"A clean coin, like a clean mind...
COIN CHEMISTRY and CLEANING
DO Understand the materials you are working with
DO Procure a Material Safety Data Sheet for the chemicals you work with. Always understand the hazards of each chemical.
DO NOT Mix acids and bases.
CLEANING: Don't do it!
NOTE: Most attempts at coin cleaning ruin a coin's condition and value. Unless you are very familiar with coin production techniques, metallurgy and wet chemistry, you should completely avoid cleaning your coin. Cleaned coins essentially become damaged or altered coins and justify much less demand from collectors, not just those alive today, but those in the next generations. An altruistic coin collector thinks not just of the immediate enjoyment of the coin, but the education and enjoyment of the coin over the next few hundred years. We are skin and bones, destined for a few short decades. Metal coins might stick around for a few thousand more years...
Many new collectors attempt coin cleaning with good intentions but poor knowledge and technique. Typical coin cleaning attempts include use of improper or harsh cleansers not meant for that particular coins chemistry or the physical wiping, cleansing, rubbing or scrubbing of the coin face which disrupts and alters the coin's natural surface, patina or toning. Often seen undesirable results include pits, unnatural fields or devices, pock marks, spots, scratches or corrosion on the coin.
When in doubt - leave it out! Unless you are quite certain what the end result of cleaning that particular coin will be, developed through hard-earned experience or study (and likely a few ruined coins in the process!), suggest you leave the coin in its current unmolested state until such time you are able to consult a more knowledgeable coin curator. Until that time, your best course of action is probably no action - leave the coin as is in the condition that first attracted you to it.
Those who can competently restore a coin do not clean the coin; rather they curate the coin. Think of the great Dutch master paintings. They are restored or curated from a damaged or dirty position to one more resembling the physical state exhibited when the painting was new. Cleaning merely alters the surface of a piece of art through, even if imperceptible at first, damaging physical and chemical alteration.
Fred Reinfeld, in his book: How to Build a Coin Collection. Revised (7th) edition, sums it up nicely:
"Since the results of cleaning cannot be accurately foreseen, and since the experts even disagree on the advisability of cleaning, it is best to refrain".
Cleaning: If you must do it...
IF (*when!) you decide to ignore our sage counsel and commence on your home coin chemistry experiment in the mother's/wife's kitchen sink or oven, a few topic-tips that follow may prove useful. When starting out an Use low value common and well circulated coins
Debris and Dirt
Do not use a stick, rose thorn or toothpick to remove grease, grime, dirt, or other verdigris from a coin surface. You are likely to scratch the coin. The material you remove will also leave behind a portion of the coin surface that looks different than the rest of the surface - providing an unwanted distraction. Typical dirt that collects in a coins surface devices can sometimes be satisfactorily removed by laying a film of Vaseline with a cotton swab or Q-Tip or very fine copper brush or a rose thorn followed by a gentle dabbing, not rubbing, of the coin surface with a dedicated (one use only) very soft and lint free (electrostatic) cloth. Soaking a coin overnight (or longer) in olive oil or warm water or a potatoes or coca cola is another fairly innocuous cleaning method for minor dirt adhering to a coin. Generally, you are trying to counteract the conditions that fouled the coin surface. For instance, had basic soil conditions created the issue, counteracting with acid treatment (coca cola, muriatic acid) may work; or not. Do not use any of these tips on a coin that holds high numismatic value. Unless you are both trained and experienced, DO NOT use acid-base chemistry to make your coins look prettier such as use of: baking soda, lye, potash, clay, vinegar, acetic acid, muriatic acid, sulfuric acid, coca cola etc. since these methods will destroy the numismatic value of your coin and potentially cause you health or safety problems.
Just blow dust away with your breath. Do not use forced air (can air, nozzle, hair dryer) as it can cause abrasions on the coin surface.
Do not use available commercial silver and jewelry polishes such as tarnish remover. These will destroy a coin's surface patina and toning and thus its value. Coin dealers may clean tarnished coins in a process known as dipping. Do not attempt this process yourself as improper dipping removes so much surface metal that the coin finish is destroyed.
After cleaning, use distilled water instead of tap water as a rinse since tap water (unless filtered using reverse osmosis) will leave spots and mineral deposits from the minerals naturally occurring in the water source. Let air dry; never use a cloth to rub a coin dry. Acetone or 99% isopropyl alcohol are used as a rinse for certain coin metals but the acetone bath must be done right. Use high quality Acetone (not the stuff you will find at Target or Walmart). See Acetone below.
Never touch any surface of a proof coin. These finishes command a high numismatic premium. Any rubbing, polishing or other touching will ruin the surface.
Collectors attached a premium for attractively toned coins. Regrettably, this premium has convinced unscrupulous individuals to doctor up coins with artificial toning for their own benefit. Also, some natural toning is simply unattractive. For this category coin, a quick 'dip' may remove the ugly tone. Additionally, professionals that have proof coins with 'haze' in the mirrored field will also dip the coins. Essentially, dips are solvents that remove a very thin layer of steel. As such, the coin is essentially now a cleaned coin. Nevertheless, a large percentage of the coin industry considers 'dipping' to be the only 'market acceptable' cleaning method. All others are considered verboten, and their detection by the Third Party Graders typically will result in the coin being returned in a 'body bag': an un-certifiable coin.
Coin Chemistry at the Coin Mine
When a silver coin
is newly minted, the force of the press forces the silver into the details of
the die and collar, creating a "fresh" silver surface. As soon as it leaves the
dies, the coin is exposed to different environments. If hermetically sealed, for
example in inert plastic and left in a very stable place, such as a temperature
and humidity controlled vault containing inert gas (or vacuum), it will
theoretically stay bright and white forever, since it will have no elements to
Once the coin leaves the mint, it undergoes various chemical and physical reactions when exposed to the atmosphere. Some of the atmospheric compounds that can react with metals include Carbonyl Sulfide (COS), Hydrogen Chloride (HCI), Hydrogen Sulfide (H2S) and Sulfur Dioxide (SO2). These gasses form naturally and occur as byproducts from decomposition or natural or synthetic materials.
Physical processes include abrasion corrosion, rust, spotting, mold and mildew, and exposure to non-gaseous environmental factors such as heat and sunlight. Some processes are both chemical and physical, such as rubbing a coin with ones's thumb or forefinger. This rubbing produces a physical effect by softening the details of the coins strike to a very small degree and also a chemical process that occurs when human fats and oils interact with the metallic surface of the coin.
Oxidation is just one chemical process that occurs when a metal is exposed to a gas. Early chemists thought that oxidation occurred when a metal was exposed to oxygen (and hence the name oxidation) although oxidation in the chemical vernacular refers to the process where a compound undergoes a reaction that removes an electron.
For our purposes, coin oxidation develops an oxide layer on the coins surface (and also on the subsurface). Electrolysis removes the surface oxide layer (or other by-products of oxidation such as carbide, hydride, halide, nitride and sulfide) . Commercially available dips such as Jewel Luster also remove a coin's surface oxide coating. However, the next effect of oxidation, like many cleaning techniques, is that a small amount of a coin's surface metal is removed. This will remove the original surface applied at the mint and the patina which has developed over time. Loss of one or both of these feature will reduce a coins eye appeal and value.
Although the oxidation reaction with silver produces a protective oxide barrier, not every oxide is protective. For instance, rust (Iron oxide) is corrosive, not protective. Cathodic protection is used to prevent rust corrosion on metal surfaces by offering a sacrificial anode for the corrosion to attack. On the other hand, zinc oxide is used to coat steel (galvanized) and iron to prevent rust oxidation. The oxide layer on silver, though protective, is not very thick and therefore allows light to penetrate the oxide layer and reach the coin surface producing luster color, luster and patina.
'Dipping' is the process of using an acid as a proton donor which is more reactive than silver. In other words, the acid removes the ugly oxidized layer. As such, the sulfur present in the Silver sulfide layer is preferentially selected to react with the dip. Many of the commercially available coin cleaners such as E-Z-est coin cleaner etc. has a very low pH, often 1.5 or 2. However, one must realize that dipping is a chemical reaction that can reduce the visual pleasantness of the coin. For example, dippping might remove original flow lines, trace elevations of metal lines on the surface of a coin produced when the mint dies strike so hard that a very fine molten metal film flows away from the strike in radial flows. These flow lines produce the original luster that collectors like to see on a coin. Patina is, essentially the chemical signature of the coin, the sum of the chemical processes the coin has undergone since being exposed to the various conditions - atmospheric and otherwise - in the world.
Removal of patina reduces the eye appeal, and thus collectability and value of a coin. Repeated dipping or other chemical alterations can produce a washed-out looking coins that is dead to the eye. Additionally, professional grading surfaces will not provide certification or grading on these coins.
Acids have a pH's of less than 7 when in an aqueous solution. There is no measurement of pH unless a compound is in aqueous solution. (For example, gasoline does not contain water and therefore has no pH). A pH less than seven is acidic, which means there are excess hydrogen ions, or protons, in solution. Protons in solution will bond with water (H2O) to form hydronium ions (H3O+).
Solutions may exist with charges but gasses do not, except under unusual conditions where they are called plasmas. In some instances, hydrogen ions (protons) will bond to another ion other than water. For instance, when a basic (pH over 7.0) solution is combined with the acidic solution, the proton will bond with an hydroxyl ion (OH-) existing in the basic solution. Hydroxyls and hydrogen combine to form water. Now, if we add a metal to the solution, the solution might strip an electron to form hydrogen by combining with protons in solution (hydrogen ions). This metal ion might not be stable and search for another proton to become neutral and thence drop out of the solution as a solid precipitate. One must be very careful when forming metal precipitates in solution since metal salts could be produced. Examples of metal salts that are produced through coin chemistry are gold chloride (hydrochlorauric acid) and silver nitrate. Metal salts are used in a wide variety of industrial applications. For instance, Barium Carbonate is a rodenticide. Although metal salts can be very dangerous they also have many helpful attributes and find wide applications such as printing and photography.
Copper, like silver, is a group 11 transition metals. However copper has significantly different reduction potential and therefore will react with many more solutions than will copper. That is why copper coins typically exhibit much more environmental damage than silver coins such as corrosion, pitting and the 'green gunk'.
For many collectors one of the features that adds eye appeal is the toning, or addition of color to the coins surface, over time.
The color exhibited by a coin is related to the thickness of the thin oxide layer on the coin surface and the wavelength of light that is reflected off the surface. Some of the light is reflects off of oxide surface back to the viewers eye, hence light is seen. The remaining portion of the light penetrates the the oxide layer to the physical face of the coin and then reflect back to the viewers eye. These different distance that the light travels is enough to create various hues visible to the naked eye. Some light may only make it part way through the oxide layer before reflecting back. Sometimes the various wavelengths combine as either constructive or destructive forces to either reinforce or cancel out certain colors. These distances of the oxide layer through which light wavelengths pass are so small that they are measure in angstroms, or nanometers (10 angstroms). Visible wavelengths with varying frequencies produce various colors.
Different wavelengths represent different spectrums of the energy band. The length of the wave, the distance between two peaks formed in a sine wave via oscillation, is used to define the energy of the wavelength. Only a portion of the energy band is visible as light to the human eye. Energy wavelengths that oscillate slower than visible light toward the red end of the spectrum include infra-red and radio waves. Waves beyond the other end of the spectrum include ultraviolet light, and radiation waves such as X-rays and gamma-rays. Radio wavelengths are measured as the number of cycles the wavelengths complete over a fixed period of time (hertz), and are experienced when hitting the human eardrum as sound. Each wavelength is a separate sound. For example, frequencies that resonate at 440 cycles per second are defined as the musical note A.
There are ways to determine the actual metallic composition, even of layered clad coins, using SEM-EDX testing.
See more on Toning.
Use of some treatment, or any - depending upon your personal integrity and professional mettle - may trigger the requirement to provide notice that your coin was worked on or otherwise modified; that the coin no longer has original surfaces.
Coin Cleaning and Curation Reference
Acetone - 100% acetone can be a valuable tool in the right hands for the right job. The right job may include detection of fake mint marks that have been added with epoxy or in cleaning the surface of the coin for haze or recent fingerprints. Acetone is a solvent, not an acid, so it does not remove metal from the coin. However, it can leave a brownish looking haze, especially along the rim, when not rinsed properly. That is, after an acetone bath - which should only take a couple seconds, the coin must then be rinsed immediately in distilled water. Distilled water is essential because we want none of the impurities, such as can be found in natural water, to remain on the surface of the coin - after all, that is the reason we dipped it in the first place. Although certain very pure alcohols can also be used as a post-acetone bath, this type of chemistry is more complicated; so keep it simple with distilled water.
Use a glass sealed container (preferably with a wide mouth so you can reach in the jar to get the coin back out by hand). You could fold up a napkin and line the bottom of the jar which will prevent the nickel from getting nicked by friction against the glass side of the jar.
The coin should then be air dried (not forced air); or, after the bath, pat (not rub, not wipe) the coin dry using tissue paper.
Dipping in acetone is done to remove plastic from a coin. Types of plastic such as PVC may form residue on a coin from improper storage or environmental conditions. Since acetone is an organic solvent, it will only react with plastic (or other organics) and NOT the coin. Use of 100% high grade acetone is essential; it's stored in metal pint or quart tins in the paint section at the hardware store, not at Walmart or Target.
After using the acetone, consider re-use for other original purposes, but do no re-use to bathe other coins. Dispose of properly, such as at the local county hazardous waste amnesty day run by the local Environmental Health Department.
CAUTION: like all organic solvents, Acetone is highly flammable. Acetone is also highly volatile. Give the product some respect, use only in areas with adequate ventilation and without any external ignition or flame source. Excessive Acetone exposure may affect your health.
Aluminum - Fake coins consisting of aluminum occasionally enter the marketplace.
Ammonia - Sudsy ammonia applied on a cotton swab q-tip can remove old dirt film from a coins surface. This is especially helpful on XF and AU coins whose luster has been obscured over time.
Artificial Toning - see Toning.
Baking Soda - Sodium bicarbonate often provides outstanding cleaning in the household. Use on coins provides much more mixed results. Nevertheless, some types of gold coins, when bathed (not rubbed on the coin) in a light paste of bicarbonate, will not leave the standard telltale cleaning signs of hairlines under magnification. However, the coin will take several years to tone and allow the return of a more natural appearance.
Blue Ribbon "Profession Coin Conditioner and Preservative". A commercial coin cleaner marketed mostly for copper coins. An oil with solvent (reportedly ether) mixed in. People report various results. The product is no longer made. Application, via rolling (not rubbing) a q-tip would typically be conducted to 'condition', or preserve, a copper coin.
Carbon Spots - Some carbon spots (mostly associated with gold or copper) can be removed via application of Nic-a-lene or Jeweluster onto the end of a softened toothpick.
CARE - A commercial coin
cleaner marketed mostly for copper coins. An oil with solvent
(reportedly ether) mixed in. People report various results.
The product is no longer made. Application, via rolling (not rubbing) a
q-tip would typically be conducted to 'condition', or preserve, a copper
Since copper comprises almost 10% of many silver coins, understanding copper chemistry is also important. Copper has also been used in most gold coinage. Acetone can work on certain copper coins.
This site contains a fair amount of information on copper chemistry:
Cyanide - Cyanide has been used very effectively to mine and treat metals. Years ago coppersmiths treated copper with potassium cyanide. Potassium cyanide has also been used by coin counterfeiters in attempts to add a matte-like finish to cupro-nickel coins. Current uses primarily occur in the metal etching and plating business since cyanide in combination with strong acids can keep precious metals in solution.
Working with cyanide can be perilous. Cyanide salts are particularly dangerous. For example, sodium cyanide is used in the gas chamber because when sodium cyanide, or any other cyanide salt is mixed with a strong acid, hydrogen cyanide is released. For this reason cyanide salts are also poisonous when ingested, because the acid in your stomach will also liberate hydrogen cyanide.
Deller's Copper Cleaner - Appears to be petroleum jelly and sulfur powder.
Electoplating - Electroplating is the deposition of a metallic coating onto an object by putting a negative charge onto the object and immersing it into a solution which contains a salt of the metal to be deposited. The metallic ions of the salt carry a positive charge and are attracted to the part. When they reach it, the negatively charged part provides the electrons to reduce the positively charged ions to metallic form.
Electrolysis - For our purposes, coin oxidation develops an oxide layer on the coins surface (and also on the subsurface). Electrolysis removes the surface oxide layer. Commercially available dips such as Jewel Luster also remove a coin's surface oxide coating. However, the next effect of oxidation, like many cleaning techniques, is that a small amount of a coin's surface metal is removed. This will remove the original surface applied at the mint and the patina which has developed over time. Loss of one or both of these feature will reduce a coins eye appeal and value.
One method of electrolysis includes placing a metal plate in a container of water, then placing ones silver on top of the submerged plate. The silver will un-tarnish over time.
Electrolysis depends upon electrochemical potentials. The difference in electrochemical potentials between silver sulfide and aluminum sulfide, where an electron transfers FROM neutral aluminum through a paste solution such as sodium carbonate TO the silver sulfide, reduces the silver and frees the sulfur ion. The free sulfur ion reacts with aluminum foil to form the aluminum sulfide compound.
This process works well for silver, not so well with silver alloys. Additionally, the effectiveness depends upon presence of other oxidation compounds and hydrides on the coin surface.
Electrowinning - From Finishing.com (http://www.finishing.com/faqs/howworks.html)
eZest - A chemical dip; likely a diluted solvent. Not recommended whatsoever.
Gold - Jewelers use a commercial gold jewelry cleaner called 'Jeweluster' that has thiourea as an active agent. Ammonia will also suffice.
Grease - Light grease and putty will often successfully hide marks and surface flaws such as small planchet defects and scratches. The use requires a cautious touch, but the technique can be quite successful. Oil based grease and putty can be removed and hence typically do not permanently damage the coin. However, when done wrong - the look proves quite distracting and will seriously lower sale price. Of course, the technique also should prevent third party grading and instead garner a body bag for altered surfaces.
Halogen lamp - AKA Bankers Lamp. Halogen light is used by coin collectors to determine if lines seen on a coin face are obtuse (raised) or incuse (gouged). Raised lines indicate die polish present as the coin left the mint and do not detract from the coin. However, incuse lines indicate potential cleaning, 'whizzing', or other surface alterations which do detract from the value, if not the beauty, of the coin.
Hydrogen Sulfide - One of several gasses that occasionally become trapped in a plastic slab and cause toning over time.
Liver of Sulfur - Solid potassium sulfides. These will readily oxidize copper and silver. Dissolve a small lump of liver of sulfur in a couple of cups of hot water and then dip the coins in this solution. You might keep the solution heated (185 or so), this will aid oxide formation. If the coin has previously been 'dipped' (a thin layer of metal of the coin has been stripped with sulfuric acid), you may have to wait even longer. Your goal in this process is to have the coin darken, to replicate the 'aged' look - which is natural toning. Next, dip in tap water mixed with baking soda with a final rinse in de-ionized water. You could pat dry with jewelry clothe, or use rottenstone or baking soda paste on a dampened q-tip. Now very lightly rub excess black tarnish from the higher devices of the coin.
Mineral Oil - Light viscosity oils will mechanically loosen dirt and grime off of a coins surface. Unfortunately, they will darken the coins patina if the oil is allowed to 'sour'. All hydrocarbons begin a lacquer process over time, you don't want to use weathered oils nor do you want to leave the oil on a coin too long. Experiment with times, starting with just a few hours building up through days and weeks. Mineral oils can leave dull, hazy finish on coins.
Mineral Spirits -
MS 70 - A commercial coin cleaning product; likely a medium soap.
Polyvinyl Chloride (PVC) - Use an acetone bath; air dry or very light cotton swap pat dry. Rinse with de-ionized water. Another technique is to use 1:10 diluted ammonia followed by a soapy/hotwater bath (for an hour or so). Use Dawn dishwashing liquid for the soap. Sometimes adding windex to the ammonia also helps (additional surfactants). PVC residue on bagged coins may result from storage in a plastic-line bag (don't do this).
Elemental Silver, unadulterated in its pure form, is a semi-precious white metal. Silver is quite is a an excellent conductor of both heat and electricity and is very ductile and malleable. Silver is highly reactive.
In chemical reactions silver forms Ag+ ions and a compound. The compounds are typically formed with the halide element of (Fluorine, Chlorine, Bromine, Iodine) and hydrogen sulfide or sulfoxides to form "silver salts". These compounds exhibit different physical properties, such as adsorption and reflection of light, and therefore appear different that pure silver - typically darker. Two type of reactions affect silver coinage: formation of films and coin surface alteration due to contact with the atmosphere. Contact with the atmosphere or the fats and oils in a human hand will create an oxidation film.. Additionally, the oxidation reaction will consume some silver from the surface of the coin.
The first neutral, molecular silver(I) alkoxide carbene complex!
Painting and Waxing - Use of a light polyurethane spray is a rather new (2006) form of coin doctoring that is hard to spot initially. The techniques require practice, technology and technique and as such aren't quite that common yet. Coin doctors often use this coin alteration method to hide or cover hairlines on a coin, especially proof coins.
Solvents - DCE, TCA
Scratching - For moderate to
moderately heavy scratching, Use 50 g/l of thiourea in about 1-2%
sulphuric acid on a trial basis. Heavy scratching, in the past, has been
ameliorated with putty. (When that nasty scar across her cheek just WON'T go away).
Solder - For removing solder from a coin used as old jewelry, consider using a copper braid to produce flux that will then absorb the old solder.
Thioureic acid - Used in products such as 'Jewel-Luster' et al.
Tri Sodium Phosphate -
Toning - Toned coins show various colors reflected in their fields or devices as opposed to an appearance uniform in color. Beware of artificial toning gained through nefarious application of heat or chemicals to produce toning. This gentleman provides a very good write up on the science of toning:
Ultrasonic Cleaners - These mechanical devices use sound waves to mechanically vibrate soil and grime off of surfaces. They do work but can have unintentional consequences, such as leaving a drab and pale surface appearance on your coin. One thing to consider is placing a coin at the bottom of the container on top of a small piece of gauze. This way, when the ultrasonic cleaner is turned n the coins will vibrate against the gauze surface, not ht glass surface. This will reduce the large number of micro marks on the coin pick-up points (the raised features); these micro marks combine to look like polish - which detracts from the coin appearance.
Xylene - Can be used to clean light corrosion on copper, using a fine horsehair brush for application. Ortho and Meta Xylene are both suspected carcinogens so use in a well ventilated area.
Question: Someone gave me a bunch of Buffalo Nickels that looked like they were cleaned with Tabasco Sauce. Why did they do that, and what should I do to make them look more natural?
He used Tabasco b.c. he only partially knew what he was doing and b.c.:
1) Contains vinegar, a weak acid
2) Contains capsaicin and/or other astringents
Both acids and astringents are used to ‘etch’ metal. In the case of the buffalo nickel people use acids to bring out the date, known as acid dates.
To understand how to remove an item, you must understand the chemical properties. The first place to go is the MSDS for each:
The way to neutralize an acid is to either dilute (using neutral) or modify (using basic/alkali).
Using malt vinegar won’t work that efficiently, since you are using a vinegar to work on a vinegar. Now, that is fine if you are using a stronger acid to remove a weaker acid, but you still need to neutralize and remove the acid, which requires neutral or base.
So, you need to conduct a physical removal process, such as electrolysis, or a chemical acid/base chemical reaction. If you aren’t that familiar with chemistry probably not safe for you to do your own acid/base chemistry.
My suggestion is soap and water, as you said, and artificially re-tone by storing (for a long time – with added heat – against sulfur paper.
But, just for kicks. These buffs are only worth several cents each – no monetary reason to further mess with the coin.
Good luck. Be safe. Have Fun.
Coin Preservation Handbook. Charles Frank. Published by Coinguard Industries. 1964. Hardcover. Fine. Very minor sulfur staining inside back cover from where old paper was stored; small old price written on inside cover. Hard book to find, important reference work. Reference.
Methods of Chemical and Metallurgical Investigation of Ancient Coins (edited by E. T. Hall and D. M. Metcalf, 1972),
Copper spots: Since US gold coins are a cu/au alloy, many have a copper spot - areas of copper toning. Look for these as indication of authenticity.
Cast Forgeries: Most forgeries are cast forgeries, unsophisticated and easily (usually) detected and therefore the mot common type of coin fraud.
Cast forgeries typically have bubbles. Realize that a cast forgery, even a high quality forgery with just minimal bubbles and very good diagnostic and design detail, will likely be slightly undersized and underweight. Weight is a major determining factor for authenticity. Die-struck counterfeits are the most common type of gold counterfeit.
The most experienced coins dealers that I have done business with have reported that he least counterfeited gold bullion coin is the US Gold Eagle followed by the Canadian Maple Leaf and then the SA Kreuggerand. The most commonly counterfeited gold bullion coins are the British Sovereigns (die-struck) and Middle Eastern coins including Saudi Guineas, and many Turkish and Iranian gold coins.
The first place to begin when ensuring authenticity of gold bullion is to check size, weight and mass. A Fisch detector proves most valuable here as it measures both mass and size (called dimensions for coins) ; use the Two-Rand Fisch to authenticate sovereigns.
Unfortunately, one thing we need to start looking out for are gold coins that are die stamped. The core is tungsten core and the plating is gold. These can be machined to the right specs and therefore extremely difficult to detect by density or rotational inertia. However microscopic analysis, ultrasound and X-ray with thermal conductivity may work at detection methods. Tungsten has historically been both and expensive and difficult to turn into coin-worthy planchets. As the Chinese increase their sophistication and mastery of machine and milling techniques, this will increasingly become a more vexing problem.
One way to side step the issue, and another reason to buy smaller denomination coins, is to avoid one-ounce coins in favor of half or quarter-ounce bullion coins.
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