There’s something captivating about an old coin glinting like new. Whether it’s a dusty penny pulled from a garage drawer or a century-old silver dollar discovered in a metal detecting session, the urge to restore its original shine is universal. But when it comes to coin cleaning, collectors tread a fine line.
For numismatists, cleaning is often a taboo subject. A single scratch or altered patina can dramatically reduce a coin’s value. Yet outside the collector world, many people are simply trying to make old coins presentable for display, resale, or casual enjoyment.
That’s where the ultrasonic cleaner enters the conversation. Designed to remove dirt from intricate surfaces without abrasive contact, ultrasonic technology sounds like the perfect solution. But can it safely be used on coins? Or will it damage more than it helps?
Let’s dig into the science and nuance behind ultrasonic coin cleaning—where it works, when it doesn’t, and how to do it right.
What Is an Ultrasonic Cleaner and How Does It Work?
Ultrasonic cleaners are precision devices that use high-frequency sound waves—typically ranging from 20kHz to 200kHz—to create microscopic bubbles in a liquid bath. This process is known as cavitation, and it’s surprisingly powerful. When these bubbles collapse, they release energy that dislodges dirt, grime, oils, and other contaminants from the surface of objects immersed in the bath.
The Principle Behind Ultrasonic Cleaning
What makes ultrasonic cleaning unique is that it reaches into the smallest crevices and tight spaces—a key advantage for items with intricate surfaces like jewelry, surgical tools, and yes, coins.
Here’s how the process typically works:
- You place an object (in this case, a coin or group of coins) into a basket suspended in the cleaner’s tank.
- The tank is filled with a cleaning solution—often water with a mild detergent or specialized solvent.
- Once the unit is turned on, transducers beneath the tank emit high-frequency vibrations.
- Cavitation bubbles form and implode, generating micro-jets of cleaning energy that clean surfaces thoroughly and evenly.
Because ultrasonic cleaning doesn’t rely on mechanical scrubbing or harsh chemicals, it’s often seen as a gentler option—one that removes debris without grinding or scratching. However, that doesn’t mean it’s risk-free.
Are Ultrasonic Cleaners Safe for Cleaning Coins?
This is the question that divides collectors, restorers, and casual users alike.
Technically, yes—coins can be cleaned in ultrasonic cleaners. Many hobbyists and even some commercial operations use ultrasonic devices to remove dirt, oil, and residue from coins. In fact, when used properly, ultrasonic cleaning can be much safer than aggressive hand-scrubbing or dipping in strong chemicals.
But the key phrase here is “used properly.”
The composition and condition of the coin determine how safe ultrasonic cleaning really is. Coins are made from a wide variety of metals, including:
- Copper and bronze, which are soft and prone to surface pitting.
- Nickel and clad alloys, used in modern circulation coins.
- Silver, which can tarnish and react to certain cleaning solutions.
- Gold, which is soft and easily scratched but chemically stable.
Ultrasonic cleaning is generally less abrasive than physical cleaning methods, but it can still cause surface dulling, loss of patina, or even micro-abrasions on certain metals. Especially with antique, collectible, or numismatically valuable coins, any cleaning at all—ultrasonic or not—can reduce their value in the eyes of dealers or grading services like PCGS or NGC.
Cleaning ancient bronze coins
On the other hand, if you’re dealing with:
- Dirty modern coins for resale in bulk,
- Corroded metal detecting finds, or
- Coins with encrusted grime or oil from decades of neglect,
Then ultrasonic cleaning may be not just safe—but ideal.
Coin cleaning
When to Use Ultrasonic Cleaning on Coins
While ultrasonic cleaning can be effective, its value really depends on your goals for the coin. If your intention is to restore a coin to perfect shine regardless of collector value, ultrasonic cleaning is a practical solution. But if you’re preserving history or hoping to have a coin graded professionally, you’ll need to tread carefully.
There are clear scenarios where ultrasonic cleaning shines—no pun intended.
One such case is with modern circulation coins that are simply grimy from years of handling. These coins often have little numismatic value and are cleaned not for preservation, but for appearance, resale, or use in collections and displays. For example, coin dealers who resell bulk coin lots on marketplaces like eBay or Etsy often use ultrasonic cleaners to remove built-up dirt and fingerprints.
Another scenario is when coins are recovered from underground or underwater environments, like metal detector finds or excavation sites. These coins tend to be encrusted with dirt, mineral deposits, or corrosion, making manual cleaning slow and ineffective. In these cases, ultrasonic cavitation can reach under layers of grime without further damaging the coin’s structure—provided the solution and settings are carefully chosen.
Even foreign currency or old souvenir coins from travel collections can benefit. Many of these coins carry sentimental rather than financial value, and ultrasonic cleaning can help revive their shine without much risk.
However, there’s a golden rule: if you suspect a coin has significant historical, collector, or resale value, consult a numismatist before cleaning it at all. Many valuable coins have been accidentally devalued by well-meaning owners armed with cleaning tools and good intentions.
Best Practices for Cleaning Coins in an Ultrasonic Cleaner
So you’ve decided your coins are suitable for ultrasonic cleaning. Great—but how do you do it safely and effectively?
The first thing to understand is that ultrasonic cleaning doesn’t require intense chemicals or high-powered settings to work. In fact, gentler is better.
Start by choosing your cleaning solution carefully. Plain distilled water often works well, especially when paired with a small amount of non-abrasive, neutral pH detergent. Avoid bleach, vinegar, or strong acids—they can discolor or corrode certain metals, especially copper-based coins.
For frequency, 40kHz is considered the standard for general ultrasonic cleaning and works well for coins. If you have access to a multi-frequency machine, you can experiment with higher frequencies like 68kHz for gentler action, especially for soft metals or delicate patinas.
Cleaning duration should also be controlled. A few minutes—typically between 2 to 5 minutes—is usually enough. Prolonged cycles can begin to etch the coin’s surface, especially with repeated sessions. If coins are very dirty, it’s better to clean them in shorter intervals with fresh solution rather than one long run.
Temperature control matters, too. Ultrasonic action is more effective at warm temperatures, but don’t exceed 40°C (104°F) when cleaning coins. Higher heat can soften protective coatings or accelerate unwanted chemical reactions with the coin metal.
If you’re cleaning multiple coins, place them in the basket so they don’t overlap or touch. Cavitation can cause them to vibrate against each other, resulting in nicks or scratches.
After cleaning, rinse the coins in clean distilled water and pat them dry with a soft, lint-free cloth. Avoid air-drying unless you’re in a humidity-controlled space, as tap water residue can leave mineral spots or water stains.
Coins You Should Never Clean Ultrasonically
As tempting as ultrasonic cleaning may be, it’s not for every coin.
Rare, antique, or professionally graded coins should never be cleaned ultrasonically—or by any method—without expert guidance. Even if ultrasonic cleaning doesn’t visibly scratch the surface, it can subtly alter the coin’s luster or remove important patina that contributes to its historical value.
Grading companies like NGC (Numismatic Guaranty Company) and PCGS (Professional Coin Grading Service) often classify cleaned coins as “damaged,” significantly lowering their market value—even if the cleaning job looks flawless to the naked eye.
Also, be cautious with coins made from soft or reactive metals, such as:
- Copper: Easily discolored or pitted
- Brass: May lose surface sheen or oxidize
- Aluminum: Can be etched by cavitation or cleaning solutions
Coins that have lacquer coatings, gilding, or paint—often seen in commemorative or souvenir coins—may suffer peeling or clouding in the ultrasonic bath.
And finally, if a coin has any signs of structural compromise—cracks, corrosion holes, or lamination issues—ultrasonic vibrations may worsen the damage.
If in doubt, treat those coins as museum artifacts, not DIY cleaning projects.
Collector and Expert Opinions on Ultrasonic Coin Cleaning
In the world of coin collecting, few topics ignite as much debate as cleaning. For traditional numismatists, even the gentlest cleaning is often frowned upon. But in practical terms, expert opinions vary—especially when ultrasonic cleaning is done with care and clear purpose.
Numismatic organizations like the American Numismatic Association (ANA) generally advise against cleaning collectible coins at all, noting that even minor alterations to a coin’s surface can reduce its grade and resale value. According to the ANA, “A coin’s surface tells its story—its journey through time and circulation. Cleaning removes that story.”
But outside of the collector’s cabinet, ultrasonic cleaning has gained support in academic, conservation, and commercial settings. For example, archaeological teams often use ultrasonic baths to safely remove soil and calcium deposits from large hoards of coins, especially when the goal is identification rather than valuation.
In metal detecting and hobbyist forums like CoinTalk or TreasureNet, users regularly report success with ultrasonic cleaning for coins that have been buried for decades. Many emphasize using distilled water only, and avoiding harsh cleaning agents that can leave behind damage invisible at first glance.
One of the most cited authorities on the subject is The British Museum, which has published guidelines on cleaning historical coins, stating that mechanical or ultrasonic cleaning may be used for thick encrustations—only when the historical context has been preserved or digitally recorded.
So while numismatic purists may still recoil at the idea, there is growing acceptance that ultrasonic cleaning has its place—especially for coins with practical or educational, rather than monetary, value.
Comparing Ultrasonic Cleaning with Other Methods
To understand why ultrasonic cleaning has grown in popularity, it helps to compare it with other coin-cleaning methods.
Manual scrubbing, for instance, is time-consuming and risky. Using brushes or cloths to remove dirt often causes micro-scratches, particularly on soft metal surfaces like silver or copper. Even cotton swabs can leave marks under pressure.
Chemical dips, another common method, can be even more destructive. Products containing thiourea or acid may strip away oxidation and dirt but often remove protective patina or leave the coin with an unnatural shine. Worse, chemical residues may linger and corrode the coin over time.
Electrolysis—popular in extreme restoration—uses electric current to remove corrosion. While effective for cleaning ancient or heavily encrusted coins, it’s notoriously unpredictable. The process can strip entire layers of metal, leaving coins pitted or uneven in color.
In contrast, ultrasonic cleaning offers a middle ground. It provides deep cleaning without physical contact and can be tailored to different metals and contamination levels using variable frequency, heat, and solution chemistry.
In fact, professional conservationists often recommend ultrasonic cleaning as a first step before any invasive methods are attempted. If used correctly, it can restore a coin’s surface gently, preserving its structural integrity.
The Long-Term Effects: Will Ultrasonic Cleaning Devalue My Coins?
This is the question that matters most to collectors. And the answer depends not just on the cleaner—but on the coin and context.
For modern, common coins, ultrasonic cleaning will likely not affect value. If anything, a clean, shiny appearance may improve resale or appeal. Many bulk coin dealers routinely clean batches of coins to make them look more presentable in mixed lots.
However, when it comes to vintage, historical, or professionally graded coins, any cleaning—including ultrasonic—can result in lower value. Coin grading companies are extremely sensitive to signs of cleaning. Even if the surface remains unmarked, changes in toning or luster may trigger a “cleaned” designation on the coin’s certification label.
Ultrasonic cleaning also carries the risk of altering surface chemistry. Even gentle cavitation can affect oxidation layers or interact with previous coatings, leading to dullness or slight textural shifts. These changes might not be obvious to the naked eye but will be noticed by experienced collectors or appraisers.
That said, the risk is largely avoidable if:
- You clean only coins without numismatic or collector value
- You use gentle solutions and short time intervals
- You avoid excessive heat or strong chemicals
- You test on a low-value coin before cleaning rarer pieces
In short, ultrasonic cleaning won’t devalue a coin if it has no collectible value to begin with. But if a coin might one day be graded or resold to a serious collector, it’s best left untouched.
What Types of Ultrasonic Cleaners Are Best for Coins?
Not all ultrasonic cleaners are created equal—especially when it comes to coins. Choosing the right machine can make a big difference in both cleaning performance and the preservation of your collection.
For coin cleaning, bench-top ultrasonic cleaners with a tank capacity between 1.5 to 6 liters are ideal. These units are large enough to clean multiple coins at once, but compact enough for desktop or home use. Brands like Branson, Elma, and Yunyisonic offer lab-grade units that include frequency tuning, degassing, and digital timers—features that provide more control and safety.
If you’re working with highly detailed or fragile coins, opt for a cleaner that operates at higher frequencies, such as 60–80kHz. These frequencies produce smaller cavitation bubbles, which are less aggressive and better suited for intricate surfaces or aged metal.
On the other hand, if you’re cleaning modern, circulation-grade coins covered in stubborn grime or soil, a standard 40kHz unit will do the job well. For large-scale operations, such as dealers who handle bulk lots, industrial-grade ultrasonic tanks with heater and filtration systems may be worth the investment.
And don’t forget accessories: a coin basket made of stainless steel or fine mesh will prevent your coins from resting on the bottom of the tank, where vibration intensity is highest and most damaging.
How to Safely Store Coins After Ultrasonic Cleaning
Cleaning your coins is only half the story. What you do after the ultrasonic bath matters just as much—especially if you want to prevent re-tarnishing or corrosion.
Start by rinsing your coins thoroughly with distilled water after ultrasonic cleaning. This removes any remaining cleaning solution that might react with air or metal. Tap water should be avoided for final rinsing, as it can contain chlorine, minerals, or impurities that may stain or damage coins over time.
Then, dry the coins gently using a lint-free microfiber cloth or by patting with clean cotton. Avoid rubbing, as even clean cloth fibers can cause micro-abrasions. For highly sensitive metals, consider drying with compressed air or letting them air-dry in a humidity-controlled space.
Once dry, store coins in acid-free holders, coin flips, or archival-quality albums. For long-term storage, opt for inert materials like Mylar or polyethylene. Avoid PVC-based sleeves, which can leach chemicals over time.
If you’re building a display or organizing a collection, it’s wise to include silica gel packets or a dehumidifier in your storage container to prevent moisture buildup.
And always handle coins by the edges—never the faces—to preserve their cleaned surfaces and prevent skin oils from affecting the finish.
Final Take: Is Ultrasonic Cleaning Right for Your Coins?
Ultrasonic cleaning has become a valuable tool for coin enthusiasts, hobbyists, metal detectorists, and even some commercial dealers. It offers a non-invasive way to bring dull, dirty, or buried coins back to life—without scrubbing, harsh chemicals, or endless effort.
But it’s not a one-size-fits-all solution.
If you’re working with common coins, souvenirs, or metal detector finds, ultrasonic cleaning is safe, fast, and effective when done with care. If you’re handling rare or potentially valuable coins, it’s best to seek professional advice or leave them in their original state.
The secret lies not in the cleaner—but in how you use it.
With the right settings, solution, and patience, you can harness ultrasonic technology to refresh your collection without compromising its story, value, or integrity. And that’s what smart cleaning is really about.