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,
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.
Click on image to enlarge
EXTRA VIRGIN olive oil from the
first pressing of the olives has the lowest acidity level, usually
under 1 percent, among different qualities of olive oil. Generally, the
cheaper the price, the lower the quality and the higher the acidity
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
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
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).
Click on image to enlarge
COINS REMOVED from the ground often
have encrusted dirt, and copper or copper alloy coins are the most
susceptible to retaining the dirt not only in the devices, but on the
surfaces. Some collectors have used olive oil to help loosen the
buildup for subsequent removal.
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
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
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
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
"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."
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
"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
"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."
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
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
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
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.