Cleaning Coins:
A Photo Essay Comparing the 12 Most-Recommended Methods

Should you attempt to clean tarnished or corroded coins? And if so, what's the best technique?



Coins found by metal detectorists and mudlarks often look like this 1920s British halfpenny — corroded and tarnished beyond recognition, and with no real numismatic value. Should you try to clean it? How?
Coin collectors will always tell you: Never clean your coins. Doing so will ruin their numismatic value.

While that is indeed true for well-preserved collectible coins — dealers will reject any coin that appears to have been artificially cleaned, because it removes the coin's attractive natural patina and suggests that you are trying to pass off a heavily worn coin as being in mint condition — it isn't necessarily true when it comes to coins found in the soil by metal detectorists or by mudlarks searching riverbanks.

What's the difference? Coins in numismatic collections are mostly taken "from circulation" — meaning they were passed from hand to hand, from pocket to pocket, but never subjected to severely harsh, damaging conditions. But coins that have been underground or underwater for decades or centuries are often in truly atrocious condition when found, sometimes so encrusted, tarnished or corroded that it's not even possible to tell what kind of coins they are. And since such badly damaged coins already have essentially no "numismatic value" — no dealer would buy any coins without visible dates or so blackened as to be unrecognizable — maybe cleaning them wouldn't be such a terrible idea after all.

Furthermore, most detectorists and mudlarks have no intention of ever selling their finds, and only want to spruce up some of their worst-condition coins for their own personal enjoyment.

So: I agree that you should never clean extremely old, rare, or valuable collectible coins that are already in decent condition. But coins that are in such terrible condition as to be unreadable, or which are so common that they wouldn't be particularly valuable even if in perfect condition? It couldn't hurt to see if you could clean them up a bit.

Well, actually, it could hurt, if the cleaning method was too harsh. The goal is to clean the outer layer of extreme tarnish or corrosion from the surface of the coin without actually damaging the coin itself in any way. And best of all would be to leave a little bit of the tarnish, to retain the coin's natural-seeming patina, and not end up with a coin that appears artificially shiny.

Tarnish? Patina? What's the Difference?

"Tarnish" and "patina" are actually two different words for the same thing. Most metals, when exposed to air, eventually develop an extremely thin layer of molecules on their surface which chemically combine with reactive elements in the atmosphere: usually oxygen, sulfur or carbon. This layer, once formed, becomes inert and ends up protecting the inner part of the metal from further damage. What distinguishes the two terms, then? It's purely subjective. When we don't like the appearance of this outer layer, especially when it's darker, we tend to call it "tarnish." When we like the appearance of the outer layer, especially when it's lighter, we call it "patina." There are actually four levels of coin discoloration, presented here from mildest to most extreme:

• Toning — An extremely light level of colorful patination that is sometimes deemed desirable by coin collectors.

• Patina — An aesthetically appealing moderate level of patination that gives the impression of antiquity without actually obscuring any of the coin's features. On copper-based coins, it appears light brown or pale golden-red.

• Tarnish — A dark, uneven or disfiguring degree of patination. On copper-based coins, it appears dark brown or even black.

• Corrosion — A more destructive metallic degradation which has begun to reach the inner portion of the metal, often aided by moisture. On copper-based coins, it appears green.

Because tarnish is not merely a layer of dirt but instead a layer of metal molecules that have bonded with oxygen or sulfur molecules from the air, whenever you remove tarnish you are admittedly removing metal from the coin itself. But, if done properly, it's such a microscopically tiny amount that the "damage" cannot be detected even under great magnification.
The problem is that there is no agreed-upon best method for cleaning coins. Keep in mind that the discoloration on tarnished coins is not "dirt," and thus can't be cleaned with mere soap and water; tarnish is instead a layer of oxidized metal which requires more advanced procedures to remove. Many techniques are recommended online — passed along like folk wisdom or Internet rumors without any real evidence as to whether or not they really work or how safe they are. How can you possibly decide which is the best technique without risking accidentally ruining a coin? 

This photo essay answers that question for you. I researched all of the most commonly recommended coin-cleaning techniques, and then performed rigorous experiments using each one, documenting the results with photographic proof. This essay will answer the question: What is the best way to clean severely tarnished coins? The answers might surprise you.

The 12 Most Commonly Recommended Coin-Cleaning Techniques

Dozens of methods for cleaning coins at home are described on various websites, but they all fall into three basic categories:

• Chemical Cleaning with Mildly Acidic Solutions
By far the most common recommendations for cleaning coins involve applying some kind of acid to them — in most cases mild or diluted acids such as citric acid (lemon juice), acetic acid (vinegar), oxalic acid (Bar Keeper's Friend), sulfamic acid (Tarn-X) or hydrochloric acid (a.k.a. "muriatic" acid).

• Mechanical Cleaning
Less common are cleaning techniques based solely on mechanical (i.e. non-chemical) brute-force removal of the outer layer of tarnish, through polishing, rubbing or scrubbing the coin with various types of brushes, felt buffers or other cleaning devices.

• Electrochemical Reactions
Lastly, a few of the recommended cleaning methods rely on electrochemical reactions — placing the coin in an environment in which a chemically induced mild electrical current carries away the tarnish molecules to a "sacrificial anode," generally made of aluminum.

All of these methods are examined in the 12 experiments documented below.

These experiments were all performed on heavily tarnished United States pennies, dated between 1959 and 1970, made from 95% copper and 5% tin or zinc (depending on the year), which were chosen because they are commonplace and of no real numismatic value. Note: Different coins from different countries or from different years might be made from different alloys and therefore may produce different results than those shown here, but overall I think the results will be similar with any type of coin.

I present the results below ranked from worst (not recommended) to best (recommended).

In each case, the photo on the left shows the coin BEFORE cleaning; the photo on the right shows it AFTER cleaning.

The test results below each photo pair are color-coded: Effects in RED are unwanted; effects in YELLOW are neutral or a mix of good and bad; effects in GREEN are desirable.


Boiling in Lemon Juice With Aluminum Foil
    Before     After

Too harsh Strips off all the patina Changes the color of the metal Erodes away corroded areas

Since tarnish is a layer of oxidized metal, and since most acids eat away at most metals to some small degree, various types of acidic solutions are recommended to dissolve away the tarnish on coins. The trick is to find an acidic solution that is so mild that it removes only the outermost layer of tarnish molecules and not the pure unoxidized metal of the coin itself. Citric acid (in the form of lemon juice) is often recommended for this purpose. Heat accelerates the chemical reaction, so some people recommended putting heavily tarnished small metal objects (such as coins) into lemon juice that has been heated to boiling in a pot. Adding a scrap of aluminum foil to the pot is thought to intensify the reaction even further by creating an electrochemical current which carries away the molecular components which comprise the tarnish. While this technique did indeed very rapidly remove all the tarnish (as the photos reveal), it also went way overboard by completely stripping off all the natural patina, and then chemically altering the color of the remaining copper to an unnatural pinkish hue. Furthermore (as discussed below in the afterword about heavily corroded older coins), the heated citric acid will quickly start to eat away at and dissolve any areas that have any corrosion (which originally appears green on copper-based coins), leaving small pits and craters.

NOT RECOMMENDED UNDER ANY CIRCUMSTANCES



Boiling in Lemon Juice
    Before     After

Too harsh Strips off all the patina Changes the color of the metal Leaves discolored areas

This is the exact same technique as in the first test, but without the addition of aluminum foil to the boiling lemon juice. As in the first test, boiling the coin in plain lemon juice quickly removed all the tarnish, but just as quickly stripped off any remnant of patina and transformed the copper to a strange pink hue. Furthermore, it left irregular discolored stains on various parts of the coin that weren't there to begin with. It probably would have also eaten away at any corroded area, but this particular penny didn't have any green corrosion at the start of the experiment. And as you will see in the Epilogue below, both of these lemon-juice techniques (along with most of the other acid-based techniques) did not achieve lasting effects; within weeks, both of these coins began to acquire new spots of tarnish, further disfiguring them.

NOT RECOMMENDED UNDER ANY CIRCUMSTANCES



Rubbing With a Paste Made From Flour, Salt and Vinegar
    Before     After

Removes most the tarnish Strips off most the patina Changes the color of the metal Leaves discolored areas

This technique — of mixing some drops of vinegar into a few spoonfuls of flour mixed with a little salt, then rubbing the paste onto a coin — is frequently touted as a reliable old-fashioned and safe recipe for cleaning tarnished metal, without having to use harsher store-bought cleaning products. It is basically the same as many similar recommendations to place the coin in vinegary foods such as Worcestershire sauce or ketchup. In all cases it's simply a way to get the metal into contact with acetic acid, the primary component of vinegar. However, while my experiment showed that this method does indeed remove the tarnish, it also altered the color of the underlying exposed copper to a vaguely pinkish hue, and left various unsightly discolorations, very similar to what happens when boiling an identical coin in lemon juice. The final photo below showing all 12 pennies together reveals more clearly the washed-out, pinkish flat appearance of these first three techniques.

NOT RECOMMENDED UNDER ANY CIRCUMSTANCES



Using a "Suede Cleaning Bar" to "Erase" the Tarnish
    Before     After

Removes the worst of the tarnish Leaves some of the patina Ruins the coin with scratches and scuff marks

Various companies that make leather shoes and clothing also advertise "suede cleaning bars" — resembling rubber erasers — that purport to quickly remove stains from suede garments. Just rub the bar over the stain and it disappears, claim the manufacturers. Some online users allege that these bars (apparently made from rubber mixed with trade-secret cleaning chemicals) also remove tarnish from metal objects such as coins. I tried rubbing one on a copper penny, and discovered that it did remove the tarnish somewhat, while leaving part of the patina intact — a good result. The technique also allows users to clean as little or as much as they want, rather than the "all or nothing" risk of the chemical-based methods. However, disastrously, upon close inspection the bar seems to have extensively scratched the surface of the coin, probably with abrasive grains embedded in the rubber. It was a little hard to confirm, but a close examination reveals many scratches in the "After" picture that don't exist in the "Before" picture. This visible damage to the coin renders any positive aspects of the technique irrelevant.

NOT RECOMMENDED UNDER ANY CIRCUMSTANCES



Dipping in Muriatic Acid, Then Cleaning With Baking Soda
    Before     After

Completely removes all tarnish and patina Does not change the color of the metal Leaves the coin with a bright unnatural sparkle Quickly eats away at any imperfections

Mild solutions of hydrochloric acid are often sold in hardware stores under its old-fashioned name "muriatic acid," labeled to be used only for "concrete etching." Some people use it to remove "flash rust" (a thin layer of quickly-formed rust) from damaged tools and car parts. Be warned: Although the consumer version of muriatic acid is greatly watered down, it is still very corrosive, so handle with caution, always outdoors. For cleaning the penny, I poured a very small amount (about one teaspoon) of the acid into a small ceramic cup, dropped the coin in and quickly muddled it around with a Q-Tip cotton swab. After about 20 seconds, I then neutralized the acid by dumping a large tablespoon of baking soda into the cup, causing it to foam and fizz. I then quickly washed off the coin and further rubbed it down with fresh baking soda to remove any residual acid that might be on it. Result? All the tarnish was completely removed (good), as was all the patina (bad), and the underlying copper color was unchanged from its natural hue (good), unlike the strange color produced by the first three examples above. However, the coin was far too bright and sparkly (moreso than the photo reveals), and had the appearance of having been very obviously artificially cleaned. Furthermore (as discussed below in the afterword about heavily corroded older coins), this technique is catastrophic for any coins that have active corrosion (not present in this example), as the acid deeply eats away at any parts where the metal was already weakened. If you have an otherwise good-condition recent coin that you want to be bright and sparkly for a few days, then muriatic acid does the job; otherwise it is far too strong and abrasive.

NOT RECOMMENDED



Washing with Liquid Bar Keeper's Friend
    Before     After

Completely removes all tarnish Completely removes all patina Does not change the color of the metal Leaves the coin with a bright artificial shine

Not all cleaning products are the same; some really do have ingredients which make them more effective than competitors. One such product is Bar Keeper's Friend, which is (as far as I know) the only consumer cleaner than contains a mild solution of yet another type of acid, called oxalic acid. It is very effective at removing stains and rust from metal items, which is why it has occasionally been recommended online as a coin cleaner. I used a dish sponge to rub the liquid form of Bar Keeper's Friend into a penny, and it very efficiently removed all of the tarnish — but, unfortunately, almost all of the patina too. As with muriatic acid, Bar Keeper's Friend did not change the natural color of the penny, and was even slightly superior in that it did not produce the same kind of unwanted reflective sparkle that muriatic acid did. Even so, the final result was once again far too "clean" and shiny for it to look like a normal penny, and again had the appearance of having been very obviously artificially cleaned. Note: In subsequent experiments, the oxalic acid also ate away at weakened corroded areas of severely damaged coins, though not as strongly as muriatic acid. There are also online warnings to avoid using Bar Keeper's Friend on anything silver, as it can eat away at the surface (which didn't seem to happen with this copper coin, though). Use only if you want a completely shiny coin. Also note the Epilogue below, which reveals that coins cleaned with this product will rather quickly begin to develop new blotches of tarnish.

NOT RECOMMENDED



Rubbing with a Paste Made from Bar Keeper's Friend Powder
    Before     After

Completely removes all tarnish Completely removes all patina Does not change the color of the metal Leaves the coin with a bright artificial shine

The results of this experiment — mixing a few drops of water into a spoonful of Bar Keeper's Friend powder — were essentially the same as with the liquid form of Bar Keeper's Friend. After I rubbed the paste onto the coin, all tarnish and patina were completely removed, the copper color was left unchanged and natural, and the coin displayed a bright clean shine that no normal uncleaned coin would ever have. If you do seek a completely shiny coin, the paste/powder form of Bar Keeper's Friend might be slightly preferable to the liquid version because with the paste you can more easily control how much cleaning you do. But it's still not recommended if you are trying to retain a natural appearance with some patina. Also note the Epilogue below, which reveals that the shine resulting from oxalic acid doesn't last very long, and after just a few weeks the coin will again start to become tarnished.

NOT RECOMMENDED



Rubbing with Tarn-X
    Before     After

Completely removes all tarnish Removes most, but not all, patina Changes color of the metal, but only very slightly

Tarn-X is a consumer product specifically advertised for removing tarnish from metal items, including coins. Its primary active ingredient is yet a different type of acid, called sulfamic acid. There are many warnings online as to its toxicity and bad smell, so if you decide to use Tarn-X, always do so outdoors while wearing gloves or other hand coverings. For this experiment, I placed the penny on a small ceramic plate, moistened the end of a Q-Tip cotton swab with Tarn-X, then rubbed this on the face of the coin, which I held in place with a wooden chopstick. After repeating this process on its other side, I rinsed the coin with water. In less than a minute, all tarnish had vanished from the coin. Most of the natural-looking patina had also been removed, but unlike with the experiments mentioned above, a small amount of patina remained. Also, the Tarn-X very slightly altered the color of the remaining copper to be a little bit more pink than normal, but not nearly as extreme as in the first three experiments on this page. The overall result of cleaning with Tarn-X made the coin look somewhat unnatural, but not as severely so as in all the experiments above. Ultimately, I can't recommend it, but this was the "best" (i.e. least destructive) of the "not recommended"s. Also note the final photo below of all 12 coins together, which reveals that any coin cleaned with Tarn-X will quickly begin to develop new unsightly tarnish spots.

NOT RECOMMENDED



Soaking in an Aluminum Pan with Boiling-Hot Water, Baking Soda and Salt
    Before     After

Removes most tarnish Leaves most patina Does not noticeably change color of the metal Causes the coin to became re-tarnished within weeks

Place the coin in either an aluminum pie pan or a glass/ceramic cup or bowl that has been lined with aluminum foil. You can also lightly encase the coin in a scrap of aluminum foil to speed up the process, though this is not technically necessary. Sprinkle a spoonful of baking soda over the coin, add a few shakes of salt, then pour boiling water into the container so the coin is thoroughly immersed. Then sprinkle a bit more baking soda and salt into the water and stir with a wooden chopstick. The exact amount of baking soda, salt and water is not particularly important, but do make sure that the aluminum foil and water do not touch any other metal object during the soaking process, as this would interfere with the electrical reaction. After a few minutes, check the coin; if it needs more cleaning, refresh the container with more boiling water and more baking soda. After you deem the coin to be approximately as clean as you want, gently sponge it off in soapy water to remove any residue of baking soda and salt. The end result is a coin with more clearly visible features yet which does not look artificially cleaned. Note: This method is almost always recommended for cleaning the tarnish from silver, but almost never for cleaning copper, bronze, or other alloys found in low-denomination coins. But I found that (at first) it seemed to work quite well — for copper-based coins, at least. This process is unique in that it is neither mechanical removal nor chemical removal, but instead an electrochemical process in which the metallic compounds comprising the tarnish are conducted from the surface of the coin to the aluminum foil by a very weak electrical current generated by the two different metals in a conducting liquid medium. I'm not entirely sure why the process seems to remove some of the darker parts of the tarnish while leaving the more appealing parts of the patina — I suspect it has something to do with the various copper compounds from which the tarnish/patina are composed, with the electrochemical reaction carrying away the darker compounds (perhaps sulfur-based) while not as successfully carrying away the oxygen-based compounds. Whatever the reason, at least immediately subsequent to cleaning, the coin was lightened, yet some amount of the appealing patina remained, resulting in a better-looking coin that still appeared somewhat "natural." HOWEVER, although this method seemed effective immediately after the experiment, just three weeks later (as revealed in the Epilogue below) the coin was already covered in a new layer of dark tarnish. So, ultimately, I cannot recommend this method, even though the initial results seemed promising.

NOT RECOMMENDED



Scrubbing with a Soft Metal-Bristle "Suede-Cleaning" Brush
    Before     After

Removes most tarnish Leaves most patina intact Does not change the color of the metal May or may not leave microscopic abrasions

This is the most surprising — and and likely to be most controversial — result of my tests. Brushes designed to clean suede (a kind of soft leather) are sometimes made with two sections: an outer ring of synthetic nylon bristles and an inner core of very fine, thin brass bristles. You can see a picture of one here. (The other type of suede brushes that have only nylon or natural bristles will have little to no effect on tarnish.) Hold the coin in the palm of one hand, add a small amount of soapy water, and scrub the coin with the kind of suede brush that has some brass bristles. Continue on both sides of the coin for as long as needed (from one minute up to several minutes), depending on how tarnished it was and how much discoloration you want removed. The end result? Most tarnish will be removed, while most of the natural patina and color will remain, without any visible damage to the coin. One might think that the brass bristles would necessarily scratch the surface of the coin, but even under close examination I could not see any scratches that were definitively caused by the brush. It could be that the brass bristles do cause very small scratches only visible at extreme magnification, but I saw none. The coin, after having been scrubbed with the suede brush, was cleaner yet still looked fairly "natural," with a normal-seeming patina. Make sure to use a suede brush that has the finest possible brass-wire bristles. This method's only downside is that it can take up to several minutes, depending on each coin's original condition. Note that these brushes are not the same thing as the "suede bars" described above.

RECOMMENDED



Rubbing with a Paste Made from Baking Soda and Lemon Juice
    Before     After

Removes most tarnish Leaves some patina intact Does not change the color of the metal Highlights imperfections

This technique — of mixing a paste from lemon juice and baking soda — is usually recommended online alongside the previously mentioned technique of combining vinegar with flour and salt, as if the two methods were basically equivalent and interchangeable. But my experiments showed that they are not similar at all in their effects. While the vinegar recipe produced unsightly results, this recipe had among the best results of all the cleaning experiments I tested. First, place a few spoonfuls of baking powder in a small dish or cup, then squeeze in fresh lemon juice drop by drop (only a few drops are required), and stir the mixture until it is a thick paste. Then, using your fingers, rub a small amount of the paste onto both sides of the coin. Rinse off, and repeat with more paste. Continue repeating until you have removed the desired amount of tarnish. This process allows you to only clean off as much as you want, and not go overboard. With the earlier boiling-lemon-juice experiments, the cleaning-power of the citric acid was overly amplified by heat; but in this one, the citric acid is mostly neutralized by the alkali baking soda, so it is much more mild and controllable. In the example shown here, I did go a little overboard and cleaned it too much, producing a surface in which the imperfections and scratches were somewhat highlighted. But overall the color and patina of the cleaned penny looked much more natural than in any of the experiments described above.

RECOMMENDED



Polishing with a Wool Felt Buffer
    Before     After

Removes tarnish to whatever degree desired Leaves patina mostly intact Leaves the color of the metal unchanged Can require a great deal of effort

This technique is essentially nothing more than intensively polishing or rubbing the coin until the tarnish is mechanically removed. If done by hand it would take weeks for each coin. But it is much quicker and more effective to use a "rotary tool" such as a Dremel or an electric drill to which has been attached a wool felt-tipped mandrel: a small conical piece of felt at the tip of a slender shaft or drill bit. Because this method uses no acids, chemicals, soaps or liquids of any kind, the color of the underlying metal remains unaltered. Also, you can pause or stop at any time to check whether you've cleaned the coin sufficiently to your liking — without going too far. Attach to your rotary tool the felt-tipped "mandrel" or "buffer wheel" (different manufacturers use different names for the same thing), turn it on to get it spinning, and then gently apply the tip to a coin that is held in place with a wooden chopstick or something similar. Before first trying this, I feared that the polishing would simply make the coin shiny without actually removing any tarnish, but the opposite proved true: Polishing eventually removed the tarnish (little by little) without imparting an unnatural shiny appearance to the coin. The primary drawback to this method is that it may take a long time, depending on the condition of the coin. I polished the example shown here for nearly fifteen minutes before stopping (although I was using a handheld electric drill instead of a proper rotary tool, which would have done the job much more quickly). Also be careful to use a felt polishing tip that has not worn away; if the supporting metal shaft touches the coin while the tool is spinning, it could damage the surface. Although this technique was admittedly the least effective, leaving more tarnish than any other technique, it was also the least destructive, producing no unwanted results or aftereffects. And since the goal of the overall experiment was to improve the appearance of coins without harming them in any way, the mildness of this polishing technique means that it is one of the few I can recommend.

RECOMMENDED



Epilogue: Three Weeks Later

This image shows all twelve coins, three weeks after the photos above were taken (arranged first-to-last in the same order as on this page), showing how they all look in direct comparison with each other, and also revealing the long-term aftereffects of the experiments (click to see the photo in high resolution):



The odd pinkish hue of the first three coins in the top row is clearly visible here, as is the overly shiny appearance of the first three coins in the middle row.

Furthermore, all six of these coins (as well as the final coin in the middle row) have very quickly begun to tarnish again, with each of them acquiring unsightly new blotches that only get worse over time.

All seven of these freshly discolored coins were the ones cleaned with acidic solutions; so we can conclusively confirm their "Not Recommended" status with the proviso: Never use any cleaning method involving straight acid of any kind. Even if the coin appears brilliantly shiny immediately after cleaning, it will rapidly re-tarnish and end up looking worse than when you started.

The most surprising revelation in this three-weeks-later photo is that the coin at the bottom left — which had been electrochemically cleaned by soaking in an aluminum pan with boiling-hot water, baking soda and salt — had turned very dark with a new layer of unsightly tarnish, mere days after the experiment. There must be some metallurgical explanation for this, but it is disappointing nonetheless.

The top-right coin looks the same as it did immediately after the experiment, but it was ruined to begin with (scratched by the suede-cleaning bar).

Which leaves just the final three coins in the bottom row, all of which appear completely unchanged three weeks after the cleaning experiment. And all three of these coins were the ones cleaned by the "Recommended" methods — i.e. the methods which were the least destructive and which retained the coins' natural-seeming appearance to the greatest degree. Although these three methods may seem to the casual observer to be the least impressive, that actually was the goal of the experiment: to find a way to clean the coins somewhat without going overboard and removing all of the appealing patina.

While it is true that one of these recommended methods (lemon juice and baking soda) does contain an acid, the alkali nature of the baking soda neutralizes the citric acid in the juice, so the resulting paste is probably no longer acidic, or at least only so mildly acidic that it does not cause any of the unwanted effects of the other acid-based techniques.

So, this photo confirms my final recommendations:

The three best coin-cleaning methods are

• Scrubbing with a Soft Metal-Bristle "Suede-Cleaning" Brush

• Rubbing with a Paste Made from Baking Soda and Lemon Juice

• Polishing with a Wool Felt Buffer


Remember: Never clean extremely old, rare, or valuable collectible coins. And there is no requirement to clean your coins at all, if you don't want to. These recommendations are only for those people who have already decided that they want to clean heavily tarnished coins which otherwise have no real numismatic or historical value.



Final Warning: Corrosion + Acid = Disaster

As a demonstration of just how damaging acid-based cleaning methods can be, for one last experiment I selected a severely corroded 1944 British penny which was found with a metal detector, having spent many decades buried in damp soil.

Knowing full well that I was almost certain to ruin the coin, I wanted to see what would happen if I tried some of the strongest cleaning methods on it — boiling in lemon juice, then cleaning with oxalic acid (Bar Keeper's Friend).

This is what the coin looked like at the start of the experiment:

It was not merely tarnished, but severely corroded with a layer of green "verdigris," or copper carbonate, the result of many years exposed to outdoor conditions. The obverse was completely obscured, while the reverse was barely recognizable as a penny at all.

After being boiled in lemon juice for exactly one minute, then being rinsed and dried, the coin looked like this:
All the corroded copper simply dissolved away, so that the features of the coin have been inverted from their normal state: instead of being in "relief" (i.e. standing out from the background surface), all of the design features are now "incuse" (i.e. indented below the background surface). This happened because the exposed relief features were the areas of the coin most affected by the corrosion, and thus the weakest. The end result is a coin whose features all sink inward (like valleys) instead of extending outward (like hills, as on any normal coin).

Furthermore, the coin had a completely dull "matte" appearance and an odd rusty color.

I then took this coin and "cleaned" it once again, this time by rubbing it with the liquid version of Bar Keeper's Friend. The final result looked like this:
Bar Keeper's Friend "fixed" the matte surface and odd color, by making the coin shiny and restoring its copper hue. But now it looked even more bizarre than before: a gleaming shiny penny that was nonetheless still severely worn and which had all of its design features extending inward. The only good thing one could say about the experiment is that afterward the coin was finally recognizable as a 1944 penny. But it was so thoroughly ruined, and so strangely artificial-looking, that I can conclusively state that using acid-based cleaning methods on corroded coins is just about the worst thing you can do to them, and should be avoided at all costs.

In memoriam: Let us express our gratitude to the poor 1944 penny, which sacrificed itself for science, to save all its fellow coins from a similar fate.