Soaking

Coins removed from the ground often have encrusted dirt, with copper or copper alloy coinage the most susceptible to retaining the dirt not only in the devices, but on the surfaces.

Gold and silver coins are less likely to retain dirt residue, but silver issues are more likely to suffer the effects of pitting from the soil's mineralization.

Copper and zinc have a higher tendency to corrode or slowly dissolve over prolonged periods in soil.

"Soaking bronzes [and many pure copper coins] in olive oil is the oldest and most reliable and least destructive cleaning technique that I know of," said Beauford of the procedure accepted by many numismatists.

"You cannot leave an object in olive oil too long," he said. "This technique has been used for hundreds of years, and it is the only cleaning technique of which I have never heard a condemnation.

"It is, however, also the slowest technique I know of. You may not see any results for a month or more, and complete cleaning of an article can take many months.

"Some encrustations will not respond at all to this technique. It is, however, the only technique other than microscopic cleaning with hand tools that I personally will use on expensive or fine coins."

Beauford explains the cleaning process can be accelerated by warming the olive oil and keeping it at a constant temperature, but not too high, since olive oil is flammable.

"To use olive oil, place the object to be cleaned in olive oil in a closed container," Beauford explains. "To hasten the process, use a toothbrush to occasionally gently scrub the coin. Change the oil monthly or bimonthly."

In addition to Maltby, though, some warn that the use of olive oil can be harmful to the coin's surfaces.

In a March 27, 2007, online posting at http://ancients.info/forums/showthread .php?t=1190, user C. Hinton explains that olive oil has an acidity ranging from 1 percent to 3 percent depending on grade.

Olive oil gets very sticky when it dries, and it is good at infiltrating cracks and crevices and therefore hard to get off, according to Hinton.

"Its acidity, in theory, will continue to slowly eat away the coin long after cleaning if it is not eliminated or neutralized," according to Hinton.

"Extra virgin olive oil" is oil is from the first pressing of the olives with no chemicals or hot water added during processing. The acidity is up to 0.8 percent. About 70 percent of Greece's olive oil is considered to be "extra virgin."

"Virgin olive oil," also from the first pressing, is not of the same exceptional quality as the extra virgin oil. The acidity is up to 2 percent.

According to Hinton, olive oil has traditionally been the most common method of cleaning used on ancient bronze coins.

You can start off by soaking the coins for at least one week in olive oil.

If you can let them soak longer the oil will penetrate better and loosen more of the dirt, he writes.

After soaking the coins in oil, take them out and clean them off with some soapy water, which should remove most of the oil residue, according to Hinton.

According to the Web site, www.dirty oldcoins.com/restore/cleaning.html, olive oil can be used to help dissolve encrustations, dirt and other deposits.

"We usually mean soaking in olive oil in the lock-it-up-and-forget-it category of coin cleaning," according to the Web site. "However, dissolving can also mean the use of acids such as lemon juice or vinegar which are more properly used to clean silver coins.

"Other than this particular case, the use of acids is emphatically not recommended for the same reason as polishing. You are wearing away at the metal in the hopes of carrying off the dirt too. Bad, bad.

"An alternative to olive oil, though no quicker, is mineral oil," according to the dirtyoldcoins.com Web site. "The main advantage of mineral over olive oil is that it doesn't stain the coin dark. It is, in fact, a great way of preserving a freshly cleaned coin from future corrosion.

"Just apply a very light coat and the oil acts as sealer and protectant."

Online information

An online discussion forum at www.forumancientcoins.com suggests placing bronze or copper coins in a container of olive oil, preferably not virgin olive oil, and allowing them to soak for a while.

How long depends on how dirty the coins are, according to the Web site, and can take anywhere from a few weeks, to a few years.

On another Web site, www.ancient-roman-coins.com/coincleaning.htm, soaking in olive oil or mineral oil is suggested only in cases where soap and water fail in removing the dirt or encrustation.

"The down side is that the olive oil will darken the patina because of the caustic or acidic base," according to the Web site. "Mineral oil works as well and darkens the metal quite a bit less."

John E. Ryan, at www.geocities.com/brass_e_ryan/Brass.html, claims that cleaning ancient coins with an advance distilled water soak or olive oil treatment often softens deposits, thereby facilitating the cleaning with brass tools.

"If one is finding deposits resistant while cleaning, interrupt the brass tools cleaning with another softening treatment," according to Ryan.

"If you use olive oil, be sure to remove the oil with solvent, trisodium phosphate (TSP), or non-detergent Ivory Soap before resuming the tool work."

A similar treatment method can be found at www.farleyco67.com/How_to_clean_an_old_coin.html.

Extended oil bath

Soaking a dirty or encrusted coin in olive oil is one method suggested at www.romancoinscenter.com/Cleaning/Clean.php, although the site states that the decision whether to use it depends on the severity of the buildup.

"If the coins have the white encrustation that looks like lime rock, I drop the coin into olive oil for an extended soak of six months to a year or more," according to the Web site.

"In my experience the only thing that will react to the lime encrustations is olive oil.

"Olive oil does have the drawback that it will weaken some patinas after a moderately long soak (more than one or two months). I have read that mineral oil could be used also for this purpose, but I haven't tried it yet."

Romancoinscenter.com continues: "After a coin has soaked in olive oil for an extended time and I think it is time to pick on it again, I will remove it from the Olive Oil and give it a quick boil in water mixed with detergent (I use Gringotts #1), and then let it soak in the Gringotts #1 for a week or so, before I start picking on it again.

"This is the easiest and least messy way that I have found to get rid of the olive oil after a soak."

At http://ancientpeddler.blogspot.com/ 2007_02_01_archive.html, it is suggested to soak an encrusted ancient coin in olive oil for a minimum of 12 hours. Tri-sodium phosphate, which is suggested to remove olive oil residue, is also discussed as a diluted soaking solution. Precautions must be taken in using TSP or any similar compound, according to the Web site.

"The degree to which the coins can be cleaned using any method is contingent on the quality of coins you have purchased and the degree of encrustation," according to the Web site.

At his metal detecting page at www.njminerals.org/metaldetecting2.html, chemist Chris Thorsten explains different methods for cleaning excavated coins, including soaking in olive oil.

On one metal detecting trip, Thorsten explained he dug up an 1870s Indian Head cent and what appeared to be an early large cent, either a Liberty Cap piece or Draped Bust cent, but whose identity was indiscernible because of the dirt and encrustation.

Thorsten first soaked the coins in WD-40 for an unspecified period before placing the coins in separate containers holding olive oil.

"The first phase of oil soaking was in WD-40," Thorsten explains. "I just sprayed some into the container and saw dirt lifting away in just a few minutes. I let the coins sit for about eight hours. I removed the large cent and looked at it under a microscope while I used a toothpick GENTLY to tease away the cakes of dirt. I put the coin back in the WD-40. After about another eight hours, I did the toothpick treatment again, followed by a degrease and a prolonged soak in olive oil that hasn't ended yet. The final treatment will be a degreasing and a brief soak in distilled water."

Even the U.S. Mint's Web site at www.usmint.gov/kids/campCoin/collect orsWorkshop/coinCourse/04.cfm suggests using vegetable or olive oil to wipe common copper coins with a soft cloth if they need it, using a mild soap and water on nickel or silver alloy coins.

The Bank of Canada's Currency Museum at www.bankofcanada.ca/currencymuseum/eng/collection/become.php recommends never to clean rare coins or coins you intend to sell. "It is safe to clean some of the more common coins.

"To clean copper coins, rub them with vegetable or olive oil and wipe with a soft cloth. Surface dirt on a nickel or silver coin may be removed by immersing the coin in a solution of warm water and liquid dishwashing soap.

"Afterwards, rinse the piece thoroughly in clean water and pat dry."

Olive oil has been used for years by collectors who report good results. Whether to use it, however, is a decision of the individual collector, who should weigh the advice of others with the potential of reaching the desired result of removing debris without harming the coin.

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